Consistent fasting may lead to weight loss over time. However, fasting can lead to nutritional deficiencies and low energy intake, negatively affecting health in the long term.

While some people may lose weight by not eating (fasting) for short periods, prolonged fasting can be dangerous and is not a safe weight loss method.

Restrictive dietary practices can lead to people not getting enough calories (energy) and key nutrients essential for proper body functions. This may lead to poor energy, focus, and physical performance, loss of muscle, and impair a person’s metabolism.

This article examines whether people can lose weight by fasting, what happens to the body when not eating, and the benefits and risks of fasting. It also explores intermittent fasting, drinking water, and safe ways to fast.

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Yes, it is possible to lose weight by eating less than usual. Calorie restriction induces a negative energy balance, which leads to short-term weight loss.

However, in response to severe calorie restriction, the body reduces its resting metabolic rate, a process known as metabolic adaptation. As a result, the body uses less energy for key functions, lowering a person’s overall energy expenditure.

As the body adapts to function with a reduced energy intake, it can become difficult for a person to maintain their initial weight loss. A 2024 review found that following calorie-restrictive diets, weight gain in the 4 to 6-month timeframe is common, with many people returning to pre-intervention weights.

Research from 2020 also details that skipping meals can affect the nutritional quality of your diet and negatively affect health over time.

Food consumption is essential to human life. Stopping eating can have severe short-term and long-term effects.

When the body cannot get energy from food, blood sugar (glucose) levels drop. In response, the liver can release stored glucose into the bloodstream to raise these levels, although these stores are limited.

Following this initial depletion, the liver can produce glucose during the first 18 to 24 hours after not eating, a process called gluconeogenesis.

Low blood sugar levels can lead to:

In periods of prolonged fasting or other caloric restriction, the body may pivot to alternative energy sources to fuel various organs, including the brain. For example, the brain relies exclusively on glucose; in its absence, the body can release ketones to replace this fuel.

The body also stores glucose in the muscles, which it can use to provide energy for movement during times of low blood glucose.

Long term

If the body does not receive adequate nutrition over an extended period, it can enter a state of starvation, during which the body alters its normal function to reduce calorie expenditure and maintain energy balance.

Long-term calorie restriction is dangerous and can lead to malnutrition, muscle loss, and metabolic slowdown.

Controlled, structured fasting can have health benefits, but severe caloric restrictions are dangerous. People should consult a medical professional before making significant dietary changes.

Under medical supervision, fasting may provide health benefits alongside weight loss. Research suggests that occasional 24-hour fasting may improve cardiovascular health.

A 2019 review looked at health outcomes associated with fasting in both human and animal studies between 2016 and 2018. Researchers suggest that specific fasting diets appear to protect against:

Other 2019 research suggests that fasting can trigger significant gut microbiota changes, which may have beneficial health effects.

Fasting may also promote autophagy, the body’s breaking down of old cells that can be potentially harmful.

Autophagy may benefit the following conditions:

If someone continues not to eat over time, it can negatively affect their health.

Low blood sugar

If a person does not eat enough carbohydrates or delays or skips meals altogether, blood sugar levels could drop. This may lead to:

Nutritional deficiency

Reducing food intake without adequate planning or supplementation can lead to inadequate mineral and vitamin intake, which can risk nutritional deficiency.

Severe caloric restriction can also lead to electrolyte imbalances.

Mental health

Short-term fasting can also increase negative emotions, such as:

Muscle loss

In cases of severe, prolonged fasting, the body begins to break down muscle tissue and other organs for energy during starvation. This occurs due to protein catabolism and cachexia.

These processes severely harm health, the immune system, and physiological function.

Cardiovascular risks

Research into the effects of fasting on cardiovascular health is ongoing. However, a 2024 study from the American Heart Association (AHA) found that intermittent fasting has associations with an increased risk of death from cardiovascular disease, and that it did not lead to a longer life span as previous studies suggested.

Intermittent fasting is a form of time-restricted fasting that people use for weight loss. This method involves fasting for a certain number of hours and consuming all one’s calories during that period.

There are many different forms of intermittent fasting, such as:

  • Time-restricted fasting: In this, a person consumes all of their calories in a portion of the day and then fasts the rest. For example, in 16:8, a person may fast for 16 hours per day, and consume all calories in the other 8 hours.
  • Alternate day fasting: In this, people alternate between regular calorie consumption and days either fasting or on a low calorie limit.
  • 5:2 fasting: In 5:2 fasting, a person eats a regular amount of calories five days a week and a lower amount for two days.
  • One meal a day: One meal a day, or OMAD, involves a person consuming all daily calories in one meal and fasting for the rest of the day.

While the focus is often on food timing rather than calorie counting, intermittent fasting is still a form of calorie restriction. If people find it a sustainable way to reduce overall caloric intake, it can be an effective weight management tool. However, it may not be suitable for all.

Research from 2024 suggests that intermittent fasting may be an effective strategy to manage weight. A 2023 scoping review showed that intermittent fasting improves the metabolic balance by syncing it with the circadian rhythm. It may also have a rapid effect on the gut microbiota.

Fasting is not suitable for all people. It is essential to speak with a healthcare professional before following a calorie-restrictive diet. This is especially important for those:

Help is available

Eating disorders can severely affect the quality of life of people living with these conditions and those close to them. Early intervention and treatment greatly improve the likelihood of recovery.

Anyone who suspects they or a loved one may have an eating disorder can contact the National Alliance for Eating Disorders, which offers a daytime helpline staffed by licensed therapists and an online search tool for treatment options.

For general mental health support at any time, people can call the Substance Abuse and Mental Health Services Administration 24 hours a day at 1-800-662-4357 (or 1-800-487-4889 for TTY).

Many other resources are also available, including:

While some people may lose weight by not eating (fasting), it may affect the food quality and nutritional value.

Under medical supervision, fasting may provide health benefits, such as improved cardiovascular health and protective effects against diabetes, cognitive decline, and some cancers.

However, not eating may cause low blood sugar levels, causing tiredness and dizziness. It may also negatively affect mental health and cause nutrient deficiencies.

People should consult a medical professional for safe ways to fast. Tips include fasting for short periods, keeping hydrated, not fasting when unwell, and reducing physical activity.