What intermittent fasting really does for your body and brain

Fasting is a common practice, whether for religious reasons or with the goal of losing weight, preventing chronic disease, or even slowing aging. Those who fast for health reasons typically practice intermittent fasting—consuming calories only during a specific window of the day.

Some scientists say the growing popularity of fasting is being fueled by research suggesting that, when practiced appropriately, certain forms can benefit overall health. “Intermittent fasting can reduce high blood pressure, improve cognition, and prevent and even reverse chronic diseases including diabetes, cardiovascular disease, and cancers,” says Mark Mattson, a neuroscientist at Johns Hopkins Medicine who has studied intermittent fasting for 25 years and has co-authored much of the research on the topic.

But some of the science remains nuanced, and the practice may be less impactful than is sometimes portrayed. “Although some studies have suggested fasting can reduce insulin resistance and inflammation, sometimes these benefits are overstated,” says Duane Mellor, senior specialist dietitian at the University Hospitals of Leicester NHS Trust in England.

At the same time, some harms attributed to fasting—such as claims that it disrupts hormones—are often similarly overstated. “There's a lot of misinformation about this by social media influencers, but there's no evidence to show that fasting leads to hormonal disruptions in most women,” says Krista Varady, a professor of nutrition at the University of Illinois Chicago and a co-author of related research.

Here’s what the research actually shows—and how to approach fasting safely.

Weight loss is the most consistent benefit

No matter the motivation, "the main benefit of fasting is weight loss," says Varady.

Indeed, in a peer-reviewed clinical trial by researchers at the University of Sydney, “participants practicing intermittent fasting lost about 8 percent of body weight and approximately 16 percent of fat mass over six months,” says Luigi Fontana, one of the study’s authors and a professor of medicine and nutrition there. A systemic review of 27 trials similarly found significant weight loss and concluded that “intermittent fasting shows promise for the treatment of obesity.”

There is also emerging evidence suggesting fasting may be easier to sustain than traditional calorie-restriction approaches. A small 2025 study found that overweight participants who practiced two months of intermittent fasting were more likely to maintain the pattern years later. “This is important because most people who go on a calorie-restricted diet are often unable to sustain the low-calorie diet and [go on to] regain weight,” says Mattson.

But there are caveats. “Meaningful weight loss only occurs if intermittent fasting is properly implemented without overcompensating ‘feast’ hours or days,” says Fontana. There is also concern about lean body mass loss. “When fasting, the body may lose lean mass such as bone and muscle along with fat,” he adds. However, this can often be offset by adequate protein intake and weightlifting to preserve muscle.

(Scientists identify one fitness habit that may matter most for longevity.)

Cholesterol and heart health benefits

Varady notes that the weight-loss benefits of fasting can help with “lowering metabolic disease risk indicators like blood pressure and cholesterol levels.” A small 2025 six-month intermittent fasting trial shows “improvements in triglycerides and LDL/non-HDL cholesterol,” says Fontana, who co-authored the study.

These improvements occur, in part, because weight loss reduces visceral (abdominal) fat and decreases the liver’s production of fat-carrying particles that circulate in the bloodstream—both of which positively affect lipid profiles.

Mattson adds another potential mechanism at play for reducing blood pressure: “Intermittent fasting increases the activity of the parasympathetic nervous system,” he says. This branch of the nervous system helps the body relax, digest food, and conserve energy after periods of stress or activity, and greater parasympathetic activity can lower heart rate, reduce vascular resistance, and counteract chronic sympathetic “fight-or-flight” activation that’s associated with hypertension.

Again though, there are caveats. “Improved cholesterol and blood pressure generally only occurs if an individual loses over 5 percent of their body weight and if their blood pressure and cholesterol levels were high before starting the diet,” says Varady.

Blood sugar and diabetes: Real potential and real risks

Research also shows that fasting can influence glucose regulation. This occurs partly because fasting lowers circulating insulin levels, increases insulin sensitivity, and shifts the body toward greater fat oxidation and ketone production—meaning the body begins burning stored fat for energy instead of relying primarily on glucose from recently eaten carbohydrates.

Mattson says intermittent fasting can also enhance cellular stress resistance, stimulate autophagy—a natural cellular “cleanup” process in which the body breaks down and recycles damaged or unnecessary components—and improve mitochondrial function. Together, these changes may improve glucose regulation and the body’s ability to maintain stable blood sugar and metabolic balance during periods of stress, illness, or dietary changes.

Other studies show additional benefits for people with insulin resistance or type 2 diabetes in particular, including reductions in fasting glucose, lower hemoglobin A1c levels, decreased visceral fat, and, in some cases, reduced need for diabetes medications.

(Can fasting help you live longer? Here’s what the science says.)

Still, fasting is not risk free for people with diabetes. Mellor warns that “people who take insulin or drugs that cause the body to make more insulin should speak to a doctor before starting a fasting diet,” largely because hypoglycemia remains a real risk if medications are not adjusted appropriately.

Brain benefits and mental health

Many people also report sharper thinking or improved mood while fasting, but “clinical trials have not been able to show the same benefits” in healthy individuals, says Mellor. At the same time, research does show benefits of intermittent fasting for people with epilepsy, Alzheimer’s disease, and multiple sclerosis—both in terms of symptoms and disease progression.

This suggests that fasting can influence brain health—an effect Mattson largely attributes to increased production of neurotrophic factors such as brain-derived neurotrophic factor and the activation of adaptive cellular stress responses that strengthen neural networks involved in learning and memory.

Still, “there remains very limited evidence for cognition benefits,” notes Varady, and much of the mechanistic research has been conducted in animals, which may not directly translate to humans.

Who should be cautious with fasting

Despite so many clear and potential benefits, fasting is not appropriate for everyone. “Caution is warranted for frail older adults or anyone at risk of sarcopenia,” which is age-related progressive loss of muscle mass and strength, “because loss of lean mass can be amplified without adequate protein intake and resistance training,” Fontana explains.

And because it can trigger restrictive behaviors and obsessive thinking about food, "people with eating disorders should also avoid fasting,” adds Valter Longo, director of the Longevity Institute at the USC Leonard Davis School of Gerontology.

(Eating disorders are on the rise in older women—and menopause is playing a role.)

Fontana also advises that pregnant or breastfeeding women abstain from fasting due to increased energy and nutrient demands needed to support fetal growth and milk production.

But women needn't avoid fasting generally. While some online claims argue that fasting disrupts hormones, Varady counters that several high-quality human trials show no significant changes in sex hormone concentrations, even after a year of continuous time-restricted eating. Mellor agrees, noting that the only research suggesting fasting can negatively affect sex hormone levels comes from rodent studies, “but human studies have not shown the same effect.”

Still, if fasting leads to “chronic under-fueling, sleep disruption, menstrual cycle changes, or excessive stress,” in either sex, it should be modified or discontinued, says Fontana.

How to fast smarter

One key scientific takeaway is that fasting should never stand alone. “No type of fasting is a short, magic fix for health,” Fontana says. “Without improving diet quality and maintaining regular physical activity—especially resistance training—fasting is not a substitute for a comprehensive healthy lifestyle and may lead to unfavorable health changes.” 

Mellor agrees and recommends choosing an approach that fits your lifestyle: “If you are trying to change your diet, it needs to be in a way that works for you.” He also stresses hydration and recommends avoiding fasts that restrict liquids altogether. Adequate hydration is essential for maintaining blood pressure and circulation and research shows that even mild dehydration can lead to headaches, dizziness, moodiness, and reduced cognitive performance.

To fast more effectively, the experts suggest prioritizing adequate protein intake, consuming fiber-rich whole foods during eating windows, monitoring for symptoms such as dizziness or excessive fatigue, and consulting a healthcare professional if you have chronic conditions or are taking any medications.

Patience and consistency are also key. “One important bit of advice for anyone planning to initiate intermittent fasting,” Mattson offers, “is to know that it takes several weeks to a month for your brain and body to adapt so that you are no longer hungry during the fasting period and improvements in health indicators become clear.”