Published: November 7, 2024
In this blog post, we are discussing whether intermittent fasting is a good approach to lose excess body weight and fat mass, and whether it can improve our insulin sensitivity, insulin secretion, and blood sugar control. I will also review several trials that have investigated the degree to which intermittent fasting can bring type 2 diabetes into remission. And I’ll close by sharing practical tips to consider when choosing a specific form of intermittent fasting, and how to implement it.
Different Types of Intermittent Fasting
Intermittent fasting simply means that we make a conscious effort to not eat or drink anything with calories for a defined period of time, or to consume considerably less than we usually would.
Time-restricted eating, or TRE, is a type of intermittent fasting where we eat only in a narrow window of most commonly 6-10 hours during the day, allowing us to have an extended fasting period of 14-18 hours overnight. In my opinion, TRE is a separate case from what we will discuss in this blog post here, because in TRE, we are not ever consciously restricting our calorie intake. We just restrict our eating window, and in that regar, it should be considered separately. I have, therefore, made two separate blog posts about the impact of TRE on body weight and fat mass as well as blood glucose regulation previously.
The other forms of intermittent fasting that we are going to discuss in this blog post (see figure above), all share that they include fasting on some days. Let’s call these fast days. Calorie intake is restricted on these days, either to zero kcal but more typically to around 500-800 kcal per day. If we only had fast days, we’d call this continuous calorie restriction. But because what we are talking about here is intermittent fasting, fast days are usually interrupted by what we may call feast days, during which we can eat to satiety. We also call this ad libitum. I think it’s important not to misunderstand the term ‘feast day’. This simply means you don’t need to count and restrict calories; it doesn’t mean that you should stuff yourself as much as possible on these days.
One of the most common and most intensely studied forms of intermittent fasting is alternate-day fasting, or ADF. In ADF, fast days and feast days alternate. There are different ways of doing this. On fast days, we could totally fast and not eat or drink anything with calories, but the more common version is one where we eat something around 500-800 kcal on these fast days. So we count and consciously restrict our calorie intake on fast days, but then get a break on feast days because we can eat as much or as little as we want. We also get a break from calorie counting on feast days. The idea here is to make it easier to create a calorie deficit and stick to it by alternating fast days and feast days. The question, obviously, is whether this works to create a calorie deficit, or if people just eat so much more on feast days that they won’t lose weight. We’ll get to that later in this blog post.
A related concept that is a bit more relaxed is the 5:2 diet in which we pick two non-consecutive days of the week during which we fast, and then we feast on the remaining five days.
A bit stricter would be the 4:3 diet, which is similar to alternate-day fasting.
A bit less strict would be the 6:1 diet, in which we fast just one day a week and eat normally for the remaining six days.
We can take this concept of incorporating fast days into our normal eating habits in any way we like, really. One study, for example, that we are going to talk about later used 5 consecutive days of fasting followed by 10 days of eating normally, a 15-day rhythm that one can repeat as often as desired. That would, on average, be fasting a bit more than 2 days per week.
Impact of Intermittent Fasting on Body Weight and Fat Mass
So the obvious question is, do these different types of intermittent fasting lead to weight loss? The quick answer is: yes, they pretty universally lead to a reduction in overall calorie intake and a loss of body weight.
How much weight can we expect to lose with intermittent fasting? From the published papers in this area, I have prepared the graph below to summarize the degree of weight loss in kg per week (y-axis) relative to the number of fast days per week (x-axis). Each blue dot represents the average weight loss from one published randomized controlled trial. In all studies considered here, calorie intake averaged between 500 and 800 kcal per day on fast days. In contrast, calorie intake was ad libitum, meaning to satiety, on the remaining days of the week. So, those fasting for three and a half days per week, that is alternate-day fasting. And fasting for 3 days per week is the 4:3 diet, while fasting for 2 days per week represents the 5:2 diet.
Let’s first focus on alternate-day fasting. You see that in all studies, participants lost weight with alternate-day fasting. In some studies, this averaged just a quarter of a kg, or half a pound, per week (Bhutani et al. 2013; Trepanowski et al. 2017; Ezpeleta et al. 2023). In most studies, weight loss averaged about half a kg, or one pound (Varady et al. 2013; Klempel et al. 2013; Hoddy et al. 2014; Varady et al. 2015), but there were also two studies in which weight loss approached one kg, equivalent to about two pounds, per week (Stekovic et al. 2019; Cai et al. 2019). In general, and with some exceptions, weight loss per week is higher in shorter-term studies over 4 or 8 weeks, whereas it is lower in longer-term studies of 12 to 24 weeks. That is simply because our body adjusts to weight loss, and the rate of weight loss slows down over time as we lose weight.
Now, how about the 4:3 diet and the 5:2 diet? These data are a bit unexpected, in that there are several studies where people fasted less frequently and still lost more weight. Quite a bit more weight even. Why is that? Well, the four circled studies all tested a combination of intermittent fasting and a high-protein diet. As we have discussed previously, a higher protein intake makes our meals more satiating per calorie, and calorie intake and body weight spontaneously drop. What we can see clearly is that combining intermittent fasting with a high-protein diet leads to greater weight loss than intermittent fasting alone (Coutinho et al. 2018; Arciero et al. 2022; Arciero et al. 2023; Guo et al. 2024). That is an interesting observation, and I’ll get back to this later in the blog post, but for now, let’s ignore these studies to figure out how much weight we can expect to lose just by fasting intermittently.
What we are seeing is more or less what we would expect: the rate of weight loss per week is lower if we fast intermittently on fewer days of the week. One can expect to lose an average of half a pound to a pound per week with the 5:2 diet (Carter et al. 2018; Sundfor et al. 2018; Hottenrott et al. 2020; Gray et al. 2021; Holmer et al 2021; Li et al. 2024; Wang et al 2024) and – as one would expect, a bit more with the 4:3 diet (Hutchinson et al 2019; Maroofi et al. 2020; Razavi et al. 2021) or alternate-day fasting.
There is one particular study that is worth talking about in a bit of detail: Hutchison and colleagues took a group of 88 women and randomized them into four groups (see figure below). Over the course of eight weeks, all participants received all of their food. Two of the groups were given food at 100% of their caloric needs, while the other two groups were given food at 70% of their caloric needs. And in each of these pairs, one group followed a 4:3-intermittent fasting diet, with three fast days and four feast days per week. The way the 4:3 diet was implemented here is that calorie intake was a lot lower on the three fast days each week, and, accordingly, higher on the four feast days each week, so that the overall calorie intake over the entire week was either at 100% or 70% of the participant’s energy needs.
As you can imagine, participants randomly grouped into the two 70% calorie groups lost more weight than those randomized to the 100% calorie groups. That makes sense. Among those who received 70% of their estimated calorie needs, weight loss was about the same, independent of whether they followed the 4:3 diet or maintained the same reduced calorie intake every day (the difference between eating 70% of calorie needs continuousy every day, or doing it using a 4:3 diet intermittent fasting approach was not statistically significant). What is particularly interesting is that among participants who received 100% of their calorie needs, those who followed the 4:3 intermittent fasting diet lost significantly more than those who had a continuously stable calorie intake. Let this sink in for a second: investigators gave participants 100% of their calorie needs every day on the 100% continuous calorie intake diet, and as a result, somewhat predictably, their weight was very stable. However, even though participants in the 100% calories with the 4:3 diet group received the exact same type and amount of food, they ended up losing a significant amount of weight. 2.5 kg of 5.5 pounds, that’s not nothing. Now, some people may say: oh, if all food was provided in this study, does that mean that calories don’t matter and you can lose weight with intermittent fasting even if you don’t eat fewer calories? I don’t think so. Because in this study, while investigators provided all of the food, they didn’t really track whether all of the food was eaten every day. I think what these findings simply mean is that people simply cannot maintain this level of calorie intake when fasting intermittently. In other words, even though participants were not supposed to restrict their calorie intake, following the 4:3 diet led to a spontaneously lower calorie intake, just like we see in free-living people, showing that reduced calorie intake and weight loss even manifest when participants are provided with enough free and prepared food in an amount that should not lead to weight loss.
Taking all of these data together, I’d say that the evidence is solid that adopting any of these intermittent fasting approaches where we restrict our calorie intake on some days powerfully leads to an overall lower calorie intake and a spontaneous loss of body weight. And if we lose body weight on intermittent fasting, about two-thirds of that weight loss comes from losing fat mass and about one-third comes from losing fat-free mass, which includes muscle tissue. That is the average, at least, and that is similar to losing weight from any other method.
The primary health benefit of losing even a few pounds of our excess body fat mass is that we tend to lose fat disproportionally from the visceral and ectopic fat depots, meaning percentage-wise, we lose more visceral and ectopic fat than the overall loss of body weight and fat mass would suggest. For example, in a study by Holmer and colleagues, patients with non-alcoholic fatty liver disease, or NAFLD, followed a 5:2 diet for 12 weeks. They lost, on average, 7.4% of their baseline body weight, but their liver fat content was reduced from a baseline of 12% liver fat to 5.9%. So that’s a reduction by 6.1 percentage points in terms of the absolute liver fat content, which is very meaningful, or – expressed differently – a relative reduction from baseline by 51%.
OK, so it’s a good thing if we lose some body weight while fasting intermittently that most of that is lost in the form of fat mass, and if we do have excess visceral and ectopic fat, we tend to lose that first. However, it comes at a bit of a price, because on average, people also lose some lean mass, including muscle mass. That is not desirable, because having more lean muscle mass is a key determinant of long-term health: for example, it helps us be physically more capable, more functional, and it increases our energy expenditure, which means with more muscle mass, we are less susceptible to weight gain; and having more muscle mass helps us regulate our blood sugar levels after a meal. We can reduce the degree of lean mass loss by doing two things: one: make sure we keep our protein intake high, both on fast days and on feast days, and we should engage in regular resistance training or at least some other form of exercise while we are fasting intermittently. Several studies have directly tested this, and show that prioritizing protein and engaging in regular exercise can help us largely preserve our lean mass (Coutinho et al. 2018; Arciero et al. 2022; Arciero et al. 2023; Guo et al. 2024; Bhutani et al. 2013; Ezpeleta et al. 2023; Hottenrott et al. 2020; Li et al. 2024).
OK, and then, lastly: do we know whether there are groups of people who have more or less success losing weight when adopting intermittent fasting, such as men vs. women, or pre- vs. postmenopausal women? The data that allow us to answer that question are pretty limited. At this point, we only have a secondary data analysis by Lin et al. that has looked into this. That study showed no suggestion of any difference in weight loss in men, premenopausal women, and postmenopausal women who were engaged in alternate-day fasting. So, at this point, the limited evidence does not suggest that intermittent fasting is more or less effective depending on your sex, age, or menopausal status.
Why Does Intermittent Fasting Reduce Body Weight?
So why do we lose weight when we fast intermittently? Why don’t we just eat more on feast days to make up for our caloric deficit on fast days?
The answer to this is because of incomplete compensation. This is an important concept that is worth explaining some more.
Let’s create an example of someone who has a normal calorie intake of 2,000 kcal per day, and they are weight stable at that intake level. Now they start alternate-day fasting and eat 500 kcal on fast days. That creates a deficit of 1,500 kcal on these fast days, and as a result, they may eat more on feast days. The kicker here is that they do eat more, but not 1,500 kcal more. The data suggest that they may eat 200 or 300 kcal more. The exact number is difficult to quantify because, in most studies, calorie intake is quantified by self-report, and self-report is not very accurate if we want to quantify someone’s calorie intake. But what we can say clearly is that the average calorie intake across each pair of two days is lower than 2,000 kcal, because we consistently see people lose weight when they follow alternate-day fasting. In other words, yes, people do compensate for the lower calorie intake on fast days, but not completely. That’s why I would call the principle that leads to weight loss with intermittent fasting incomplete compensation.
Incomplete compensation also acts in the other direction, which is important to understand. Let’s say we have a constant calorie intake of 2,000 kcal, but one day, Thanksgiving comes along, and we eat 4,000 kcal. 2,000 kcal more than normal. And in the few days following Thanksgiving, we will eat a little less, spontaneously, even without making an effort. But that reduction in spontaneous calorie intake in the days following Thanksgiving does not amount to 2,000 kcal, so we are going to gain a tiny bit of weight. And if this happens regularly for us, that’s why we will gain weight over time.
By the way, I am simplifying a bit here. As we fast or overeat, there will also be other adaptive changes, such as small changes in energy expenditure. Such that if we fast, our energy expenditure goes down a little bit, and vice versa, if we overeat, our energy expenditure will increase. So there are compensatory mechanisms to both fasting and overeating, but if we fast a lot, as in intermittent fasting, or overeat a lot, these compensatory mechanisms will be insufficient and can still lead to weight change.
So, the lesson we learn here is that intermittent fasting helps us reduce our calorie intake and lose weight because our body compensates only incompletely for the very low calorie intake on fast days. And vice versa, days during which we overeat may similarly be a key driver of excess calorie intake and weight gain, also due to incomplete compensation.
Impact of Intermittent Fasting on Blood Glucose Regulation
As I have explained in a previous blog post, carrying excess body fat in visceral and ectopic depots is one of the most common causes of insulin resistance and glucose intolerance. So it is probably not overly surprising that when people adopt some form of intermittent fasting and lose weight, their insulin sensitivity, glycemic control, and glucose tolerance tend to improve.
However, broadly speaking, two conditions need to be met to see improvements in insulin sensitivity or glycemic control in people following intermittent fasting:
- the people studied must have suboptimal blood glucose regulation at baseline, and
- they must be overweight or obese and lose a significant amount of weight and fat mass while fasting intermittently.
In general, people with overweight or obesity who start insulin-resistant experience improvements in insulin sensitivity if they lose significant amounts of weight and fat mass (Trepanowski et al. 2017; Gabel et al. 2019; Ezpeleta et al. 2023; Holmer et al. 2021; Hutchison et al. 2019).
And if they also have elevated blood glucose levels, such as elevated fasting glucose or HbA1c levels, a loss of weight and fat mass is usually accompanied by a decrease in these biomarkers of glycemic control (Yang et al. 2023; Holmer et al. 2021; Li et al. 2024).
We also have limited evidence that weight loss associated with intermittent fasting improves glucose tolerance in an oral glucose tolerance test, or OGTT (Li et al. 2024). The problem here is that most studies in this field did not conduct an OGTT or more sophisticated tests of blood glucose regulation, and that is also why we have no good data on the impact of intermittent fasting on beta-cell function, i.e., the ability of the pancreas to produce insulin. It is plausible to hypothesize, however, that if a sufficient percentage of ectopic, specifically pancreatic, fat is lost, it seems likely that beta-cell function may also moderately improve with intermittent fasting-induced weight loss.
Interestingly, improvements in blood sugar regulation are not always seen, and some people have used this to argue that intermittent fasting is not effective in improving insulin sensitivity, glucose tolerance, or glycemic control. Now, if we take a closer look at these studies that did not detect an impact of intermittent fasting on blood sugar regulation, these fall into two categories.
The first were studies done in people with normal blood sugar levels and normal insulin sensitivity at baseline (Cai et al. 2019; Arciero et al. 2023; Sundfor et al. 2018). As I have explained before, if blood sugar levels are regulated within the normal range, and someone has normal insulin sensitivity, then further improvements may simply not be possible. Or only very small improvements are possible that are difficult to detect in a trial.
The second were studies in which intermittent fasting was compared to continuous calorie restriction, where participants in both groups lost similar amounts of body weight and fat mass. In several such studies, measures of glycemic control or insulin sensitivity improved in both interventions to a similar degree (Trepanowski et al. 2017; Carter et al. 2018; Gray et al. 2021; Wang et al. 2024; Xiao et al. 2022). And if we were to look at this superficially, we may conclude that there is nothing special about intermittent fasting. And indeed, that’s an important point: all of the different types of intermittent fasting we are discussing in this blog post are simply alternative approaches to help people achieve a caloric deficit and lose excess body fat mass. People who have high blood glucose levels or who are insulin resistant at baseline see Improvements in glycemic control and insulin sensitivity to the same degree as is seen with similar weight loss from other interventions. In other words: intermittent fasting, such as alternate-day fasting or the 5:2 diet, does not seem to have weight loss-independent benefits for blood sugar regulation. That is in contrast to early time-restricted eating, which seems to have benefits for blood glucose regulation that are partly explained by the weight loss it tends to trigger, and partly independent of that weight loss. I discussed this in more detail in the last blog post <points to upper left corner>.
Now, I am concluding that intermittent fasting does not have weight loss-independent effects on insulin sensitivity even though there is one paper by Gabel and colleagues, a secondary analysis of a randomized controlled trial, that claims that alternate-day fasting improves insulin sensitivity more than similar weight loss from continuous calorie restriction. One problem with this claim is that while the weight loss between the alternate-day fasting group and the continuous calorie restriction group was not statistically significant, it probably was clinically relevant. Specifically, participants in the alternate-day fasting group lost about 3 kg or 6 pounds more weight, which could be the driver of the greater improvements in insulin sensitivity in the alternate-day fasting group. We also need to consider that this was a secondary analysis based on a subgroup of the overall study population from a larger trial.
Given that this is also the only study suggesting weight loss-independent benefits of intermittent fasting for insulin sensitivity, my conclusion is not overly affected by that one secondary analysis: my conclusion based on the entirety of the evidence is that intermittent fasting improves insulin sensitivity and glycemic control by helping people shed excess body fat, in other words, through a mechanism that involves losing weight. If you follow alternate-day fasting or the 5:2 diet and you don’t lose any weight, you should not expect to see meaningful improvements in your insulin sensitivity or glycemic control. Similarly, if you have prediabetes, diabetes, or insulin resistance for reasons other than excess visceral and ectopic fat, you also should not expect to see improvements with intermittent fasting. In other words, if you are insulin resistant and potentially glucose intolerant because you have a very low muscle mass, such as in sarcopenia, or because of certain medications you are taking, or because you have a chronic inflammatory condition, your insulin resistance may not improve even if you do lose weight while fasting intermittently.
Now, it is important to point out that in some studies where participants started out insulin resistant, even though they lost some weight, insulin sensitivity did not change (Bhutani et al. 2013; Maroofi et al. 2020). One potential reason for these outliers may be that the degree of weight loss may have been insufficient to lead to a meaningful reduction in visceral and ectopic fat mass. In other studies, some but not all measures of insulin sensitivity were improved by intermittent fasting-induced weight loss (Hutchison et al. 2019).
So even though the evidence is not 100% consistent, in general, if you are overweight or obese, and insulin resistant and potentially also glucose intolerant, you can expect to lose weight when fasting intermittently, and have a good chance of improving your insulin sensitivity, glucose tolerance, and glycemic control. That is because losing weight and excess body fat may directly address the root cause of your insulin resistance, namely excess visceral and ectopic fat.
Can Intermittent Fasting Reverse Type 2 Diabetes?
If intermittent fasting can improve insulin sensitivity, glucose tolerance, and glycemic control, does this mean we could even reverse prediabetes or type 2 diabetes by fasting intermittently? We have some intriguing data from randomized controlled trials in people with type 2 diabetes that I’d like to share.
The first is a study by Guo and colleagues that was published earlier this year. These investigators randomized 405 adults, so this was a big study, with overweight or obesity and newly diagnosed type 2 diabetes to one of three intervention groups. The first group received the anti-diabetes medication metformin daily. Metformin has several mechanisms of action, but its main one is probably that it increases insulin sensitivity. The second group received an antidiabetes drug of the SGLT2-inhibitor class. These medications cause the kidneys to excrete more glucose, thereby lowering blood glucose levels and also basically getting rid of some extra calories. The third group received advice to follow the 5:2 diet, where they restricted their calorie intake to 500-600 kcal per day on two fasting days per week while eating to satiety the remaining five days of the week. Participants in the 5:2 diet group also replaced some of their other food intake with a high-protein meal replacement shake. So this was not just an intervention comparing anti-diabetic medications to the 5:2 diet alone. They tested 5:2 plus a slight increase in protein intake. So, let’s keep that in mind.
Participants followed these different interventions for 16 weeks, and then there was an additional follow-up assessment eight weeks later, meaning 24 weeks after the baseline. Over the 16-week intervention period, participants in the 5:2 diet group lost an average of 9.7 kg or 21 pounds, which was more than those in the metformin group at -5.5 kg or 12 pounds and those in the SGLT2-inhibitor group at -5.8 kg or 13 pounds. So this is similar to what we have discussed earlier: intermittent fasting helps people reduce their calorie intake and lose weight, and based on what we know about high-protein diets and their effects on satiety, the increase in protein consumption in the 5:2 diet group probably also contributed to this weight loss effect.
Now we are getting to the really interesting finding though: HbA1c, the primary endpoint of this trial, was lowered more in the 5:2 diet group, with -1.9 percentage points, than in the metformin group, with -1.6 percentage points and the SGLT2-inhibitor group at -1.5 percentage points. At the end of the 16-week intervention period, 80% of patients in the 5:2 diet group maintained an HbA1c level of less that 6.5%, which is the threshold for the diagnosis of diabetes. So, in other words, at least their HbA1c levels indicated that 80% of participants were no longer diabetic! That was more than in the metformin group, where that figure was 60%, and also more than in the SGLT2-inhibitor group at 55%. And this is a considerable difference given that the reduction under the 6.5% threshold was achieved only on anti-diabetes medication in the two control groups, while participants in the 5:2 diet group were not taking any anti-diabetes medications. Fasting glucose also decreased most in the 5:2 diet, with -30 mg/dL, which compared favorably to the -20 mg/dL seen in the other two groups. HOMA-IR decreased in all three interventions, with no difference between them. These effects were even stronger among participants who were fully compliant with the intervention, and mostly persisted at the 24-week follow-up visit, that is 8 weeks after the conclusion of the intervention.
There is a second study by Yang and colleagues. These investigators enrolled 72 adults with type 2 diabetes. They randomized them to an intermittent fasting intervention compared to a control group that did not fast or restrict their calorie intake. The form of intermittent fasting studied here was a bit different from that used in most other studies: participants were asked to restrict their calorie intake to 840 kcal per day for five consecutive days, followed by 10 days of eating to satiety. On fast days, foods were provided and consisted mostly of high-fat but also very high-fiber meal replacements. This 15-day pattern was then repeated for a total intervention duration of 3 months, at which time participants were studied again. Investigators also followed up with participants at 6 and 12 months to see to which degree any benefits would persist.
At the end of the 3-month intervention period, the intermittent fasting group had lost substantially more weight than the control group, -5.9 kg or 13 pounds, compared to -0.3 kg or 0.7 pounds. Of course, this was to be expected because this was the only group restricting its calorie intake. Diabetes remission, defined as HbA1c <6.5% without medication, was achieved in 17 of 36 participants in the intermittent fasting group, and 16/36 were still in remission at the 12-month follow-up time point, that is, nine months after completion of the intervention. This compares to remission in 1/36 in the control group at the 3-month time point, of which 0/36 were still in remission at 12 months. Average HbA1c was reduced more in the intermittent fasting group, by 1.8 percentage points compared to an increase of 0.4 percentage points in the control group. And fasting glucose was reduced to a greater degree in the intermittent fasting group compared to the control group, by a whopping 33 mg/dL or 1.8 mmol/L.
To me, these are extremely encouraging results. People who have type 2 diabetes and are overweight or obese usually are to a large degree insulin resistant and glucose intolerant because of excess body fat, specifically visceral and ectopic fat. And it does make a lot of sense that the best approach to addressing this root cause of the underlying insulin resistance would be to try to get rid of the excess body fat. Any intervention that can do this would be expected to benefit insulin sensitivity and glucose tolerance, with the potential to reverse type 2 diabetes fully in a large percentage of affected individuals, and certainly also prediabetes in most individuals. One trial discussed here used the 5:2 diet with an additional increase in protein intake. The other used this unusual form of intermittent fasting with five consecutive fasting days followed by ten ad libitum days. Still, it would be my strong guess that any form of intermittent fasting that you can maintain long enough to lose enough of your excess body fat should be able to substantially improve your insulin sensitivity, glucose tolerance, and glycemic control.
Summary & Conclusions
To summarize, there are various forms of intermittent fasting in which calorie intake is restricted on some days and unrestricted on other days. The most common ones are alternate-day fasting and the 5:2 diet. What they all have in common is that overall calorie intake is reduced even if we fast only on some days, due to incomplete compensation for the calorie restriction. As a result, we lose body weight and fat mass.
In general, the degree of weight loss is roughly proportional to the number of fasting days per week. So alternate-day fasting, where we fast three and a half days per week, is more effective than the 5:2 diet, for example, where we only fast two days per week.
As with weight loss resulting from other interventions, body weight lost due to fasting intermittently consists mostly of fat mass, but also of lean mass, which includes muscle mass. Fat mass is disproportionately lost from visceral and ectopic depots, i.e., even modest weight loss can lead to meaningful reductions in visceral fat mass and ectopic fat stores such as liver fat.
If we start with excess body fat that has resulted in insulin resistance and potentially glucose intolerance, such as prediabetes or type 2 diabetes, then we can expect that any sufficiently large loss of excess body fat will lead to an improvement in insulin sensitivity, glucose tolerance, and glycemic control. And if we lose most of our excess visceral and ectopic fat, this has the potential to even bring type 2 diabetes into remission in many patients. Prediabetes due to excess visceral and ectopic fat should be reversible in all or almost all cases, even though we need to acknowledge that this has not been explicitly tested.
Now, there are some important additional things to consider regarding how exactly to go about fasting intermittently and how to potentially make it even more effective. So next, let’s talk about …
How to Implement Intermittent Fasting
Let’s say you want to implement some form of intermittent fasting, but you are not sure which one is best for you and how to go about it.
Now, my very first suggestion is that if you have a preexisting condition, such as diabetes or a history of hypoglycemic episodes, talk to your doctor first before you start intermittent fasting.
Other than that, I suggest you see the various forms of intermittent fasting, including time-restricted eating, as discussed in the last two blog posts, as options.
If you have a normal body weight, and your main goal is the prevention of weight gain, then I suggest picking a moderate form of time-restricted eating, such as 10-hour TRE. Alternatively, as a weight maintenance protocol, you could also do something like the 5:2 diet for one month once a year, for example, or as is necessary to prevent weight gain.
If you have some excess body weight and fat mass, and particularly if you are also insulin resistant and even glucose intolerant, with a condition such as prediabetes or type 2 diabetes, then my suggestion would be to do a more restrictive form of time-restricted eating, such as 8-hour early TRE with an eating window in the earlier part of the day. Or experiment with one of the other forms of intermittent fasting discussed in this blog post here.
Decide how many days per week you want to be fasting, and then restrict your calorie intake to somewhere between 500 and 800 kcal on these fasting days. Start with one or two fasting days per week for a few weeks, and see how that goes. And if you cannot tolerate the very low-calorie intake on the fasting days in these forms of intermittent fasting, maybe give time-restricted eating a shot. It’s worthwhile to experiment with this to find a form of intermittent fasting that you can tolerate and stick with long enough to make a difference.
Now, in terms of practical implementation, when should you eat your food on your fasting days? Most of the studies discussed here suggested that participants eat only one meal of 500-800 kcal around lunchtime. However, one study specifically compared alternate-day fasting with one meal around lunchtime, one meal as a dinner, or several smaller snacks throughout the day. All of these approaches led to similar reductions in body weight and fat mass. In other words: it doesn’t matter when you consume your food on your fasting days, as long as you stick to your caloric deficit.
Other than that, I recommend for any dietary approach that leads to weight loss, be it intermittent fasting or continuous calorie restriction or even if you are using weight loss medication, that you prioritize a wide variety of nutrient-dense whole foods to ensure that you meet your micronutrient needs. If we eat fewer calories, that means that we need to meet our daily needs for vitamins and minerals from a smaller amount of foods, and these foods, therefore, need to be rich in these micronutrients. So, eating a variety of whole, unprocessed, or minimally processed foods, prioritizing vegetables, fruit, legumes, meat, eggs, fish and shellfish, and dairy products would be the way to go here. And doing that may also enhance the rate of weight loss when fasting intermittently because a diet rich in these foods has a lot of other properties that can help with weight loss.
In the same vein, consider that whenever we lose body weight, we tend to lose mostly fat mass, but we also tend to lose some lean muscle mass. That is also the case for the various forms of intermittent fasting. It would therefore be highly recommended to increase the percentage of calories consumed in the form of protein by making sure to always include a solid serving of a protein-rich food in each meal.
Just as important, if not more so, is regularly engaging in exercise of any kind, specifically resistance or high-intensity interval training. Several studies have tested the impact of intermittent fasting on body weight and body composition, and have found that eating a high protein diet and exercise both can reduce the loss of lean muscle mass while losing weight when fasting intermittently. Now, doing this may slow down your overall weight loss, because now you are losing less muscle tissue; do not get discouraged by that. Losing fat mass, specifically in our visceral and ectopic depots such as our liver is what we are aiming for, and if you can do that while preserving your muscle mass as much as possible, then that’s even better for our long-term health.
Another important point to consider: whenever we fast for an extended period of time, the ability of our pancreatic beta-cells to secrete insulin as part of the first-phase insulin response is reduced. That means that while you are doing intermittent fasting, make sure not to consume meals rich in naked, highly glycemic carbs. These could lead to a blood sugar spike. Instead, always prioritize lower-glycemic index sources of carbs such as vegetables, fruit, and legumes, and consume these with or after some non-starchy vegetables or salad, as well as some protein.
And my last point: if you are serious about losing weight, you do not just have to rely on intermittent fasting. My strongest recommendation would be to COMBINE your choice of intermittent fasting with other strategies that we know reduce calorie intake and body weight, such as eating a diet with a higher protein and fiber content, but a lower energy density, and minimizing the consumption of ultra-processed foods, liquid calories in the form of alcoholic and sugar-sweetened beverages, hyperpalatable foods, etc. Take a look at my YouTube playlist Keys to a Healthy Body Weight for more information.
Studies Not Considered
Several studies were not considered here due to major limitations. These included a lack of an adequate control group (Varady et al. 2009; Kalam et al. 2019; Kang et al. 2022; Heilbronn et al. 2005; Ekberg et al. 2024; Eshghinia et al. 2013), insufficient or problematic reporting of study results (Varkeneh et al. 2022; Chair et al. 2022), or a very small sample size leading to a lack of statistical power (Heilbronn et al. 2005; Da Silva et al. 2023; Borer et al. 2024).
Take care!
References
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One Response
I am quite interested in studies in which a total fast for the day (or multiple) days is included. I have found that it is much easier to eat nothing, instead of eating a small amount. If I eat nothing, I am seldom very hungry.