DEAR MAYO CLINIC: A friend told me that she is fasting to lose weight. I’ve tried many diets over the years without much success, so I’m thinking about trying fasting, too. Is intermittent fasting a healthy way to lose weight? Is this just skipping a meal occasionally or is there more to it?
ANSWER: One diet trend that shows no sign of going away soon is intermittent fasting. That’s when you voluntarily abstain from food or beverages other than water for a certain amount of time. Some fasting is for religious reasons, while others fast for weight loss.
The three most popular approaches to intermittent fasting are:
- Alternate-day fasting
Eat a normal, healthy diet one day and then completely fast or have one small meal the next day. Usually, the small meal is fewer than 500 calories.
- 5-2 fasting
Eat a normal diet five days per week and fast for two days per week.
- Daily time-restricted fasting
Eat normally but only within an eight-hour window each day.
Recent research has found that using intermittent fasting for weight loss may have some benefits in the short term.
It appears that fasting for a short time can produce ketosis, which is a process that occurs when the body doesn’t have enough glucose for energy, so it breaks down stored fat instead. This causes an increase in substances called ketones. This, coupled with fewer calories consumed overall, can lead to weight loss. Research suggests that alternate-day fasting is about as effective as a typical low-calorie diet for weight loss.
Fasting also affects metabolic processes in the body that may work to decrease inflammation, as well as improve blood sugar regulation and physical stress response. Some research shows this may improve conditions associated with inflammation like arthritis, asthma and multiple sclerosis.
Little long-term research has been conducted on intermittent fasting to examine how it affects people over time. As a result, long-term health benefits or risks are unknown.
Intermittent fasting can have unpleasant side effects, like hunger, fatigue, insomnia, irritability, decreased concentration, nausea, constipation and headaches. Most side effects go away within a month.
Sticking with an intermittent fasting routine can be easier for some people rather than trying to watch calories every day. Other people, especially those with busy or variable schedules, have more difficulty maintaining an intermittent fasting routine.
Intermittent fasting is safe for many people, but it’s not for everyone. Skipping meals is not recommended for people under 18, those with a history of disordered eating, or those who are pregnant or breastfeeding. Athletes may find it difficult to fuel and refuel appropriately for an active lifestyle. If you have diabetes or other medical issues, it is important that you talk with your health care team before starting any type of fasting regimen.
Also, note that the key to achieving weight loss with intermittent fasting is not to overeat during your eating windows. As with any weight loss plan, eating fewer calories than you expend remains the basis for losing weight.
Also, keep in mind that shortening your eating window may make it difficult to get the necessary vitamins and minerals your body needs. While on an intermittent fasting diet, it is important to eat nutritional meals made from quality, healthy ingredients, such as fruits, vegetables, whole grains, low-fat dairy and lean protein.
Intermittent fasting can be dangerous if taken too far. A technique called dry fasting restricts food and fluid intake. This can result in severe dehydration and pose serious health concerns. Malnutrition can occur if the caloric restriction is too severe, such as averaging fewer than 1,200 calories per day long term.
I always recommend patients speak to their primary health care team prior to beginning any fasting routine to ensure there are no concerns related to other medical conditions or overall nutrition. — Romi Londre, R.D.N., Clinical Dietetics, Mayo Clinic Health System, La Crosse, Wisconsin