Does intermittent fasting work?

Top view of a plate with vegetables and a glass of water

IN THE DIET WORLD THESE DAYS, intermittent fasting (IF) may be a popular trend, but 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 isn’t about deprivation, but about splitting up your calories differently than the three-square-meals a day plus a snack routine.

The reason intermittent fasting 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.

Three of the more well-known IF plans are:

  • 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.
  • Time restricted fasting: fasting every day for 12 hours or longer.

When I have a client who’s interested in intermittent fasting, I usually advise 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’re 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.

Most recently, a study published in the New England Journal of Medicine showed no significant difference in weight loss between groups fasting and those following a calorie-controlled diet. The study looked at Chinese adults following a fasting routine or just eating fewer calories — 1,200 to 1,500 a day for women, and 1,500 to 1,800 a day for men. Both the fasting and calorie-controlled groups lost an average of 14-18 pounds, with neither group having an advantage over the other. There was also no difference in blood sugar levels, insulin sensitivity, cholesterol or blood pressure between the two groups.

What does this study show? The far-reaching benefits of fasting may not be superior to reducing overall calories. Perhaps some people have an easier time eating fewer total calories if they restrict the total time they eat, though. Likely, this fasting approach works well for some people and not so well for others.

If you want to experiment with fasting, try to go into it slowly. 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 and over time, can slow your metabolism.

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

Who is a good candidate for intermittent fasting?

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, children, those with eating disorders or a history of them, 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. More research is needed to be able to point people in the right direction.

Remember: Many different weight loss methods can work for any number of people. Before you start any plan though, think about how sustainable it is and if it is healthy. Don’t get so caught up in fasting that quality of diet is an afterthought.

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