Fasting has been practiced for centuries for religious and cultural reasons. In recent years, it has also become popular as a health strategy for weight loss, disease prevention, and possibly slowing aspects of aging. Many people now follow intermittent fasting, a method that involves consuming calories during a limited window of time each day and fasting for the remaining hours.
Research suggests that fasting may provide several health benefits, including improvements in blood pressure, metabolic health, and certain disease risk factors. However, scientists emphasize that the effects of fasting are complex and may vary from person to person. Some benefits may also be less dramatic than they are sometimes portrayed online.
What Is Intermittent Fasting?
Definition of Intermittent Fasting
Intermittent fasting is an eating pattern that alternates between periods of eating and fasting. Unlike traditional diets that focus primarily on what foods people eat, intermittent fasting focuses on when people eat them. People who practice intermittent fasting usually consume food within a restricted time window, such as eight hours per day, and fast during the remaining hours. Researchers are studying how limiting eating times may affect metabolism, weight management, and disease risk.
Why Intermittent Fasting Has Become Popular
Growing interest in intermittent fasting has been driven by research suggesting that certain fasting patterns may improve health when practiced properly. Neuroscientist Mark Mattson, who has studied intermittent fasting for more than 25 years, notes that fasting may help reduce high blood pressure, improve cognition, and potentially help prevent or even reverse chronic diseases such as diabetes and cardiovascular disease. However, experts caution that some claims about fasting are exaggerated. While research is promising, the scientific evidence is still evolving.
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The Most Reliable Benefit: Weight Loss
Why Fasting Helps People Lose Weight
The most consistent benefit of intermittent fasting is weight loss. Nutrition researcher Krista Varady explains that many health improvements linked to fasting occur because people lose weight. Limiting the hours during which people eat often reduces total calorie intake, which can lead to gradual weight loss over time. However, weight loss occurs only if people do not compensate by eating significantly more during eating periods.
Evidence From Clinical Trials
Scientific studies support the weight-loss benefits of intermittent fasting. A clinical trial conducted by researchers at the University of Sydney found that participants practicing intermittent fasting lost about 8 percent of their body weight and around 16 percent of their fat mass over six months. A systematic review of 27 clinical trials also reported significant weight loss and concluded that intermittent fasting may be a promising approach for treating obesity. Researchers also note that fasting may be easier for some people to maintain compared with strict calorie-restriction diets.
What Intermittent Fasting Does to the Body
Changes Metabolism
When the body goes without food for an extended period, insulin levels decrease, and the body begins to burn stored fat for energy rather than relying primarily on glucose from recently eaten carbohydrates. This metabolic shift increases fat oxidation and can lead to the production of ketones, molecules derived from fat that can serve as an alternative energy source for the body and brain.
Activates Cellular “Cleanup”
Fasting may also stimulate autophagy, a natural process in which cells break down and recycle damaged or unnecessary components. Scientists often describe autophagy as a type of cellular housekeeping. This process may improve cellular health and stress resistance, although much of the detailed research has been conducted in animals rather than humans.
May Improve Heart and Metabolic Health
Intermittent fasting may improve several markers associated with cardiovascular and metabolic health. Studies show that fasting may help reduce blood pressure, LDL cholesterol, and triglycerides, and visceral (abdominal) fat. These improvements are often linked to weight loss and changes in metabolism. However, experts emphasize that these benefits typically occur only when people lose a meaningful amount of weight or already have elevated risk markers.
Blood Sugar and Diabetes
Research suggests that fasting can improve insulin sensitivity and help regulate blood sugar levels. Some studies have shown improvements in fasting glucose and hemoglobin A1c levels in people with insulin resistance or type 2 diabetes. However, fasting may also pose risks for people who take insulin or medications that increase insulin production. These individuals should consult a healthcare professional before attempting a fasting diet because of the risk of low blood sugar (hypoglycemia).
What Intermittent Fasting Does to the Brain
Potential Effects on Brain Health
Some people report improved mental clarity or mood while fasting. Scientists believe fasting may influence brain health through mechanisms such as increased production of brain-derived neurotrophic factor (BDNF), enhanced cellular stress resistance, and improved mitochondrial function. These changes may help strengthen neural networks involved in learning and memory. However, clinical trials in healthy individuals have not consistently shown clear improvements in cognitive performance.
Effects on Neurological Conditions
Research suggests intermittent fasting may benefit certain neurological conditions, such as epilepsy, Alzheimer’s disease, and multiple sclerosis. Some studies report improvements in symptoms or disease progression, although more research is needed to confirm these effects.
What Scientists Still Don’t Know
Limits of Current Research
Although intermittent fasting shows promise, scientists stress that the evidence is still developing. Some benefits reported in early research may not be as large or as consistent as often claimed. Much of the mechanistic research explaining how fasting works has been conducted in animal studies, which may not always translate directly to humans.
Hormones and Fasting
Some online claims suggest that fasting disrupts hormones, particularly in women. However, current human studies generally do not show significant hormonal disruption in most women who practice intermittent fasting. Researchers say misinformation about fasting and hormones often spreads through social media rather than scientific evidence.
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Who Should Be Careful With Intermittent Fasting
Intermittent fasting is not appropriate for everyone. Experts recommend caution for people with eating disorders, pregnant or breastfeeding women, frail older adults, or those at risk of muscle loss, and individuals taking insulin or certain diabetes medications. In these cases, fasting may pose health risks and should only be attempted under medical supervision.
