The Physical and Health Effects of Fasting

Intermittent fasting has become a popular approach to weight management and overall health, with research suggesting benefits for metabolism, brain function, and insulin sensitivity. But what actually happens inside your body when you fast? From your gut to your breath, here’s what science says about the effects of fasting from head to toe.

How Fasting Affects Your Weight

Fasting can lead to weight loss but not necessarily more than traditional calorie restriction.

"Research shows that fasting results in weight and fat loss," says Samantha Gitlin, RD, CDN, a registered dietitian in New York City. "But people don’t lose more weight from fasting than they do with traditional low-calorie diets."

A JAMA Internal Medicine study found that alternate-day fasting was no more effective for weight loss than simply reducing calories. The key reason fasting works? It limits the eating window, which naturally reduces calorie intake.

What Happens to Your Appetite?

It seems counterintuitive, but fasting may actually decrease hunger in the short term.

"Fasting has been shown to lower levels of ghrelin, the hunger hormone," says Whitney English, RDN, a Los Angeles-based nutritionist.

However, prolonged fasting can trigger rebound overeating once the eating window opens. That’s because the body, designed for survival, may push you to consume extra calories to compensate for the restriction.

The Brain’s Surprising Response to Fasting

Many people report feeling more alert and focused during fasting periods, and science may explain why.

"This enhanced brain functioning is likely due to the body switching from glucose to ketones for fuel," English explains. This process, called intermittent metabolic switching, may help the brain resist stress and even improve cognitive function.

A Nature Reviews Neuroscience study suggests that this adaptation may have helped early humans stay sharp during food shortages.

How Fasting Impacts Gut Health

Animal studies indicate that intermittent fasting positively alters gut bacteria, increasing beneficial microbes and supporting immune function.

However, human data is limited. "Restricting nutrients to the gut may limit its workload and shift the microbiome toward healthier bacteria," says Amy Rothberg, MD, director of a weight management clinic at the University of Michigan. "But this is still speculative."

Fasting and Insulin Sensitivity

For people without diabetes, fasting lowers insulin levels, improving the body's ability to process glucose. This effect is associated with:

• Lower blood sugar levels

• Improved cholesterol and lipid profiles

• Better blood pressure regulation

However, results vary. A JAMA Internal Medicine study found that alternate-day fasting did not significantly improve metabolic risk factors compared to daily calorie restriction.

Bad Breath? Fasting Could Be the Culprit

If you notice bad breath during fasting, you’re not imagining it.

Fasting reduces saliva production, leading to a buildup of bacteria that causes odor. Additionally, when the body burns fat for fuel, it releases ketones like acetone, which can cause a fruity-smelling breath.

Who Should Avoid Fasting?

While fasting may offer health benefits, it’s not for everyone. Experts do not recommend fasting for:

• Pregnant or breastfeeding individuals

• People with a history of disordered eating

• Those with diabetes or blood sugar regulation issues

Additionally, if fasting leads to overeating, fatigue, or stress, it may not be the right approach for you.

A More Balanced Approach to Fasting

Rather than extreme fasting, some experts recommend aligning meals with your natural circadian rhythm.

"Stop eating after dinner around 6 to 8 p.m. and start again at breakfast between 6 to 8 a.m.," English suggests. This form of fasting is more sustainable while still offering potential health benefits.

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