Calorie Restriction and Fasting:
What Human Studies Actually Show About Eating Less and Ageing

Eating less without malnutrition, known as calorie restriction, extends lifespan in many animals, and it has inspired a wave of fasting trends: intermittent fasting, alternate-day fasting, "fasting-mimicking" diets and multi-day fasts. Human evidence is more modest but real. In CALERIE, the first long-term randomised trial of calorie restriction in healthy, non-obese adults, participants aimed to eat 25% less but achieved about 12% over two years. They lost about 10% of their body weight and improved blood pressure, LDL cholesterol, insulin sensitivity and inflammation, and a later analysis found their pace of biological ageing slowed by 2-3%. However, fasting isn't magic. In a one-year trial, alternate-day fasting produced similar weight loss to simply eating less every day, but more people dropped out. In monkeys, long-term calorie restriction reduced age-related disease, although effects on lifespan differed between studies. For most people, the benefits come from eating somewhat less and maintaining a healthy weight, in whatever pattern they can sustain. This article explains what's known about calorie restriction and the main fasting approaches.

Key numbers

What calorie restriction and fasting mean

• Calorie restriction (CR): eating consistently less than you would freely choose, typically 10-30% fewer calories, while getting all essential nutrients.
• Time-restricted eating: eating within a daily window, such as 8-10 hours (see Eating Windows and Longevity).
• Alternate-day fasting: alternating days of little or no food with normal eating days.
• 5:2 diet: two low-calorie days a week.
• Fasting-mimicking diet: a few days a month of a low-calorie, low-protein plant-based diet.
• Prolonged fasting: water-only or very low-calorie fasting for several days, which carries risks and needs medical supervision.

In animals, calorie restriction lowers insulin and growth-factor signalling, increases cellular repair processes such as autophagy and reduces inflammation. Whether these translate into longer human lives is still unknown.

Evidence strength

Moderate: modest calorie restriction improves cardiometabolic health in non-obese adults. The CALERIE trial, funded by the US National Institute on Aging, randomly assigned 218 healthy adults under 50, of normal weight or slightly overweight, to calorie restriction or their usual diet for two years. The target was a 25% reduction, but participants achieved about 12% on average. Even so, they lost about 10% of their body weight, mostly fat, and had lower LDL cholesterol, triglycerides and blood pressure, better insulin sensitivity and glucose control, and markedly lower inflammation. The researchers concluded that "modest calorie restriction may reduce the risk of heart disease and diabetes even in healthy adults who are not obese".

Limited: calorie restriction may slow biological ageing. A 2023 analysis of CALERIE blood samples, published in Nature Aging, found that calorie restriction slowed participants' pace of ageing by 2-3% as measured by DunedinPACE, an epigenetic measure of how fast the body is ageing. The researchers estimated that this could translate into a 10-15% lower risk of death, an estimate based on other studies. However, two other epigenetic "clocks" showed no change, and the study couldn't show effects on actual disease or lifespan.

Moderate: in monkeys, calorie restriction reduces age-related disease. Two long-term rhesus monkey studies, at the US National Institute on Aging (NIA) and the University of Wisconsin, initially seemed to disagree about lifespan. A 2017 joint analysis concluded that both found fewer age-related diseases with calorie restriction, and the differences in survival were partly explained by diet: the Wisconsin control monkeys ate a high-sucrose purified diet, while the NIA monkeys ate a more natural diet, and the age at which restriction started differed.

Moderate: fasting schedules work mainly by reducing calories. A one-year trial in JAMA Internal Medicine randomly assigned 100 adults with obesity to alternate-day fasting, daily calorie restriction or no change. Weight loss was similar with both approaches: 6.0% with alternate-day fasting and 5.3% with daily restriction at one year, compared with controls. But 38% of the alternate-day fasting group dropped out, compared with 29% for daily restriction, and their LDL cholesterol was higher. The authors concluded that alternate-day fasting may be less sustainable for most people.

Limited: fasting-mimicking diets. In a 2017 trial in Science Translational Medicine, 100 healthy adults followed a fasting-mimicking diet for five consecutive days a month (about 1,100 calories on day one and 700 on days two to five) for three months, or ate normally. The diet group lost an average of 2.6 kg, their systolic blood pressure fell by 4.5 mm Hg and a growth factor, IGF-1, decreased, with larger benefits in those starting with obesity or higher blood pressure. There were no significant changes in fasting glucose, cholesterol or CRP. The diet products were supplied by a company in which some researchers held equity, and longer, independent trials are needed.

Important limitations and risks.

• Long-term human evidence on lifespan doesn't exist, and animal results may not apply to people.
• Sustaining calorie restriction is hard: CALERIE participants achieved only about half their target.
• Too much restriction can cause harm, including muscle and bone loss, low energy, menstrual problems, feeling cold and poorer immunity.
• Fasting isn't safe for everyone: see the table below.

Recommendations by situation

Practical notes

Calorie restriction is the most consistent anti-ageing intervention in animals, and in people, modest restriction improves heart and metabolic health and may slightly slow measures of biological ageing. Fasting schedules mainly work by helping people eat less; they aren't superior to daily calorie reduction, and some are harder to stick with. For most people, the practical lesson is to avoid habitual overeating, keep a healthy weight, eat a high-quality diet and choose an eating pattern you can maintain for life (see Eating Patterns for Longevity). Older adults should prioritise protein and strength rather than eating less (see Protein in the Sri Lankan Diet).