Intermittent Fasting: What the Evidence Actually Says

Intermittent fasting (IF) is an eating pattern that cycles between periods of eating and voluntarily abstaining from food.

The short version

• There are many benefits touted for intermittent fasting, but some have more evidence behind them than others.
• There are several different approaches to intermittent fasting, which give people flexibility in choosing a schedule that may suit them.
• Intermittent fasting may help some people lose weight and improve certain metabolic health markers, but it does not consistently work better than other calorie-reduction approaches. Long-term evidence is still limited, and the best eating pattern is one that is nutritionally balanced, safe, and sustainable for the individual.
• IF is not suitable for everyone. You should consult your doctor or healthcare provider before starting intermittent fasting or any weight-management plan.

What intermittent fasting actually is

The focus of intermittent fasting is primarily on when you eat rather than what you eat.

• Most intermittent fasting approaches determine when you eat rather than prescribing specific foods. However, nutrition quality and overall calorie intake still matter.
• What is allowed during fasting periods varies by approach. Generally, water and zero-calorie beverages such as plain coffee, unsweetened tea, and diet soda are okay to consume during fasting periods.
• IF is different from prolonged fasting, which is often described as abstaining from food for more than 48 hours, although definitions may vary.
• During eating periods, it is still important to consume a balanced, nutritious diet. Foods rich in fiber and protein, including fruits and vegetables, may also help with fullness.

Types of intermittent fasting:

• Time-restricted eating: eating takes place within a limited daily window. For example, the 16:8 approach involves fasting for 16 hours and eating within an eight-hour window.
• Alternate-day fasting: alternating between regular eating days and fasting or substantially reduced-calorie days.
• The 5:2 approach: people eat normally on five days of the week and substantially reduce their calorie intake on two nonconsecutive days—often to around 25% of their usual energy needs.
• Religious fasting: religious fasting practices may differ significantly from dietary fasting approaches. During Ramadan, for example, Muslims abstain from both food and drink between dawn and sunset for 29 or 30 days.

How intermittent fasting works

One of the main theories behind IF focuses on circadian biology. The human body has an internal clock that operates on an approximate 24-hour cycle (circadian rhythm) and influences a number of processes, such as sleep. Read more on circadian rhythms here.

Feeding and fasting can affect the body’s circadian clock. One theory is that certain intermittent fasting schedules, particularly those that concentrate food intake earlier in the day, may align more closely with the body’s circadian rhythms and offer potential metabolic benefits. Eating the majority of calories earlier in the day rather than late at night may also exert health benefits in much the same way.

As a fast continues, insulin levels fall and the body gradually relies more heavily on stored fat for energy, producing ketones in the process. The timing and extent of this metabolic shift vary depending on factors such as fasting duration, activity level, and a person’s usual diet.

What the research suggests

Weight loss and body composition

One of the main reasons people try intermittent fasting is to lose weight. A shorter eating window may naturally result in fewer calories being consumed over the week. Overall, intermittent fasting can support weight loss for some people, but it does not consistently outperform other approaches that create a similar calorie deficit. Its main advantage may be that some people find the schedule easier to follow.

Health markers

Reductions in C-reactive protein—a blood marker of inflammation—total cholesterol, and triglycerides have been reported across several studies. However, these positive results are difficult to separate from the effects of weight loss, as many participants in these studies also lost weight.

Aging

Autophagy is a cellular process that removes and recycles damaged components. Fasting has been shown to influence autophagy and oxidative stress in animal and laboratory studies. However, researchers do not yet know whether typical intermittent fasting schedules meaningfully increase autophagy or extend lifespan in humans.

Research suggests that calorie restriction and intermittent fasting may affect biological processes associated with aging. In humans, however, the long-term effects remain unclear, and studies designed to measure lifespan directly would be extremely difficult to conduct.

Who should (and who should not) try intermittent fasting?

Intermittent fasting may not be appropriate for some people with diabetes, particularly those who take insulin or medications that can cause low blood sugar. Anyone with type 1 or type 2 diabetes should speak with their healthcare provider before trying it, as medication and blood sugar monitoring may need to be adjusted. Intermittent fasting is also generally not recommended during pregnancy or breastfeeding, for children and adolescents, or for people with a current or previous eating disorder unless advised and supervised by an appropriate healthcare professional. People with other medical conditions or medications affected by food intake should consult their healthcare provider first.

During fasting periods, particularly when starting out, some people may experience headaches, dizziness, low energy, or difficulty concentrating. Staying properly hydrated may help with some symptoms, but anyone experiencing persistent or severe symptoms should stop fasting and consult a healthcare professional.

Edited by The Digest team