BY THE OPTIMIST DAILY EDITORIAL TEAM

“The main benefit of fasting is weight loss,” says Krista Varady, a professor of nutrition at the University of Illinois Chicago and a co-author of related research, and the data supports her.

In a peer-reviewed clinical trial at the University of Sydney, “participants practicing intermittent fasting lost about eight 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 systematic review of 27 trials found significant weight loss and concluded that intermittent fasting shows promise for treating obesity.

What makes this finding useful is what a 2025 study found about adherence. Participants who practiced two months of intermittent fasting were more likely to maintain the pattern years later. Mark Mattson, a neuroscientist at Johns Hopkins Medicine who has studied intermittent fasting for 25 years, explains why that matters: “Most people who go on a calorie-restricted diet are often unable to sustain the low-calorie diet and go on to regain weight.” An eating pattern people can keep may outperform a better one they abandon.

But it is not automatic. “Meaningful weight loss only occurs if intermittent fasting is properly implemented without overcompensating ‘feast’ hours or days,” Fontana says. There is also the risk of losing lean body mass. “When fasting, the body may lose lean mass such as bone and muscle along with fat,” he adds, though this can often be offset by adequate protein intake and resistance training.

What happens to cholesterol and blood pressure

Weight loss itself drives many of the cardiovascular benefits researchers attribute to fasting. Varady notes that it can help with “lowering metabolic disease risk indicators like blood pressure and cholesterol levels.” A 2025 six-month trial showed improvements in triglycerides and LDL cholesterol, occurring in part because weight reduction decreases visceral fat and lowers the liver’s production of fat-carrying particles in the bloodstream.

Mattson points to a second mechanism: “Intermittent fasting increases the activity of the parasympathetic nervous system.” This is the branch responsible for rest and recovery, and greater activity there can lower heart rate and reduce vascular resistance, working against the chronic stress response associated with high blood pressure.

The caveat is specific: “Improved cholesterol and blood pressure generally only occurs if an individual loses over five percent of their body weight and if their blood pressure and cholesterol levels were high before starting the diet,” Varady says. For people whose markers are already in a healthy range, the effect is likely to be smaller.

Blood sugar and diabetes: meaningful benefits, real risks

For people with insulin resistance or type 2 diabetes, the evidence is stronger. Fasting lowers circulating insulin levels, increases insulin sensitivity, and shifts the body toward burning stored fat rather than relying on glucose. Studies show reductions in fasting glucose, lower hemoglobin A1c levels, and, in some cases, a reduced need for diabetes medication.

But fasting is not risk-free for people managing diabetes. Duane Mellor, senior specialist dietitian at the University Hospitals of Leicester NHS Trust, is direct: “People who take insulin or drugs that cause the body to make more insulin should speak to a doctor before starting a fasting diet.” The risk of hypoglycemia is real when medications are not adjusted to account for reduced food intake.

Brain health: promising mechanism, limited clinical evidence

Many people report sharper thinking while fasting, and Mattson identifies a plausible mechanism: fasting stimulates production of neurotrophic factors such as brain-derived neurotrophic factor and activates cellular stress responses that may strengthen neural networks involved in learning and memory. Research also shows benefits for people with epilepsy, Alzheimer’s disease, and multiple sclerosis, in both symptoms and disease progression.

In healthy adults, however, the clinical picture is thinner. “There remains very limited evidence for cognition benefits,” says Varady. Much of the mechanistic research has been conducted in animal models, and results have not consistently translated in human trials.

Who should approach this carefully

Fasting is not appropriate for everyone. Fontana notes that caution is warranted for frail older adults or anyone at risk of sarcopenia, the progressive age-related loss of muscle mass and strength, because losing lean mass can be amplified without sufficient protein and resistance training. Valter Longo, director of the Longevity Institute at USC, adds that “people with eating disorders should also avoid fasting” due to the risk of reinforcing restrictive behaviors. Pregnant and breastfeeding women should also abstain, given increased energy and nutrient demands.

Women generally, though, do not need to avoid fasting. Persistent claims that it disrupts hormones have not held up under scrutiny. “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 Varady. Multiple high-quality human trials have found no significant changes in sex hormone concentrations, even after a year of continuous time-restricted eating.

What fasting actually requires to work

The researchers agree on one thing clearly: fasting does not work in isolation. “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.”

Practical guidance from the research team includes prioritizing protein intake, eating fiber-rich whole foods during eating windows, staying hydrated, and watching for symptoms like dizziness or excessive fatigue. Anyone with chronic conditions or taking medications should consult a healthcare professional first.

And allow time. “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,” Mattson says. The research is promising for many people, but the results depend on building the surrounding habits alongside the fasting window itself.

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