What Really Happens to Your Body When You Fast

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Periods of fasting affect much more than your weight.
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For those looking to lose weight, a meal plan that incorporates intermittent fasting (aka time-restricted feeding) may sound like a smart choice. After all, prolonged calorie restriction has been linked to benefits for brain function, gut health, energy levels and metabolism. But how much do we really know when it comes to fasting's effects?

"Fasting activates an evolutionary program in which our body diverts nutrients normally used for growth and reproduction toward mechanisms for repair and survival," says Whitney English, RDN, a Los Angeles-based nutritionist and founder of the practice Whitney E. RD.

Translation: Abstaining from food for set periods of time may result in weight loss, but it also rewires your whole system. Here's what we know about fasting's effects on the body, from head to toe.

Your Weight

"Research shows that fasting results in weight and fat loss," says English. But, there's a catch. "People do not lose more weight from fasting than they do with traditional low-calorie diets." In other words, simply reducing the amount of food you eat may be just as effective as halting your intake altogether during certain hours.

A July 2017 study published in JAMA Internal Medicine, for example, found that alternate-day fasting did not result in any more weight loss compared to regular calorie restriction among metabolically healthy, obese individuals.

So why are meal plans that incorporate fasting often touted as no-brainer means to shed pounds? "They work simply by limiting the number of calories consumed," says Amy Rothberg, MD, director of the Metabolism, Endocrinology and Diabetes Investigational Weight Management Clinic at the University of Michigan. "Consider the average American who usually consumes meals and snacks over 12 hours every day. Limiting consumption to eight or even fewer hours generally will limit caloric intake."

But fasting may do more than simply curb intake. A June 2012 study published in Cell Metabolism found that mice given a high-fat diet during an eight-hour feeding window (a regimen that is often referred to as time-restricted feeding or TRF) were protected against adverse metabolic outcomes, such as fatty liver disease and hyperinsulinemia (high levels of insulin in the blood), compared to mice that consumed the same amount of calories over the course of the day.

Read more: The Beginner's Guide to the 5:2 Fasting Diet for Weight Loss

Your Appetite

"Counterintuitively, fasting has been shown to decrease the hunger hormone ghrelin," says English. So those who fast from, say, 8 p.m. until noon the next day may not feel the hunger pangs you'd expect at breakfast time.

The only problem? Once you return to your regular eating patterns — or switch back and forth between fasting and non-fasting days — there's a good chance you may go overboard.

That's because the human body is biologically hardwired to sufficiently fuel itself in order to avoid famine. As a result, neurotransmitters and appetite hormones could drive you to overeat after an extended period of deprivation, according to Harvard Health Publishing.

Your Brain

Here's a surprising-but-true fact: Some people report feeling more alert and focused while adhering to meal plans that incorporate fasting. "This enhanced brain functioning is likely brought about when our bodies switch from using glucose for fuel to using ketone bodies as an energy source," explains English. "Researchers call this 'intermittent metabolic switching' and believe that it adapted to help humans remain in a state of optimal functioning in order to hunt and gather food during periods of hunger."

This switch may help bolster neural networks' "resistance to stress, injury and disease" and potentially improve cognition and mood, per a February 2018 study published in Nature Reviews Neuroscience.

Your Gut

Animal studies suggest that intermittent fasting regimens can modify the makeup of the microbiome by increasing the proportion of health-promoting microbes in the gut, and they may even bolster animals' immune response to pathogenic bacterial infections like salmonella typhimurium.

Human data, however, remains lacking. "Limiting nutrients to the gut may limit the work that it needs to do, and also may change the [human] microbiome in favor of more healthy bacteria," says Rothberg. "But this is speculation."

Read more: Everything You Need to Know About Prebiotics—the Secret Ingredient to Boosting Your Gut Health

Your Insulin Sensitivity

In healthy individuals, eating triggers the release of insulin, the hormone that shuttles glucose (the product of carbohydrate digestion) into cells to be used for energy or stored for later. When we take a break from eating, insulin levels drop. The result? Improved insulin sensitivity, which is protective against diabetes.

"Limiting calories also translates to reduced blood glucose, improved lipids and reduced blood pressure," says Rothberg. "This has been shown in a number of studies of low-calorie and very low-calorie diets, and time-restricted feeding has some of the same impact."

While animal studies have shown that time-restricted feeding patterns can help to reverse the progression of metabolic diseases in mice, findings from clinical trials conducted in humans report mixed results. A randomized clinical trial published in JAMA Internal Medicine in July 2017, for example, found that alternate-day fasting did not improve cardiometabolic risk factors (think: insulin resistance, LDL cholesterol levels) any more than daily calorie restriction.

Read more from our 'What Really Happens to Your Body When' series.

To Fast or Not To(o) Fast?

There's no one-size-fits-all answer, but depriving the body of food for extended periods of time is unlikely to be sustainable — or enjoyable — for most people. If taken too far, fasting may lead to an unhealthy relationship with food. For this reason, The Academy of Nutrition and Dietetics discourages anyone with a history of disordered eating to adhere to meal plans that include fasting.

Women who are pregnant or breast-feeding are advised not to fast. The same goes for diabetics, since long-term energy restriction can result in extreme fluctuations in blood glucose levels.

Finally, if you're fasting solely to shed weight, remember this: "Calorie restriction always backfires," says English. "People end up gaining the weight back and then some." Instead, the nutritionist recommends trying what she calls "commonsense fasting" for optimal health.

"Align your eating routine with your circadian rhythm, eating during the day and fasting at night," suggests English. "This looks different for each person, but generally just means that you stop eating after dinner around 6 to 8 p.m. and you begin eating again at breakfast between 6 to 8 a.m." Simple as that.

Read more: The Dos and Don'ts of Intermittent Fasting

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