The Effects of Fasting on the Body, From Head to Toe

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 what really happens when we fast?

"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 the effects of fasting on the body, from head to toe.

First on the list of what happens to your body when you fast? Your weight.

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

A July 2017 study 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 people with obesity.

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 curb intake. A June 2012 study 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 ate the same amount of calories over the course of the day.

Here's what happens to your appetite during a fast: "Counterintuitively, fasting has been shown to decrease the hunger hormone ghrelin," English says. 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.

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," English says. "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 in ​​Nature Reviews Neuroscience.​​

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," Rothberg says. "But this is speculation."

For people without underlying conditions like diabetes, 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," Rothberg says. "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.

The July 2017 study in ​​JAMA Internal Medicine​, 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.

It's possible you'll notice your breath stinks while fasting. Indeed, intermittent fasting can lead to bad breath, according to April 2019 research in the ​Journal of Applied Oral Science​.

So, why do you have bad breath when fasting? Bad breath and intermittent fasting can go hand-in-hand because fasting decreases salivary flow, per the ​Journal of Applied Oral Science​ research. This can lead to a build up of bacteria, which, along with the decomposition of food particles, can cause stinky breath.

Fasting can also up the concentration of acetone in your breath, per April 2020 research in the ​International Journal of Molecular Sciences​, which can likewise contribute to fruity smelling breath.

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.

People who are pregnant or breast-feeding are advised not to fast. The same goes for people with diabetes, as 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," English says. "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," English says. "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."