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5 Ways Intermittent Fasting Affects Glucose Levels

Madison Holt, MS, RDN, LD

Published in Fasting

4 min read

April 13, 2021
January 24, 2023
a plate of meat, fish, nuts and veggies
a plate of meat, fish, nuts and veggies

Intermittent fasting (IF) is the intentional restricting of our mealtimes to maximize health. Media coverage tends to paint IF as the latest fad diet … except it’s neither a fad nor a diet. IF existed long before it became trendy, and it can be unique in that it lacks some of the bizarre or arbitrary rules of other diets.

Intermittent fasting really isn’t a “diet.” Think of IF like the dietary cousin to restricting kids’ screen time. Limiting computer time keeps our kids from gorging on YouTube and helps their developing brains. IF does the same for our bodies.

How Intermittent Fasting Affects Glucose Levels

When it comes to IF and blood glucose, the results are so much more than just calories and weight loss. IF is a boon for numerous bodily systems. It lowers blood glucose. It increases our bodies’ sensitivity to insulin. It reduces inflammation. It makes our digestive and metabolic processes more adaptable, and it can even change when and how our bodies make blood sugar.

1) Lower Insulin Levels

While our body does produce some insulin throughout the day, the majority of insulin release happens when we eat. Counting meals and snacks, the typical adult eats six or seven times a day.

This means our insulin levels spike six or seven times a day. IF, by reducing the frequency of eating and snacking, lessens our insulin spikes and reduces our body’s overall insulin levels. So, why is this good?

Well, in addition to its role in regulating our blood sugar levels, insulin plays a part in cell growth, and both DNA and RNA synthesis. Dangerous increases in insulin levels not only impact our blood sugar, but can hinder cell growth and bodily regeneration.

2) Increased Insulin Sensitivity

IF’s increased insulin sensitivity goes hand-in-hand with reduced insulin levels. The more our insulin spikes, the more our body becomes accustomed to insulin (like needing more coffee in the morning the longer you drink it). So the next time we eat, our body needs to spike even more insulin to get the job done. 

Additionally, reductions in adiposity (the scientific term for losing body fat) increase our sensitivity to insulin. And wouldn’t you know? One of the benefits of IF is that the weight we lose tends to be fat. This reduces our “adiposity” and increases our insulin sensitivity. 

Even with no weight loss, with no reduction in our body fat, science suggests IF still increases our bodies’ sensitivity to insulin. The bodily changes triggered by IF tend to “reprogram” our metabolism. In this scenario, our bodies are like a computer on the fritz—turning it off and back on again usually fixes the problem.

IF is like a power switch, resetting how our bodies respond to insulin. Increased insulin sensitivity means our bodies require less insulin to lower our blood glucose.

two empty plate and a plate of bacon, a fried egg, avocado and tomatoes and a clock

3) Lower Inflammation

Our bodies become inflamed as a response to stressors. We often think of stressors as germs or viruses, but the process of metabolizing food is an energy-intensive process that puts oxidative stress on our bodies.

Too much oxidative stress makes it difficult for our bodies to detoxify; it’s why so many products and supplements tout themselves as “antioxidants.” IF, it turns out, is a kind of all-natural antioxidant.

Because those who practice IF intentionally limit when they eat, they also, by extension, limit the oxidative stress of metabolism. Limiting this kind of stress is key; a number of diseases—Alzheimers, Parkinson’s, and, most pertinently, diabetes—are thought to arise from excess inflammation and oxidative stress.

Lowering inflammation through IF can reduce our susceptibility to these diseases and lessen our symptoms if we already have them. 

4) Improved Metabolic Flexibility

As we talked about above, humans aren’t really designed for “three square meals a day.” We evolved over hundreds of thousands of years to subsist through cycles of feast and famine. Our ancestors couldn’t just call for fast-food delivery when they got the munchies.

The food they did get wasn’t always nutritionally sound, either, meaning the body jealously stored whatever nutrients it could find. IF simulates our evolutionary feast and famine cycles, but on a much smaller scale. It trains our bodies to smartly use it’s metabolic resources.

For example, if your stomach is empty and your body needs energy, IF trains it to metabolize fat reserves as nature intended. This tends to stabilize our blood sugar levels, flattening peaks and lifting valleys. This kind of metabolic flexibility also reduces the side-effects of hunger, meaning we won’t get “hangry” if we’re a few minutes late for dinner.

someone writing a meal plan

5) Early Time Restricted Eating

One of the hidden superpowers of IF is how it boosts our natural circadian rhythms. Deep within everyone’s brain is a master clock that, taking into account external data such as sunlight, nudges our bodies toward certain activities.

This circadian rhythm, as it's called, is like having a smartphone pre-set with a whole day’s worth of alarms—wake up, eat lunch, etc.—but much less annoying. Our circadian rhythm includes a glucose/insulin timer. Sensitivity to insulin actually decreases as the day wears on.

It takes our bodies longer to process a piece of toast at 9 PM than it does at noon. Those who practice early time-restricted feeding IF, (sticking with a 16:8 pattern but eating only between 10AM and 6 PM, say), boost insulin’s circadian effectiveness.

Overall insulin sensitivity rises. Mean glucose levels fall. By shifting our eating window earlier in the day, we are aligning our food intake with when our body is naturally most insulin sensitive, reducing spikes in insulin and blood sugar.

A Quick Background on Intermittent Fasting

Let's have a look at a few of the different reasons that intermittent fasting has become a popular dieting method.

It's Simple

The secret to IF’s appeal is its simplicity. You merely don’t eat for a certain number of hours per day (or days per week). Those who practice 16/8 IF, for example, fast for 16 hours and then allow themselves food for eight hours.

While a 16-hour fast sounds extreme, remember, you’ll probably be asleep for eight of those hours. And, really, we shouldn’t be eating late at night anyway, right? Paired with a sensible meal plan, IF can be a powerful tool for controlling blood sugar levels.

Fad diets fail because they ask us to juggle a thousand different rules. Is this vegetable Keto-approved? Are these burgers Paleo? IF replaces the maze of rules with one simple code: only eat during a set window of time.

It's Evolutionarily Normal

The way most Americans eat—breakfast when we wake, lunch near noon and dinner near sunset—is a new-ish invention. Three meals just happened to fit the Industrial Revolution workday. Even in modern times, “three square meals'' isn't universal.

Non-diabetics with hypoglycemia, for example, often eat many smaller meals a day to regulate blood sugar. Many farming cultures eat five or six meals a day. The Greeks and Spanish are infamous for their 11pm “dinners.” 

For most of human history, we ate what we could when we could. We have body fat for a reason, after all. When our ancestors failed hunting Mammoths, fat kept them alive. When they found a walnut grove, our ancestors gorged to build up fat reserves. Simply put, our forebears lived in a perpetual state of intermittent fasting.

Ages of hunter-gathering shaped our bodies. From an evolutionary standpoint, intermittent fasting is more “normal” than breakfast-lunch-dinner. Add in issues brought on by our modern processed foods, ultra-rich and sugar-heavy, and it becomes clear that maybe IF, biologically speaking, is the way to go.

a scale, a tape measure, dumbbells and a bowl of veggies and fruits

It May Support Weight Loss

This is the hook for most people to try IF. Systematic studies show a strong correlation between IF and weight loss. Between 2001 and 2009, 27 different trials examined the interplay of IF and weight loss. Participants lost weight in all 27 trials. That’s an astounding result. 

The asterisk here (there’s always an asterisk, right?) is that the study of intermittent fasting is a relatively nascent field. Established science generally stretches back decades and centuries. Scientists need much more data before definitely linking IF and weight loss.

Still, participants losing weight in 27 of 27 IF trials is a very promising start.

a person showing a stop sign with one hand and holding a clock with another hand

It May Increase Longevity

The other interesting aspect of IF is its potential to extend human lifespan. Obviously, weight loss lowers our risk for a whole host of diseases. Less disease means longer life. IF offers more than that, though. Emerging science suggests that IF affects our bodies on a cellular level, improving health and increasing lifespans.

Autophagy is the process by which our cells break down old materials. Think of it as cellular recycling. Enzymes break down cellular waste and use the product to build fresh new cells. Science suggests intermittent fasting increases autophagy, making our cells more efficient recyclers. This makes us, on a cellular level, “younger.”

IF also increases mitochondrial efficiency. Those who still remember their paper plate/jelly bean cell projects from middle school will proudly tell you the mitochondria is “the powerhouse of the cell.” Mitochondria produce the chemical energy our cells rely on.

As we age, our mitochondria become less efficient. Studies show, however, that IF reverses cellular aging, giving us the mitochondria of a (relative) teenager.


Intermittent Fasting May Not Be Suitable For Everyone

a person holding a lass f water and smiling

Though the benefits of IF are alluring, it must be said that this kind of deliberate restriction isn’t for everyone. Here are a few types of people who may want to consult their doctor before beginning this type of diet.

1) People Who Are Underweight or Suffer From Eating Disorders

Twitter Founder and CEO Jack Dorsey claims eating one meal a day (OMAD) gives him a laser-like focus. If Dorsey were a woman, though, especially a young woman, would we call his habits “intermittent fasting” or would we call them an “eating disorder?”

The line can be blurry. And for those who have suffered past eating disorders or the chronically underweight, it’s a line best avoided.

2) Pregnant And Breastfeeding People

IF probably isn’t a great idea for pregnant and breastfeeding women. IF’s caloric restriction can negatively impact fetal development. And because breastmilk nutrition is largely informed by what the mother eats, restricting diet during breastfeeding can produce less nutritious milk.

3) People With Chronic Fatigue or Burnout

The “warning label” on most every scientific study of IF is how it interplays with stress. Deliberately restricting our caloric intake puts a stress on our bodies. For most, this low-level stress is beneficial. It does, after all, help us lose weight and revitalize our cells. 

However, for those who already live under a high degree of bodily stress, or those who pair IF with other activities such as cold therapy or lots of high intensity training, extended IF can overload our body’s stress tolerance and cause negative effects.

4) People Taking Insulin Therapy or Hypoglycemic Medications

Because IF directly affects our body’s production of blood sugar, those on insulin therapy or medications that cause hypoglycemia should contact their doctor before beginning an IF regimen.

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Kara Collier, RDN, LDN, CNSC

Reviewed by: Kara Collier, RDN, LDN, CNSC

Kara Collier is the co-founder and VP of Health at Nutrisense, one of America’s fastest-growing wellness-tech startups, where she leads the health team. She is a Forbes 30 under 30 recipient, frequent podcast guest & conference speaker.