Posted May 10, 2023
In the last blog post, I shared several strategies on how to avoid blood sugar spikes when eating foods rich in carbohydrates.
Many of you had questions about this blog post or the accompanying video, including:
- whether blood sugar spikes are even a concern,
- why I don’t suggest that everyone should just go on a low-carb diet if they want to avoid blood sugar spikes, and
- whether blood sugar spikes are the result of insulin resistance.
Several of you also had specific questions about retrogradation and the second meal effect.
With this blog post, I am going to address the seven most common questions I received from you.
Because this post is directly related to the previous one, I strongly recommend you first read the prior blog post.
Let’s dive into your questions together. Let’s start with the most common one I received.
Question 1: Are Blood Sugar Spikes Even A Concern?
Let’s be very clear: if you asked your doctor about blood sugar spikes, they may respond that there is no such thing. That is because there is no clear clinical definition or diagnosis of blood sugar spikes, and even the research community so far has not clearly defined what a blood sugar spike is.
The current clinical guidelines are not overly concerned with the ups and downs of blood sugar levels after meals, as long as someone has normal fasting blood glucose, normal 2-hour glucose in a 75g oral glucose tolerance test (OGTT), and normal HbA1c.
So let me be clear that what I am sharing here is my take on this topic after researching it thoroughly. It is very well possible that other diabetes professionals have a different view of this.
That said, I define a blood sugar spike that we may want to avoid as any increase in blood sugar levels after a meal to 180 mg/dL or higher (for blood glucose, 1 mmol/L is equivalent to 18 mg/dL, i.e., 180 mg/dL is equivalent to 10 mmol/L).
How did I come up with that, and why do I feel that it may be harmful to our health if we regularly experience such spikes?
Well, my definition of a blood sugar spike is based on two lines of evidence.
First, if we look at young, healthy, and lean people with totally normal glucose tolerance, and have them wear a continuous glucose monitor while they are eating highly glycemic meals or their own normal diet, we observe that they rarely have blood sugar levels exceeding 140 mg/dL. In fact, if we consider all of the CGM data from this group of people, from two different publications, we find that about 99% of their blood sugar readings are within this range marked green in the figure below, from 70 to 140 mg/dL. We also see that while occasionally they may have blood sugar levels in the orange range, between 140 and 180 mg/dL, they basically never experience blood sugar levels exceeding 180 mg/dL. And that is independent of what they eat.
There are other studies in which some participants who did have neither diabetes nor pre-diabetes experienced blood glucose excursions to 180 mg/dL or higher, and some researchers interpret this as evidence that such excursions are part of normal physiology. I disagree with this assessment, because such excursions to 180 mg/dL or higher only occur in populations that are older and that include some people with overweight or obesity, suggesting that the higher postprandial blood sugar levels in these populations may reflect slightly reduced glucose tolerance even in people without diagnosed pre-diabetes or diabetes. For more detail on this topic, and all of the relevant references, please see my separate blog post.
Overall, the available data suggest to me very strongly that a fully glucose-tolerant body keeps blood sugar levels in the 70-140 mg/dL range, and that’s why I suggest that this is the ideal range we should try to maintain most of the time. We also see that the body very much tries to keep blood sugar levels below the 180 mg/dL level at all times. And there is probably a good reason for that, as we’ll see next.
That brings us to the second point, and that is whether there is some evidence that regular blood sugar spikes to over 180 mg/dL are associated with health risks.
We know for sure that having chronically elevated blood sugar levels, as in diabetes, is associated with substantially increased risks of a number of chronic diseases, such as cardiovascular disease and chronic kidney disease. But what about people who mostly have normal glucose levels, but spike to 180 mg/dL or higher regularly? Well, it would be ideal if we had data from tens of thousands of people who had worn a CGM for an extended period of time so that we could actually see whether those with a spiking pattern have an increased risk of disease. Unfortunately, we don’t have any such data.
However, we have some indirect evidence based on a biomarker of blood sugar spikes called 1,5-anhydro-glucitol (1,5-AG). I guess most of you have not heard about this biomarker, so let me briefly explain what this is about to help you understand why it’s important in this context.
In short, 1,5-AG is a sugar that all of us have in our blood. And under normal circumstances, the blood concentrations of 1,5-AG are pretty stable. But, as soon as your blood glucose levels exceed about 180 mg/dL, you start to excrete some of the 1,5-AG in the urine.
Why is that? Well, to really understand why our blood levels of 1,5-AG drop whenever our blood glucose levels exceed 180 mg/dL, we need to understand how both glucose and 1,5-AG are handled by our kidneys.
What you are looking at in the figure above is a nephron, the filtering unit of our kidneys. The blood flows into the glomerulus, the round shape on top. Solved in the blood are both glucose and 1,5-AG molecules, similar to how sugar is dissolved in coffee or tea. In the first step, called filtration, the glomerulus just filters out cells and large molecules, whereas much of the water and small molecules such as glucose and 1,5-AG end up in what we call the primary urine. On average, our body actually produces something like 170 liters, or 42 gallons, of primary urine per day. To enable us to do things other than drink water and run to the bathroom all day, most of the water in this primary urine is reabsorbed into the blood in step #2 called reabsorption. And in that step, the kidneys usually also reabsorb all of the glucose and most of the 1,5-AG. What is then being excreted in the urine is a much smaller volume of water that is usually free of glucose and contains only a very small amount of 1,5-AG (step #4).
The process of re-absorption of glucose has its limits, though. At blood glucose concentrations exceeding 180 mg/dL, the kidney reaches what is often called the renal glucose threshold, meaning that the kidneys are unable to reabsorb all of the glucose from the primary urine. So once blood glucose levels reach 180 mg/dL or higher, some glucose will be excreted in the urine. And when that happens, more 1,5-AG will also be excreted in the urine. And that is why blood 1,5-AG concentrations in the blood drop whenever our blood glucose concentrations exceed 180 mg/dL.
So what this means is that someone who has diabetes and who constantly has blood sugar levels over 180 mg/dL, 1,5-AG levels in blood will be very, very low. But it also means that for anyone who does not have diabetes or even pre-diabetes, blood 1,5-AG levels will be reduced if they experience frequent blood sugar spikes to 180 mg/dL or more.
And this is of interest because lower blood levels of 1,5-AG are consistently and strongly associated with an increased risk of of cardiovascular disease, chronic kidney disease, and cancer mortality, and that is even in people without diabetes and adjusted for traditional markers of blood sugar regulation, such as HbA1c or fasting plasma glucose.
Now, even though the literature on 1,5-AG is compelling, we need to consider that 1,5-AG is an indirect biomarker of blood sugar spikes. It is certainly possible that other conditions also lower 1,5-AG concentrations in blood, and maybe these conditions predispose to chronic disease. For example, if an early, pre-clinical stage of kidney disease were to affect the re-absorption capacity for 1,5-AG, then that by itself may explain at least some of the observed associations between low blood 1,5-AG levels and chronic disease risk. Therefore, we must be clear that the evidence linking blood sugar spikes, as I define them here, and chronic disease risk is certainly not conclusive at this point.
Taken together, the available evidence suggests that blood sugar levels almost never reach 180 mg/dL in young, lean, and totally healthy people, suggesting that any regular spike over 180 mg/dL is not part of normal physiology. And the data on 1,5-AG suggest that people who experience frequent spikes over 180 mg/dL have a substantially higher risk of cancer, cardiovascular disease, and chronic kidney disease, and that is independent of their fasting glucose or HbA1c values. Therefore, even though we need to acknowledge that whether someone has blood sugar spikes is not usually considered in normal clinical care, I feel that the available data strongly suggest that we would benefit from avoiding any spike over 180 mg/dL, whether we have diabetes or not.
And let me add this here quickly: there are some influencers on social media who suggest that even an increase in blood sugar of 30 or 40 mg/dL is harmful. That would mean that if you had a fasting blood glucose of 90 mg/dL and your blood sugar increased to 120 or 130 mg/dL, that this may be concerning. I do NOT agree with this at all. I do not know of any scientific evidence that such small bumps in blood sugar within the normal range are harmful. I would even go so far as to say that raising the alarm on such small fluctuations within the normal range is fearmongering and that in and of itself is potentially more harmful than a small blood sugar bump. As I interpret it, the current scientific evidence suggests the following:
- keeping blood sugar levels in the 70-140 mg/dL range most of the time,
- avoiding or minimizing any spike over 180 mg/dL, and
- keeping fasting glucose and HbA1c in the normal range, or – if you have diabetes or pre-diabetes, as low as possible.
At the same time, as long as blood sugar levels are within the 70-140 mg/dL range most of the time, I don’t see any compelling evidence that it’s worth obsessing over every little bump in the blood sugar level.
Question 2: Why Not Just Eat a Low-Carb Diet?
The title of my last blog post and video was ‘How to Avoid Blood Sugar Spikes (Without Reducing Carb Intake)’. And this triggered many people to comment that it would be better, easier, or healthier to just cut out all or most carbohydrates.
In order to keep this discussion here fairly short, I do not want to have a general discussion about the pros and cons of carb-rich foods in our diet. Let me just say this:
First, the blog post and video were intended not to suggest that people need to eat a high-carb diet to be healthy. I simply wanted to acknowledge that high-carb foods are the staple foods of most people’s diets, and the idea was to provide some strategies to help people eat these foods without spiking their blood sugar levels. I also stated very clearly that if someone has diabetes or even pre-diabetes, these strategies may not be sufficient to avoid all spikes, and it may be necessary to either stay away from highly glycemic foods or maybe even all high-carb foods entirely, and/or to take measures to improve glucose tolerance.
Second, it’s very clear, actually from the literature we discussed above, that many healthy people can eat whatever carbs they want without ever experiencing a blood sugar spike, even if they don’t use any of the strategies I presented. And I don’t know of any convincing evidence that eating carb-rich foods has negative health impacts in someone whose blood sugar levels are always in this normal range from 70 to 140 mg/dL. If any of you know of a paper that I have overlooked, I invite you to post a link to it in the comments section. Until I see some solid evidence, my take is that as long as someone is fully glucose tolerant, it’s fine for them to eat carbs. And for people who are a little glucose intolerant and experience some spikes to some foods occasionally, I feel that using the strategies I discussed will likely be sufficient to keep their blood sugar in the normal range.
We also need to consider that many of the strategies I presented that help avoid blood sugar spikes almost certainly have other, long-term health benefits. These include staying away from high-glycemic index foods such as cakes, toast with jam, french fries with ketchup, or soda while eating more protein, vegetables, and salads and going for walks regularly. It does seem very likely to me that adopting such habits would be beneficial for most people, including those who have diabetes or pre-diabetes.
Question 3: Can A Food Be Re-Heated After Retrogradation?
Many of you wondered about this, and I apologize for not explaining this aspect. To recap, when we cook a starchy food such as rice or potatoes, and eat it right away, almost all of the starch in it can easily be digested into individual glucose molecules. These small glucose molecules can easily be absorbed, and our blood glucose level rises quickly. However, if we first cool the food down, ideally in a fridge, some of the starch is converted to resistant starch. Our body can not digest resistant starch, and therefore the amount of glucose that enters our bloodstream will be lower. This means that our blood sugar response to starchy foods that have undergone one cycle of cooking + cooling is reduced.
Now, the important part: once the starch has become resistant starch, it does not convert back to digestible starch, so it’s fine to reheat it once it has been cooled down.
Another question related to retrogradation was whether bread that is cut up and frozen also undergoes retrogradation. The answer is yes. And yes, you can – and probably should – thaw it or toast it prior to consumption.
A related question that was asked a few times is whether some of the starch in frozen supermarket foods such as frozen french fries or frozen pizza could have undergone retrogradation. Now, disclaimer here: I am not a food chemist nor have I ever worked in the food industry or seen a specific paper in which the resistant starch content of frozen foods in different countries has been measured. I am taking a somewhat educated guess here: any freezing, even of a raw starchy food, probably leads to some retrogradation, but it’s almost certainly more pronounced if the food was cooked prior to being frozen. My understanding is that potato products such as french fries are usually cooked and/or fried prior to being frozen, so I would expect these to contain quite a bit of resistant starch. Supermarket pizza, on the other hand, is usually frozen without being cooked, so I think this is not a likely source of meaningful quantities of resistant starch.
A major reminder here: just having a bit of resistant starch doesn’t make a food into a health food, so I don’t want anyone to claim I recommended french fries. French fries with a bit of resistant starch are still French fries and not a healthy diet component
Question 4: How Does The Second Meal Effect Work?
Many of you were wondering about this, so I guess an apology is due for explaining this so poorly in the previous video. Let’s try again:
Imagine you sit down to eat dinner. We know that the blood sugar response to that dinner is very much dependent on what you have for dinner. If you eat a lot of ‘naked’ carbs with a high glycemic index, for example, white rice or baked potato, you should expect your blood sugar to rise a lot. If you have a small portion of a carbohydrate-rich food with a low glycemic index, for example, some lentils, and you pair them with some protein, a little bit of fat, and a large serving of fiber-rich non-starchy vegetables, your blood sugar levels will probably rise very little. Ok, I hope so far everyone can follow?
Now, the second meal effect says that the blood sugar response to a meal is also affected by what we eat at the prior meal. So, in this case, the prior meal would be lunch. If we have carbohydrates for lunch, then this will obviously cause a blood sugar rise after that lunch, but eating carbs for lunch will LOWER our blood sugar response to a high-carb dinner. I realize this may not make much sense, but it is true. All other things being equal, if your lunch has carbs, your blood sugar response to dinner will be lower than if your lunch doesn’t have any carbs. To a lesser degree, the same is true for protein and fiber. If we have protein or fiber at lunch, these will LOWER our blood sugar response to our dinner.
So, again, carbs, protein, and fiber eaten at the first meal (in this case, lunch) will lower the blood sugar response to the second meal (in this case, dinner), all other things being equal. So, ideally, if you know you are going to have a large serving of carbs for dinner, you’ll make sure to have at least some carbs for lunch as well, and ideally also some protein and fiber.
Question 5: Do Dietary Acids Other Than Acetic Acid (Vinegar) Also Lower Blood Sugar Levels?
This is a great question. Vinegar is an acid because it contains about 5% acetic acid. That may sound like a chemical, but it’s just a naturally occurring organic acid. Vinegar lowers the blood sugar response to a meal partly because the enzymes digesting complex carbohydrates operate less effectively in the presence of an acid. That means that it’ll take longer to break down starchy foods, and so the glucose molecules from the starch are absorbed more slowly and the body has more time to adjust to the incoming glucose.
Other organic acids that should – theoretically – be able to do the same thing include citric acid and lactic acid. I decided to not include these in the original video because there is much less research on the acute effects of citric acid or lactic acid on blood sugar levels. Let’s discuss the little bit of research we do have briefly.
First, citric acid. In our diet, citric acid occurs in citrus fruit, such as lemons and limes, but also in lower concentrations in oranges and grapefruits. There are at least two studies that showed lower blood sugar levels after meals that contain lemon juice. Now, note that if you drink citrus fruit juices that themselves contain sugar, such as orange juice or grapefruit juice, the net effect on your blood sugar level is likely not so great. However, lemon or lime juice may well be good options, so if you like your salad dressing with lemon juice rather than with vinegar, or if you like drinking lemon water with your meal, then that should work as well.
What about lactic acid? We consume lactic acid from foods that have undergone fermentation by lactic acid bacteria, such as sauerkraut, kimchi, yogurt, or kefir.
I was not able to find any study that tested whether the addition of sauerkraut or kimchi to a meal high in carbs acutely lowers the blood sugar response to that meal. Now, if I had to guess, I would say that a solid portion of sauerkraut or kimchi eaten with a starchy food could very well lower the blood sugar response, partly because of the acidic nature of these foods, partly because they are rich sources of fiber.
There are also no published studies that have tested whether adding yogurt or kefir to a meal affects the blood sugar response to that meal. It’s a bit more tricky to guess how adding these foods to a meal would affect the blood sugar response. On the one hand, they are slightly acidic (even though much less so than vinegar or even sauerkraut), and they contain some protein, so these two factors may lower the blood sugar response. But then, they do contain some carbs as well. Most importantly, yogurt and kefir contain calories, so if we are curious about the net effect of these foods on the blood sugar level, we must ask which other calorie-containing food we are comparing yogurt/kefir to. If we eat less bread and instead some plain yogurt, this will surely lower the blood sugar response to that meal. If we replace salmon with yogurt, the net effect may be an increase in the blood sugar response. So, in other words, this question becomes a lot more complicated whenever we are interested in the impact of a calorie-containing food on any biomarker.
Also, if you eat these fermented foods regularly, particularly in their unheated, pro-biotic state with living bacteria, they may also benefit your glucose tolerance. We’ll go into all of that in much more detail when we discuss ways to improve glucose tolerance.
Question 6. Are Blood Sugar Spikes a Sign of Insulin Resistance?
Many of you wondered about this, and the honest answer is that we don’t know for sure why it is that some people spike and others don’t. What we do see a lot, however, is that many people spike very soon after eating, sometimes just 15-30 minutes after their meal, and quite often, the blood sugar does not stay elevated for long. That would suggest to me that spiking is not primarily due to insulin resistance. Instead, it’s more likely that a spike is the result of either accelerated gastric emptying, meaning the food leaves the stomach very quickly after it’s been eaten; and/or the spike is a sign of a relatively slow insulin response. We call this a defect in the first-phase insulin response.
If you have never heard about first-phase insulin response, let’s dive into this a little bit, because it’s a core concept important in understanding diabetes, and also in understanding the impact of diet on glucose tolerance. Now, for this one, I really recommend you first read my blog post on the Regulation of Blood Sugar, or watch my video on the Regulation of Blood Sugar. If you haven’t yet, do that first and then come back here.
So, let’s assume we are totally healthy, and we eat a mixed diet containing carbohydrates regularly. Let’s say we just got out of bed, and now eat something that contains glucose, such as sugar or starch. Our body senses this incoming glucose very quickly, and within minutes, the beta-cells in our pancreas secrete pre-formed insulin. What you can imagine is that the beta-cells store a certain amount of insulin, and they can secrete this insulin very quickly. This is what we call the first-phase insulin response. This is then followed by slower production and secretion of more insulin over a longer period of time. We call this the second-phase insulin response.
Now, this is relevant because this first-phase insulin response is a major defect in people with diabetes (see figure above). Oftentimes, people with manifest diabetes do not have any first-phase insulin response. And a lacking first-phase insulin response is thought to be one of the earliest defects in type 2 diabetes.
Now, there is a situation that can also lead to a temporary defect in the first phase insulin response, and that is if you are on a low-carb diet, or if your most recent meal or meals was low in carbs. See the second meal effect above. Let’s say that you don’t eat carbs for dinner, then your beta-cells will reduce the amount of pre-made insulin that is stored, and that means that your first phase insulin response to the next high-carb meal will be a little bit diminished. If you have been on a low-carb diet for a long time, then your first-phase insulin response will be substantially diminished. This is probably temporary, but there is research to suggest that it can take several weeks of eating high-carb meals regularly until your first phase insulin response is fully restored. I am saying ‘suggest’ here because most studies do not measure the first-phase insulin response separately. They simply show persistence in glucose intolerance for some time, which is likely at least partly related to an impaired first-phase insulin response.
Well, how do we know if we have a defect in the first phase insulin response? For a normal person, it’s close to impossible to find out because you’d have to do some pretty sophisticated tests that are not usually done in clinical care. However, just imagine that someone eats a large amount of high-glycemic index carbohydrates, such as bread, rice, or potatoes. They eat this naked, that is, without any protein or fat, and let’s imagine they have a defect in their first-phase insulin response. What would you expect? I would expect a blood sugar spike, because in that person, it’ll take a while until a sufficiently large amount of insulin has been secreted. Right?
So, again, if I suffered from regular blood sugar spikes, I would make attempts to slow down my gastric emptying, and also make sure I maximize my first-phase insulin response. Strategies we discussed that slow down gastric emptying are including some protein, fat, and fiber in a meal, and also including some vinegar or lemon juice.
The main strategy to maximize the first-phase insulin response is related to the second-meal effect. If you eat carbs at a meal, for example, lunch, then your pancreatic beta-cells will be ready for more carbs at the next meal, in this case, dinner. And being ready here simply means that the beta-cells will have more insulin pre-made and stored within them so that the body can react immediately with a good first-phase insulin response when it senses glucose coming in.
I wrote “eat carbs regularly” in the figure above not because I think that everyone needs to eat a high-carb diet. This simply means that eating carbs regularly maximizes the first phase insulin response. So if you eat a high-carb meal, your blood sugar response will be lower if your prior meal or meals also contained some carbs.
If you prefer to follow a low-carb diet, just be aware that your first-phase insulin response to any occasional carbs you may eat is likely going to be lower, and that could then set you up for a blood sugar spike. So either stick to low-carb for all meals or – if you have an occasional higher-carb meal – use one or several of the strategies we discussed in the last blog post to prevent a blood sugar spike.
Now, whether or not you experience blood sugar spikes, you may still be insulin resistant, so I don’t want to suggest that insulin resistance cannot be a problem. We’ll talk about insulin resistance in many of the upcoming videos, what it is, how we can measure it, what its main cause and main contributing factors are, and what we can do about it. If that’s a major interest of yours, make sure you are subscribed to my newsletter so I can let you know whenever I publish new content. The form to subscribe can be found below this blog post.
That brings us to the last question:
Question 7. How Do Blood Sugar Spikes Relate to Reactive Hypoglycemia?
We just discussed that a blood sugar spike may be the result of an insufficient first-phase insulin response. Because there is too little insulin to help clear glucose from the blood in the first few minutes after a meal, glucose levels rise a lot, and then these very high blood glucose levels could lead to an insulin spike during the second phase of the insulin response. This insulin then drives glucose from the blood and into the tissues so fast that blood sugar levels plummet, and sometimes plummet too much and too quickly such that they can fall into the hypoglycemic territory where blood sugar levels are actually too low.
Once the blood sugar levels are lower than 70 mg/dL, that’s what we call hypoglycemia. And if that hypoglycemia is secondary to a blood sugar spike, we call it reactive hypoglycemia. Once the blood sugar is lower than 60 mg/dL, that’s something you will usually feel: a faster heartbeat, dizziness, shaking, nervousness or anxiety, and particularly a strong hunger for something to eat. That’s not pleasant, and can actually be dangerous if the blood sugar levels go too low. Reactive hypoglycemias could be a direct result of a strong insulin spike that’s secondary to a blood glucose spike, and they usually occur about 45 minutes to 2 hours after a meal.
So if you are experiencing any of these symptoms listed above in the two hours after eating a meal rich in carbs, you may want to consider that you may have reactive hypoglycemia following a blood sugar spike. Using some of the strategies I shared in my last video to avoid such a blood sugar spike should also substantially reduce the risk of reactive hypoglycemia.
References
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- Dungan et al.; 1,5-anhydroglucitol and postprandial hyperglycemia as measured by continuous glucose monitoring system in moderately controlled patients with diabetes. Diabetes Care 2006; 29: 1214-9.
- Ikeda and Hiroi. Cardiovascular disease and 1,5-anhydro-d-glucitol. Global Health & Medicine 2019; 1: 83-7.
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- Freitas et al.; Lemon juice, nut not tea, reduces the glycemic response to bread in healthy volunteers: a randomized crossover trial. European Journal of Nutrition 2021; 60: 113-22.
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- Jansen et al.; Prolonged glycemic adaptation following transition from a low- to a high-carbohydrate diet: a randomized controlled feeding trial. Diabetes Care 2022; 45: 576-84.