Why are blood sugar spikes bad for us?

High blood sugar levels are mostly associated with type 2 diabetes but in fact, regular blood sugar spikes can have a wide-reaching impact across our entire body. Despite the many short- and long-term effects of consistent blood sugar spikes, there’s good news—minimising the spike and the crash can go a long way to making you feel healthy and well.
Annabel Nicholson
min read
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Quick summary

  • A blood sugar spike happens when glucose builds up in the bloodstream and blood sugar levels increase.
  • This happens most often after eating and the bigger the spike, the bigger the crash afterwards.
  • Regular blood sugar spikes set off a series of events in the body that all lead to inflammation.
  • Chronic inflammation is the cause of diseases like stroke, heart disease, liver disease, and type 2 diabetes—these are known as inflammation-based diseases.
  • Glucose spikes can have short- and long-term health consequences ranging from fatigue, hunger, and cravings, to obesity, type 2 diabetes, and heart disease.
  • Glucose spikes from sweet foods are worse for you than spikes from savoury foods because sweet foods contain fructose.
  • Keeping blood sugar levels stable is the best way to combat the negative health consequences of blood sugar spikes.

Glucose is the main type of sugar in the blood and without it, we’d cease to exist. It’s the body’s preferred energy source and keeps everything functioning normally. There can, however, be too much of a good thing, and too much glucose can hurt us—often without us realising it. So, what is a blood sugar spike? And what does it mean for your health? Here’s everything you need to know.

What is a blood sugar spike?

Essentially, a blood sugar spike happens when glucose builds up in the bloodstream and blood sugar levels increase.

We tend to associate elevated blood sugar levels with conditions such as type 2 diabetes and while that’s certainly correct, an astonishing 80% of people without diabetes also experience high glucose spikes.[1]  

This most often happens after eating, when the food we eat is broken down during digestion. And it’s not just the sweet stuff that you need to watch our for—all carbohydrates break down into glucose, which then enters the bloodstream. We tend to ingest more glucose than we need, which causes blood sugar levels to rise rapidly and this causes a spike. To compensate, our body produces more insulin to help lower blood sugar levels. This can sometimes cause levels to dip too low and so begins the unhealthy rollercoaster of highs and lows in blood sugar levels.

We should make it clear at this stage that blood sugar spikes are completely normal regardless of whether you have type 2 diabetes or not—especially after eating a high-carb meal. A single spike isn't going to lead to long-term complications like type 2 diabetes, but they can still result in short-term health issues.

What happens in the body after a blood sugar spike?

As the blood moves around the body, glucose is delivered to cells where it’s turned into energy. This energy is used to power every single thing that happens in our bodies.

That’s the simple version of events but there’s actually a load more going on. It's nothing overly complicated but we think it's a super interesting insight into what repeated blood sugar spikes really mean.

We enter a state of oxidative stress

It all starts with something called mitochondria, which are found in nearly every cell in the body. While microscopic in size, they’re super important because it's here that glucose is turned into the energy that powers the cell. Mitochondria usually only burn as much glucose as the cell needs for energy but when we have a blood sugar spike, we’re sending too much glucose to our cells too quickly, essentially drowning our mitochondria in glucose.

Recent scientific theory suggests that when our mitochondria are drowning in glucose, our cells release tiny molecules called free radicals.[2] Despite their somewhat positive name, free radicals are a dangerous thing to have floating around because they have the potential to damage anything they come into contact with. Our bodies can deal with them in moderation but when there are too many, they become unmanageable. With repeated glucose spikes, the quantity of free radicals produced is too much and our body eventually enters a state of oxidative stress.  

Uncontrolled oxidative stress contributes to heart disease, cognitive decline, and type 2 diabetes

Picard et al., Nat Rev Endocrinol

Oxidative stress isn’t always harmful but uncontrolled oxidative stress in the long term contributes to a number of conditions such as heart disease, cognitive decline, and type 2 diabetes.[2]

Glucose bumps into other molecules in the body—and damages them

When we toast bread, we make it brown—this is known as the Maillard reaction and it happens when a glucose molecule bumps into another type of molecule, which causes a reaction. The second molecule is then said to have been ‘glycated’ and when this happens, it’s damaged forever.

This process doesn’t just apply to making our perfect slice of toast. Molecules in our body are naturally glycated all the time—wrinkles, cataracts, heart disease, and Alzheimer’s Disease are all consequences of glycation.[3-6] It’s a completely natural part of ageing and while it can’t be stopped, it can be slowed down or sped up.[7]

The more glucose we deliver to our body, the more often glycation happens and it's actually glycation that’s measured in an HbA1c test (the blood test for diabetes). The test measures how many red blood cell proteins have been glycated by glucose molecules in the preceding 3 months.   

We gain weight

Once all the glucose our body needs for energy has been taken to the relevant parts of the body, it’s essential that any excess is taken out of circulation as soon as possible to reduce this free radical production and glycation.

Our body does this by using three different internal storage units. 

  1. The liver turns glucose into glycogen and in this form, it can do no damage.
  2. The muscles can also store glucose as glycogen.

But, we typically eat much more glucose than we need, so these storage units get full very quickly—which brings us on to storage unit number three…

  1. Any excess glucose that isn’t stored as glycogen in our liver or muscles is turned into fat.[8] 

Whenever the mitochondria in our cells need more energy, our body can turn the glycogen in the liver and muscles back into glucose so it can be used. It’s only when our glycogen stores diminish that we use our fat reserves for energy—at this point we’re in fat-burning mode, which is when we lose weight.[9]

Excess glucose that can’t be stored as glycogen is turned into fat

Stryer, L., Biochemistry, 4th edition

The caveat here is that it’s only possible to burn fat if insulin levels are low—in fact, one 2021 study found that weight loss is always preceded by a decrease in insulin levels.[10] We’ve written about insulin and its role in blood sugar regulation before but in short, insulin helps lower blood sugar levels by enabling glucose to enter the cells. If blood sugar levels increase, insulin is released. As blood sugar levels decrease, insulin stops being produced and its levels also decrease. So, the steadier your blood sugar levels are, the steadier your levels of insulin will be, and your fat reserves will have more of a chance to fuel the body.

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Why are blood sugar spikes bad?

The combination of free radicals, oxidative stress, and glycation leads to a general state of inflammation in the body. A protective measure, inflammation is the result of the body trying to defend itself. Chronic inflammation, however, is harmful because it turns against our own body. It’s this kind of inflammation that is the cause of most chronic illnesses such as stroke, heart disease, liver disease, and type 2 diabetes. Three out of five people are thought to die of an inflammation-based disease.[11]

Health consequences of glucose spikes

The consequences of glucose spikes range from the short-lived to those that stick around for the long haul. 

Are you tired, too?

One of the most common feelings of modern life is that of tiredness—this isn’t necessarily the tiredness we feel after a late night or a workout, but the inexplicable feeling of fatigue despite doing all the things we ‘should’ be doing for a healthy lifestyle. 

Well, it may come as no surprise that this feeling is also attributed to glucose spikes. With repeated glucose spikes, our mitochondria become full—they’re quite literally stuffed with glucose and this means they’re unable to convert glucose into energy efficiently. As a result, the cells starve and as humans, this is felt as tiredness.[2] 

But that’s not all. Beyond this, glucose spikes can contribute to:[12]

…and that’s just in the short-term. Over time, consistent glucose spikes have been shown to contribute to more serious complications, such as:[12]

  • Aging and arthritis
  • Dementia and Alzheimer’s Disease
  • Type 2 diabetes
  • Cancer risk
  • Depression
  • Heart disease
  • PCOS and infertility
  • Non-alcoholic fatty liver

It’s a big list we know. It’s not there to scare you but instead, we hope it highlights just how important blood sugar levels to your overall health and wellbeing. 

Are some glucose spikes worse than others?

One last quick explainer before we get onto the good news (hurrah!)—some glucose spikes are indeed worse for you than others. 

We said earlier that all carbohydrates break down into glucose. That means that food like rice and pasta break down into glucose, but so does a cake or biscuit. They both cause glucose spikes but its the sweet treat that will be worse for you for two reasons:

  1. Chances are, the sweet treat has more sugar in it than the savoury starch, so will cause a bigger spike (and therefore a bigger drop).
  2. The sweet treat will contain sucrose, which contains fructose.

Glucose, sucrose, fructose—you might notice they all end in -ose and that’s how you can know they’re all sugars. We’ve written about different types of sugar before but in short, starchy carbohydrates such as rice, pasta, and bread, all break down into glucose, whereas sweet sucrose breaks down into glucose and fructose. 

Everything in this article so far explains what happens to glucose but when we eat something sweet the body has to also deal with the fructose. So, what happens?

Unlike glucose, fructose can’t be turned into glycogen—it can only be stored as fat

Softic et al., Dig Dis Sci

Well, unlike glucose, fructose can’t be turned into glycogen and stored—the only thing that fructose can be stored as is fat.[13] Fat from fructose tends to accumulate in the liver (hello non-alcoholic fatty liver disease)[14] and in between organs, as well as on the hips, thighs, and face. It also enters the bloodstream and contributes to an increased risk of heart disease.

It’s for this reason that if two foods have the same number of calories, it’s best to go for the savoury option. You’ll still increase your blood sugar levels with glucose but without fructose, fewer molecules end up as fat.[15] It’s also worth being aware that a lot of fat-free process foods contain sucrose, which as we now know breaks down into glucose and fructose, and fructose equals fat.

Keeping blood sugar levels stable

If you’ve got this far, well firstly—thank you! But secondly and most importantly, don’t let all this information get you down. Nothing in this article is aimed to scare you into never eating a biscuit again (in fact, you deserve one for spending all this time reading!). So, we’re going to end on a high because while glucose spikes can cause all these short- and long-term complications, reducing these spikes (known as ‘flattening the glucose curve’) can bring relief.

The most effective way to reverse type 2 diabetes is to flatten our glucose curves

Goldenburg et al., BMJ

And how best to do that? With diet! The food we eat may well cause glucose spikes and their associated problems, but with small, simple changes we can use diet to keep glucose levels stable and our bodies happy. 

There are a few different things you can do but here are three of the simplest ways to keep your blood sugar levels stable—these work for both people with and without type 2 diabetes. In fact, a paper published last year made it clear that the most effective way to reverse type 2 diabetes is to flatten our glucose curves.[16]

  1. Have a savoury breakfast: what you eat for breakfast can determine your cravings and hunger pangs for the rest of the day (!),[17] so consider swapping your bowl of cereal, for a savoury option like eggs to keep levels steadier.
  2. Eat your vegetables first: a recent study showed that eating your meal in a certain order could reduce the glucose spike after eating by as much as 73% and the insulin spike by 48%.[18] Research into food ordering is still ongoing but the evidence in blood sugar management is promising. The premise is simple—when possible, eat your vegetables first. This could mean eating the vegetables of your roast dinner first or a side salad before eating a bowl of pasta. The idea is that the fibre of the vegetables coats the lining of the interesting, which reduces how much glucose is absorbed and how quickly, leading to a steadier increase in blood sugar rather than a spike.[18,19]
  3. Move after a meal: research suggests that a walk after dinner, often the biggest meal of the day, can significantly reduce blood sugar levels for up to the next 24 hours.[20] But it doesn’t have to be a walk—just 10 minutes of gentle movement can help lower blood sugar levels as glucose is taken up by muscles for fuel.

The effect of blood sugar levels is astonishing but while there’s a lot to take in, the most important thing to take away from this is that with a flatter glucose curve (less extreme spikes and crashes), you can do a lot of good for your physical and mental health. 🙏


[1] Hall, H., Perelman, D., Breschi, A., et al. (2018). Glucotypes reveal new patterns of glucose dysregulation. PLoS Bio 16(7):e2005143. Accessible here.

[2] Picard M., Juster, R-P., McEwen, B.S. (2014). Mitochondrial allostatic load puts the ‘gluc’ back into glucocorticoids. Nat Rev Endocrinol 10(5):303-310. Accessible here.

[3] Ichihashi, M., Yagi, M., Nomoto, K., Yonei, Y. (2011). Glycation stress and photo-aging in skin. J Anti-Aging Med 8(3):23-29. Accessible here.

[4] Katta, A., Suryakar, A.N., Katkam, R.V., et al. (2009). Glycation of lens crystalline protein in the pathogenesis of various forms of cataract. Biomed Res 20(2):119-121. Accessible here.

[5] Soldatos, G., Cooper, M.E. (2006). Advanced glycation end products and vascular structure and function. Curr Hypertens Rep 8(6):472-478. Accessible here.

[6] Takeuchi, M., Kikuchi, S., Sasaki, N., et al. (2004). Involvement of advanced glycation end-products (AGEs) in Alzheimer’s disease. Curr Alzheimer Res 1(1):39-46. Accessible here.

[7] Kim, C-S., Park, S., Kim, J. (2017). The role of glycation in the pathogenesis of aging and its prevention through herbal products and physical exerice. J Exerc Nutrition Biochem 21(3):55. Accessible here.

[8] Stryer, L. (1995). Biochemistry, 4th edition. W.H. Freeman and Co., New York.

[9] Stryer, L. (1995). Biochemistry, 5th edition. W.H. Freeman and Co., New York.

[10] Wiebe, N., Ye, F., Crumley, E.T., et al. (2021). Temporal associations among body mass index, fasting insulin, and system inflammation: a systemic review and meta-analysis. JAMA Netw Open 4(3):e211263. Accessible here.

[11] Pahwa, R., Goyal, A., Jialal, I. (2018). Chronic inflammation. Stat Pearls [Internet]. Accessible here.

[12] Inchauspe, J. (2022). Glucose Revolution. Short Books, London.

[13] Softic, S., Cohen, D.E., Khan, C.R. (2016). Role of dietary fructose and hepatic de novo lipogenesis in fatty liver disease. Dig Dis Sci 61(5): 1282-1293. Accessible here.

[14] Geidl-Flueck, B., Hochuli, M., Németh, A., et al. (2021). Fructose- and sucrose- but not glucose-sweetened beverages promote hepatic de novo lipogenesis: A randomized controlled trial. J Heptaol 75(1):46-54. Accessible here.

[15] Silva, J.C.P., Marques, C., Martins, F.O., et al. (2019). Determining contributions of exogenous glucose and fructose de novo fatty acid and glycerol synthesis in liver and adipose tissue. Metab Eng 56:69-76. Accessible here.

[16] Goldenburg, J., Day, A., Brinkworth, G.D., et al. (2021). Efficacy and safety of low and very low carbohydrate diets for type 2 diabetes remission: systematic review and meta-analysis of published and unpublished randomized trial data. BMJ 372:m4743. Accessible here.

[17] Shukla, A.P., Iliescu, R.G., Thomas, C.E., Aronne, L.J. (2015). Food order has a significant impact on postprandial glucose and insulin levels. Diabetes Care 38(7):e98-e99. Accessible here.

[18] Chang, C.R., Francois, M, E., Little, J.P. (2019). Restricting carbohydrates at breakfast is sufficient to reduce 24-hour exposure to postprandial hyperglycemia and improve glycemic variability. Am J Clin Nutr 109(5):1302-1309. Accessible here.

[19] Nishino, K., Sakurai, M., Takeshita, Y., Takamura, T. (2018). Consuming Carbohydrates after Meat or Vegetables Lowers Postprandial Excursions of Glucose and Insulin in Nondiabetic Subjects. J Nutr Sci Vitaminol (Tokyo) 64(5):316-320. Accessible here.

[20] Borror, A., Zieff, G., Battaglini, C., Stoner, L. (2018). The Effects of Postprandial Exercise on Glucose Control in Individuals with Type 2 Diabetes: A Systematic Review. Sports Med 48:1497-1491. Accessible here.

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