Intermittent Fasting: The Science Behind the Trend | Chris Kresser
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Intermittent Fasting: The Science Behind the Trend

by Chris Kresser

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Intermittent fasting—combined with a healthy, active lifestyle, like this woman is engaging in—could lead to weight loss.
Intermittent fasting could help you lose weight, improve your metabolism, and avoid chronic disease. iStock/adamkaz

You’ve heard about some of the benefits of intermittent fasting: that it may help you lose weight, lower your risk of certain chronic diseases, and even give your brain a boost.

But after reading the latest news articles—this increasingly popular way of eating is enjoying its moment in the spotlight—you’re left asking, How exactly does it work? You’ve come to the right place for answers. Keep reading to learn what happens to your mind and body when you skip a meal … or two, or three.

You’ve heard of intermittent fasting before, but have you ever wondered how it works? Read up on the science behind intermittent fasting and find out if it’s the right choice for you. #nutrition #healthylifestyle #chriskresser

What Is Intermittent Fasting? Think Lifestyle, Not Dieting

The term “intermittent fasting” (IF) is used to describe several different approaches to the short-term abstention from food in order to improve health. The key word here is “short-term.” IF does not involve fasting for prolonged periods.

The most common methods of IF are: (1)

Time-Restricted Eating

For this type of IF, you shrink the window of time during which you eat each day. Typically, that involves extending the duration of your regular overnight fast, to anywhere from 12 to 20 hours, by skipping either breakfast or dinner. However, you can choose to have three full meals, just within a compressed time block.

One popular version of this approach is the 16:8 diet: as you might have guessed, it involves fasting for 16 hours and compressing your eating into an eight-hour time slot. You can eat as many meals as you’d like over the course of eight hours; although, of course, you should eat healthy foods during that time. You can also drink water, coffee, and other non-caloric beverages while fasting to help curb hunger. This type of intermittent fasting helps obese individuals lose weight and lower their systolic blood pressure. (2)

Another popular time-restricted approach is called the Warrior Diet. Popularized by nutritional and fitness expert Ori Hofmekler, it extends the fasting block to 20 hours and shrinks the eating window down to just four hours, specifically four hours at night. However, the approach doesn’t mandate fasting completely for all 20 hours: the Warrior Diet encourages some snacking throughout the day, by munching on things like raw fruits and vegetables. Then, it suggests a feast at night, usually in the form of a large dinner. Although time-restricted intermittent fasting has been greatly researched, this specific approach lacks the scientific studies to back it up.

Whole-Day Fasts

This type of IF is just as it sounds: fasting for 24 hours for as little as once or twice a month or as much as once or twice per week. Several variations of this method have been popularized in recent years: Eat, Stop, Eat dictates day-long fasts twice a week, while alternate-day fasting promotes fasting every other day. The 5:2 diet is similar: two days of near-fasting, consuming 500 to 600 calories each of those days, and five days of normal food intake.

Of course, you don’t have to follow any of these approaches exactly. Intermittent fasting can be as formal or informal as you like. As my Revolution Health Radio cohost, Steve, and I discussed on an episode, you may not feel like fasting in times of stress, or you may choose to interject IF more heavily into your life after overeating during holidays. I often fast when I’m flying or think I’m coming down with a cold. In this way, IF is more lifestyle than diet. Unless you follow the 5:2 approach, you technically don’t count calories or even restrict them. (Although, to get the health benefits from IF discussed below, if you’re not already eating a healthy ancestral diet, you will need to change what you eat as well as when you eat.)

I know these approaches run counter to what you’ve always been told: eat three square meals a day, every day. But our bodies weren’t necessarily designed to do that.

In fact, going for extended periods without eating was completely normal for our hunter–gatherer ancestors—if a hunt wasn’t successful, food may not have been available. We’re biologically adapted to this style of eating; we didn’t have access to a steady food supply until the agricultural revolution. (3) All this is to say that IF can be a healthy way of eating, with great benefits for your overall well-being.

While the bulk of research into IF has been performed on laboratory animals, human trials have been conducted and are only increasing. In both instances, researchers have found many benefits, including:

  • Weight loss and improvements in metabolic health
  • Increased insulin sensitivity
  • Reduction in risk for cardiovascular disease and other chronic conditions
  • Protection against memory loss and neurological disorders

What Happens to Your Cells, Hormones, and Genes When You Fast

So how does IF lead to these impressive health benefits?

Intermittent Fasting Burns Fat, Not Sugar

When you fast, your body burns fat instead of sugar for energy, which leads to fat loss and gives your brain a boost.

Like a car, your body needs fuel to function; food is that fuel. During digestion, the stomach breaks down carbohydrates into sugar that your cells use for energy—to “gas up,” so to speak. If your cells don’t use all the available glucose, it ultimately gets stored as, you guessed it, fat. During a fast, your cells switch from using glucose as their primary fuel source to using fat. Thus, your fat stores, chiefly triglycerides, get burned up for energy. This is why research has found that IF leads to weight loss, as well as an improved cardiovascular disease risk profile. Some experts suggest the shift may occur as early as 10 hours, and usually between a window of 10 to 16 hours, after not eating, while others suggest the metabolism change is most pronounced after 18 hours of fasting. (4, 5, 6, 7, 8, 9, 10, 11, 12) The breakdown of proteins for fuel won’t happen until about the third day of fasting. That means that IF can help you lose weight while maintaining muscle mass.

When your body uses fat stores for energy, it releases fatty acids called ketones into the bloodstream. Ketones play a role in weight loss, but they have also been shown to preserve brain function, even offering some security against epileptic seizures, Alzheimer’s disease, and other neurodegenerative disorders. For instance, one study of older adults with mild cognitive impairment found that a boost in ketones improved memory in just six weeks. (13) Such benefits may occur because ketones help trigger the release of brain-derived neurotrophic factor (BDNF), which strengthens neural connections, particularly in areas involved in memory and learning. Studies have also shown that IF promotes the growth of new nerve cells in the brain. (14, 15, 16, 17, 18)

IF Lowers Insulin and Improves Insulin Sensitivity

When you fast, your insulin levels drop, while your levels of human growth hormone and norepinephrine rise, helping you shed weight and resist chronic disease.

Put simply, we get a flood of insulin when we eat, while levels decrease when we fast. Insulin regulates whether extra glucose from digestion gets stored in the body as fat—another reason IF may contribute to weight loss. Research shows that IF decreases insulin dramatically and can reduce hyperinsulinemia, as well as improve insulin sensitivity. In animal studies, IF has been proven to both prevent and reverse diabetes. (19, 20, 21, 22) Research also suggests that when insulin is decreased, the body experiences a rise in FoxO transcription factors, which control genes related to metabolism. Ultimately, this process may alter gene expression in favor of healthy aging and longevity. IF also seems to lower insulin-like growth factor 1, a genetic marker for diseases like cancer. (23, 24)

It Increases Levels of Human Growth Hormone (HGH)

Studies show fasting releases HGH. (25, 26) That’s significant because, as we age, our bodies may produce lower levels of HGH—and that’s associated with an increase in fat tissue and loss of muscle mass. Research into HGH’s effect on body composition suggests it can help subjects lose weight without losing muscle. (27, 28, 29)

It Ups Norepinephrine

This spike in this neurotransmitter, which is also a stress hormone, contributes to IF’s positive impact on metabolism and helps the body break down fats for fuel. (30, 31, 32)

It Increases Your Cells’ Resiliency and Health

During intermittent fasting, your cells get a little stressed out—in a good way—making you more resilient. (This is why I sometimes fast if I think I’m getting a cold.)

As Mark Mattson with the National Institute on Aging explained to the Canadian Medical Association Journal, cells are under “mild stress” during a period of fasting: (33)

They respond to the stress adaptively by enhancing their ability to cope with stress and, maybe, to resist disease.

He likens the stress IF puts on cells to that placed on your muscles and heart during exercise: it’s an immediate shock to the system that allows your body to get stronger over time.

Fasting also seems to cause cells to initiate a waste-removal process called autophagy. During autophagy, the body cleans house and starts regenerating itself—eliminating dysfunctional, damaged cells to make room for new, healthy ones. Autophagy may offer protection against diseases like cancer or dementia. (34, 35, 36, 37) IF also appears to protect the body against the negative effects of oxidative stress and inflammation. (38)

It Improves Your Circadian Rhythm

IF can help you find harmony with your internal clock, improving your metabolism and overall health and well-being.

The light/dark cycle is a very important circadian cycle in humans, and it has major implications for metabolism. Evolutionarily speaking, we’re designed to eat during the light cycle (daytime) and fast during the dark cycle (nighttime). Our metabolism appears to be most efficient when we adhere to these natural rhythms. Yet, our modern lifestyles mean we’re awake and active long after the natural light cycle has ended. Many people eat well into the night, when they should be in the fasting and sleeping circadian phase. This evolutionary mismatch results in overeating and reduced metabolic efficiency.

Following a time-restricted approach to IF may help regulate circadian rhythms and improve metabolism. The science suggests harmonizing our eating patterns with our biological clock can lead to improved weight regulation and reduced obesity. Studies also show that, through this synchronization, IF promotes normal expression of genes involved in liver health and even restores beneficial gut bacteria. (39, 40, 41, 42)

When You May Not Want to Fast

Before you adopt the IF lifestyle, consider these caveats. Intermittent fasting may not be right for you if:

  • You’re suffering from chronic fatigue or HPA axis dysregulation
  • Your hormones are out of whack
  • You’re a woman trying to maximize fertility
  • You suffer from any type of eating disorder
  • You’re under a lot of stress at work or at home
In general, I’m a fan of IF, especially for people who need to lose weight rapidly for health reasons, lower their blood sugar, or improve insulin resistance. In fact, I teach IF as an effective health coaching strategy in my course, the ADAPT Health Coach Training Program. If you don’t fall into the categories listed above, IF may be a good strategy for improving your health.

Have you tried intermittent fasting before, or are you planning to do so in the future? Let me know in the comments.

  1. I hope IF is as good as I hear. I’ve been doing it for some time. Not for weight loss, really, but just for those less tangible items outlined above. Just turning 67 and I still work, go for long hard mtb rides, farm… whatever, but compressing the morbidity window in the next few years is the goal.

    And the 16:8 programme lets me eat all the farm fresh meat and veggies we grow I want… which is a lot. Yum. Might be easier here in Te Waipounamu Aotearoa than the US, though.

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