Does intermittent fasting work?

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While intermittent fasting (IF) may be a popular trend in the diet world these days, those trying to lose weight or improve their overall health should know that it can be a hard plan to stick to. 

The approach alternates between periods of fasting and non-fasting during a particular time period. Intermittent fasting is not about deprivation, but about splitting up your calories differently than the three-square-meals a day plus a snack routine. 

The reason IF is thought to be effective in weight loss is because it increases your body’s responsiveness to insulin. Insulin, a hormone that is released when you eat food, causes your liver, muscle and fat cells to store glucose. In a fasting state, blood glucose levels drop, which leads to a decrease in insulin production, signaling your body to start burning stored energy (carbohydrates). After 12 hours of fasting, your body runs out of stored energy and begins burning stored fat.

Two of the more well-known IF plans

  • 5:2 diet: Eat normally, but healthfully for five days of the week and consume only 500 to 800 calories on two nonconsecutive days. Women usually aim for closer to 500 and men around 800 calories. Some might opt to split their calories into two or three mini meals. 

  • Alternate days: Eat normally one day, then fast (eat fewer than 600 calories) the next, and repeat for the rest of the week.

When I have a client who is interested in intermittent fasting, I usually try to get them to do a fast overnight which is natural for our bodies. A 12- to 14-hour fast (7 p.m. to 7-9 a.m. for example) is a great place to start. Most people will wake up very hungry after a fast like this. I would propose that you eat a lot of healthy fruits and vegetables as well as legumes, whole grains and lean protein and dairy (if tolerated) so that you are getting in many nutrient-rich foods and at the same time, fueling your body during the day when you need energy the most.  

This eating pattern really works for many people for optimal health and weight management (and is nothing new, as dietitians have been recommending it for years).

The good and the bad of intermittent fasting

There are small research studies that show intermittent fasting can stress the immune system slightly and, as a result, can actually strengthen it. Potential benefits may include weight loss, improved mental state, improved fat-burning, increased growth hormone production, lowered blood cholesterol and a reduction of inflammation. Most studies though have been done in animals and may not translate the same way in humans. 

Almost anyone can fast for 10-12 hours overnight (dinner at 7 p.m. and breakfast at 7 a.m. the next morning, for example). A benefit here is less time eating late at night, which is typically better for our waistlines. 

Fasting during the daytime is much harder and may cause harm if low blood sugar makes you feel drowsy or just unpleasantly irritable. Some may experience headaches. Fasting for long periods of time, of course, can make it difficult to get enough nutrients in and over time, can slow your metabolism down.  

Another downside is some people may wind up binging on their non-fasting days, which defeats the purpose of following such a plan.

Good candidates for IF

Before starting a fasting plan, you should check with your primary health care provider, especially if you have any existing health conditions. Some people shouldn’t fast for prolonged periods of time, including pregnant women and those with certain health conditions like diabetes.

Intermittent fasting and lasting weight loss results

A 2017 statement by the American Heart Association says there’s evidence that both alternate-day fasting and periodic fasting may be effective for short-term weight loss, but there isn’t enough evidence to determine whether it’s effective long term.

Liz Weinandy, is a registered dietitian for Nutrition Services at The Ohio State University Wexner Medical Center.