Is intermittent fasting safe for me?

Most people can benefit from practicing moderate fasting (shorter than 24 hours). However, there are safety concerns for pregnant women, children, individuals with type 1 diabetes and individuals who are malnourished or not getting proper nutrition. These individuals are advised not to practice intermittent fasting and should talk to their doctor before contemplating doing so in the future.

There are also concerns for young children, although not eating after dinner and fasting overnight for 8–12 hours is probably not a concern for young adolescents. Preliminary data suggest that alternate day fasting (with 500 calories or less on fast days) may be safe and effective for obese young adolescents. However, Dr. Krista Varady recommends against IF for children under 12 years of age.

Is there a best time (of day) to fast?

Based on current research in humans, there is limited consensus around optimal meal timing for people who practice intermittent fasting. We generally become more insulin resistant as the day progresses, or less able to clear blood glucose and get it to where it needs to go in our bodies. For this reason, it’s probably best to consume most of your calories earlier in the day and to start your fasting window several hours before you go to sleep.

“Your body can’t deal with nutrients (glucose) as efficiently later in the day, so you might as well give your body a break from glucose then,” Dr. Krista Varady says. “Early in the day, your body is primed and ready to deal with an influx of nutrients.”

If you are practicing an alternate day fasting regimen in which you eat under 500 calories on your fast days, you can either eat these calories in a single meal or divide them into three small meals throughout the day. Allowing for flexible meal timing on these fast days may help you maintain your fasting practice long term.

Is there a best way to break my fast?

Scientific research studies have not yet fully evaluated the efficacy of different types of post-fast meals on metabolic health in humans.

A handful of existing studies suggest that some people may experience an acute blood sugar spike, associated with insulin resistance, following a post-fast meal high in carbohydrates. However, this acute postprandial insulin resistance may be more likely to occur in individuals not accustomed to prolonged periods of fasting (16–24 hours). If you are new to fasting, you may want to break your overnight or longer fasts with meals low in glycemic index and high in fiber and plant fats (olives, seeds, nuts, coconut, avocado, etc.) After practicing intermittent fasting for several months, your body will likely experience different metabolic reactions to an influx of nutrients post-fast. IF generally leads to improved insulin sensitivity and lower blood glucose levels over time.

What should I eat when I’m not fasting?

Fasting has metabolic health benefits independent of what you are eating during your feeding periods. However, fasting will not turn a junk food diet into a good one.

In a study of people practicing IF and eating either a regular (25% fat) or high-fat diet (45% fat), Dr. Krista Varady found no significant differences in metabolic health between these two treatment groups over time. Each group displayed similar decreases in weight and coronary heart disease risk factors including LDL cholesterol and triglyceride levels.

Varady is currently exploring whether there are any links between fasting, low-carb diets and improved metabolic health. Many people who practice IF pair their fasting schedule with a ketogenic or very low-carb diet. However, there is little scientific research from human trials to determine any benefits or issues of pairing these interventions. The problem with ketogenic dieting is that it often leads people to eat an overabundance of unhealthy animal fats as opposed to healthier plant fats.