Studies have shown that periodic fasting offers numerous health benefits: it can help prevent or reduce symptoms associated with many age-related diseases, including heart disease, cancer, and diabetes, and it may also provide some protection against dementia. Studies using animals have found that introducing a fasting diet during middle age delays the onset of symptoms associated with cognitive decline.

“Our genes are geared to being able to cope with periods of no food,” says Mark Mattson from the National Institute on Aging. “The evidence is pretty strong that our ancestors did not eat three meals a day plus snacks.”

How fasting improves health

When the digestive system is not continually engaged, the body can put more energy towards healing and repair. A fast is considered to begin approximately 10 to 12 hours after you eat your last meal. At this point, the body will have used up all the available glucose from food for energy. The body will begin drawing energy from glycogen stores in the liver and muscle. After glycogen stores are mostly used up, the body begins breaking down stores of fat for energy to produce ketone bodies to use as fuel. This state, described as being in nutritional ketosis, typically begins after 3 to 4 days on a prolonged fast. At this point, people usually report feeling lighter, with increased energy and focus. Nutritional ketosis should not be confused with ketoacidosis, which is a dangerous state marked by high levels of ketone bodies in people with diabetes.

Because your body is used to running on carbohydrates, the carb withdrawal at the beginning of a fast can often cause unpleasant symptoms, including headaches, nausea, and other flu-like symptoms. These tends to subside after a few days, and are less likely to occur after a few rounds of periodic fasting. You can help prevent or lessen symptoms by drinking lots of water and making sure you’re replenishing electrolytes.

Certain hormone levels are also affected during a fast. Production of insulin-like growth factor 1 drops and reaches very low levels after a few days. High levels of IGF-1 have been linked to tumor development, which helps explain the protective effects of fasting against cancer. Valter Longo, director of the Longevity Institute at the University of Southern California, has found that a 48-hour fast in mice slowed the growth of five different cancer types and increased the effectiveness of chemotherapy drugs, with subsequent fasts making the effects even more pronounced.

The effects of fasting on brain health

Mattson’s studies suggest that alternate day fasting, with a single meal of 600 calories on fast days, boosts production of a protein called brain-derived neurotrophic factor by 50 to 400 percent. Brain-derived neurotrophic factor is involved in the production of new brain cells, and also plays a role in memory and learning. It can also help protect brain cells from changes associated with Parkinson’s and Alzheimer’s. In mice engineered to have symptoms similar to Alzheimer’s, introducing an alternate-day fasting diet in middle age slowed the onset of memory problems by about six months — roughly the equivalent of 20 years in humans.

How to fast safely

There are several different approaches to fasting, many of which don’t actually require going without food for an extended period of time.

Alternate-day fasting refers to the practice of reducing calorie intake on non-consecutive days during the week. Fasting days may include one meal consisting of about 600 calories, and normal meals can be eaten the rest of the week.

Time-restricted fasting involves limiting food consumption to a certain period of the day, such as 10:00 am to 6:00 pm, leaving more time in between dinner and breakfast for the digestive system to rest. Mattson skips breakfast and lunch during the week, eating only one meal a day, and eats normal weekend meals with his family.

Although fasting is difficult, new research and developments may make it more accessible for the general population. Some research indicates that you can get many of the same results by reducing protein intake. And pre-packaged meal plans, such as the ProLon fasting mimicking diet, allow people to experience many of the same benefits of fasting while still eating three meals a day. Meals in the ProLon kit include plant-based soups and snacks high in healthy fats, which provide a feeling of fullness even while consuming fewer calories. Learn more about ProLon and find out if it’s right for you.