Intermittent fasting is an umbrella term for a variety of eating patterns that alternate between fasting and feasting. As a tool for both managing weight and improving health markers, intermittent fasting has become a very popular diet plan among adults in the U.S.
The intermittent fasting diet is also known as intermittent energy restriction. This approach to eating refers to food consumption schedules that cycle between periods of restricted energy intake (fasting) and periods of unrestricted energy intake (feasting). In this context, “fasting” generally means the consumption of approximately 500 calories or fewer per day, or approximately 20% to 25% of your usual energy intake. However, it might range from 0% to 40% of your usual energy intake.1 “Feasting” refers to eating as you normally would. This approach is gaining status as a weight-loss strategy alternative to traditional diets based on daily energy restriction.2
By the Day or By the Hour
Intermittent fasting is about when you eat, not what you eat. It does not involve calorie monitoring — just clock or calendar watching. It can easily fit into daily routines, with minimal disruption to family or social life. Intermittent fasting is easy for many people to adhere to while intrinsically cutting calories. Examples of intermittent fasting patterns include:
Human studies on the health benefits of intermittent fasting/time-restricted diets are limited. Yet, this pattern of eating appears to promote weight loss, ease weight management and improve metabolic health.
Intermittent fasting is as effective for weight loss as standard weight loss diets.4 Recent studies have demonstrated that among healthy adults, alternate day fasting for a 22-day period resulted in significant weight loss, reduced fat mass and a decrease in fasting insulin levels.5 Similar results were found among obese adults who followed an eight-hour window intermittent fasting pattern for 12 weeks5 and among women following the 5:2 (fasting two days per week) intermittent fasting pattern for 26 weeks.4 On average, the time-restricted eating pattern reduced energy consumption by 300 to 600 calories per day. Weight loss occurs because individuals do not fully compensate for this calorie deficit during their “feast” periods.1
Intermittent fasting improves glucose control, blood pressure and lipid profiles. Recent studies have demonstrated that adults with prediabetes or Type 2 diabetes who followed a 4:3 (fasting three days per week) intermittent fasting pattern were able to reverse their insulin resistance.4 Similarly, overweight, prediabetic men who followed a six-hour window intermittent fasting pattern for five weeks experienced improvements in measures of insulin, oxidative stress and appetite.6 Adults who followed a 12-hour window intermittent fasting period for six weeks demonstrated a significant improvement in their lipid profiles7, while obese adults who followed an eight-hour window intermittent fasting pattern for 12 weeks experienced reduced systolic blood pressure and body weight.6 And when compared to women who followed a traditional calorie-controlled diet over a six-month period, women who followed a 5:2 intermittent fasting pattern experienced similar weight loss but had greater improvements in insulin sensitivity and waist circumference.3
These improvements in metabolic health appear within two to four weeks of intermittent fasting and tend to regress over the same time period after the intermittent fasting pattern is discontinued.
The physiological mechanism through which health seems to improve with intermittent fasting is not fully understood. Yet, these (and other) studies suggest that intermittent fasting eating patterns are effective for weight loss and metabolic health in most adults, and that these health benefits are greater than can be attributed to caloric reduction alone.4
Prior to adapting any of the intermittent fasting eating patterns, there are a few points to consider.
However, this normalizes within two to three weeks.
Is It Right for You?
Intermittent fasting is one of many diet approaches. If you choose to give it a try, you can ease into an intermittent fasting pattern by starting with one of these small changes:
Intermittent fasting appears to be a good approach to weight management and personal health promotion — but only if it’s right for you!
Elizabeth Gollub, Ph.D., M.P.H., R.D., Nutrition Specialist, School of Nutrition and Food Sciences
Daniela Quan, Graduate Assistant, School of Nutrition & Food Sciences