DEAR MAYO CLINIC: Is intermittent fasting for weight loss safe? What are the risks of short-term diets that are very low-calorie?
ANSWER: Recent research has found that using intermittent fasting for weight loss may have some benefits in the short term. But at this point, the long-term effects of this type of diet or other diets that severely restrict calories are not known. To lose weight and keep it off, the best strategy is to adopt healthy eating and exercise habits that you enjoy so you can stick with them over time.
Intermittent fasting currently is a popular trend in dieting. There are several fasting methods people employ for weight loss. Some dieters decrease the amount of time they eat each day to, for example, only six hours in a 24-hour period. Others fast every other day. Another approach is called the 5:2 diet, which involves gradually decreasing daily calories to the point that you're only consuming 500 calories a day for two days each week.
A recently published article assessed a variety of research on intermittent fasting. It found that there are some benefits, at least in the short term, to fasting as opposed to just decreasing calories overall.
It appears that fasting for a short time can produce ketosis — a process that occurs when the body doesn't have enough sugar for energy, so it breaks down stored fat instead, causing an increase in substances called ketones. Fasting also affects metabolic processes in the body. These processes trigger a number of responses, including decreased inflammation, improved blood sugar regulation and better response to physical stress. The research shows intermittent fasting could have other health benefits, as well, but more study is needed.
It is crucial to note, however, that little long-term research has been done on intermittent fasting to examine how it affects people over time. So at this point, it's unclear if there are any long-term health benefits or risks related to this diet technique.
We do know that there are risks involved with certain types of intermittent fasting. For example, a technique called dry fasting that includes restricting fluid intake as well as food intake is dangerous because it can cause severe dehydration and pose serious health concerns. And if you take caloric restriction too far, that can lead to malnutrition.
As you consider weight loss and diets, keep in mind that no one approach works for everyone. But there are some basic principles that you should follow as you decide how to best achieve and maintain a healthy weight. At its core, your diet should support your health overall. There are a variety of diets that can do that, including the Mediterranean diet; the vegetarian diet; the Dietary Approaches to Stop Hypertension, or DASH diet; and the Mayo Clinic diet. All of these diets have similarities that are greater than their differences. They are based on real food and focus mainly on plant products, such as fruits, vegetables, beans, nuts, whole grains, and healthy fats like olive oil.
In addition, as you incorporate a new diet into your life, it needs to be practical, so that you can sustain it over time. If it is drudgery, or if you feel like you're suffering, it won't work in the long run. Eventually, you're likely to slide back into old habits. If you're considering a new approach to your diet — especially if it's something that could be risky if you don't do it correctly, like intermittent fasting — talk to your health care provider first for guidance on how you can manage it in a healthy way. Keep in mind, too, that choosing to adopt healthy lifestyle choices can and should be an enjoyable way to live. — Dr. Donald Hensrud, General Internal Medicine and Endocrinology, Mayo Clinic, Rochester, Minnesota