What is Intermittent Fasting?

Put “intermittent fasting” into a search engine and you’ll pull up millions of hits! Articles discuss using intermittent fasting for managing weight loss, heart disease, Type 2 diabetes and sleep problems. Some even suggest that intermittent fasting may slow the aging process. However, the science on intermittent fasting is still evolving.

Read on to find out more about the different types of intermittent fasting and current research on this topic.

Alternate-day Fasting

What is it?

Alternate-day fasting refers to rotating days of eating and days of fasting. On fasting days, no foods or beverages with calories are consumed. Calorie-free drinks, such as water, black coffee and tea are permitted. On non-fasting days, you can eat whatever you want — although following healthful eating guidelines is recommended. Theoretically, over the course of a week, you would have cut the total number of calories that you typically consume. And, over time, by eating fewer calories, the pounds would be expected to drop off.

Is it effective for weight loss?

Studies suggest that following an alternate-day fast is as effective as a typical low-calorie weight-loss plan. However, as the description reveals, this is a challenging fasting pattern, and side effects reported include strong feelings of hunger and decreased concentration on fasting days. Also, some studies revealed that people ate more than usual on non-fasting days. Studies on alternate-day fasting are somewhat limited and most have been over short periods of time. More research is needed on alternate-day fasting as an approach to weight loss.

Modified Fasting

What is it?

Modified fasting involves eating very little amounts of food on fasting days. Some modified fasts restrict intake on fasting days to 20% to 25% of needed calories. Others advocate limiting intake on fasting days to only 500 calories. Another version, referred to as the 5:2 fast, calls for fasting two days per week. Participants on this fast follow their usual eating patterns on the other five days of the week.

Is it effective for weight loss?

The results of modified fasting for weight loss are contradictory. Participants in some modified fasting weight loss studies lost more weight than those on traditional weight-loss diets, although the difference was not significant. Other studies found no difference in weight loss between modified fasting and a reduced calorie diet.  At this time, there is not enough research to know if this dietary pattern is a safe and effective way to lose weight.

Time-restricted Fasting

What is it?

Time-restricted fasting limits the intake of calories primarily to waking hours. The goal is to fast eight to 12 hours per day with the bulk of the fast occurring while you are sleeping. This is an easier fasting pattern for people who do not snack after dinner or eat breakfast immediately upon rising.

Is it effective for weight loss?

The jury is still out about the effectiveness of time-restricted fasting for weight loss. Some studies suggest it has a similar effect as alternate-day fasting. Most studies do not show a significant difference between time-restricted fasting and traditional calorie restriction on weight loss.

The Bottom Line

Intermittent fasting is not currently a recommended treatment for weight loss or any other health condition. There needs to be more research, especially regarding any negative side effects of fasting, such as the risk of nutrient deficiencies. Intermittent fasting may pose a health risk for certain individuals, so it is not recommended for people with diabetes, women who are pregnant or breastfeeding and individuals with a history of eating disorders or disordered eating. Anyone considering fasting should first discuss it with a doctor, especially if taking medications that may need to be adjusted.

For a personalized weight management plan that meets your individual needs, consult a registered dietitian nutritionist. An RDN can create a weight loss program based on your unique health and nutrition needs and goals. To find a dietitian in your area, search the Academy's Find a Nutrition Expert database.