Does Intermittent Fasting Work for Weight Loss?


There’s been lots of discussion not just about you should eat, but you should eat. The concept of intermittent fasting has been a big debate in the world of nutrition with enough substantiating evidence to argue both sides. The latest study on intermittent fasting examined the timing of the eating window and whether it could help lower appetite and improve fat burning. Here’s a look at the latest research in intermittent fasting.

What’s Intermittent Fasting?

Intermittent fasting is not a diet of food restriction, but rather a dieting pattern. The theory behind intermittent fasting is based on taking advantage of metabolic differences that take place in the body during times of “fasting” versus times of “feasting.” In contrast to the readily available energy available during normal periods of eating (aka “feasting”), periods of “fasting” require the body to rely on fat stores for energy. It’s believed that the depletion of fat stores during “fasting” periods leads to weight loss.

Fasting takes place between time-restricted feeding windows. The two main types of intermittent fasting include time-restricted eating and the 5:2 format.

During time-restricted eating, you can eat within an 8-hour window of choice. For example, it could be from 8 a.m. to 4 p.m. or from 12 p.m. to 8 p.m. During the remaining 16 hours of the day no food is consumed, only no-calorie beverages (like water and unsweetened tea).

The 5:2 format is when two non-consecutive days of the week are spent “fasting,” with only 25% of daily recommended calories eaten during those two days. If the average U.S. adult consumes 2,000 calories per day, that would mean 500 calories would be consumed on the two “fasting” days. The remaining five days of the week, you could eat your regular diet. This type of intermittent fasting is seen in “The Fast Diet” by Dr. Michael Mosley

From a nutritional perspective, there are pros and cons to following this sort of diet.


  • No food is eliminated and all food groups are allowed.
  • During days when you’re fasting, the diet promotes high-fiber foods that will help keep you satiated.
  • Proper hydration is recommended.
  • Simple rules to follow. (You just need to know the time.)
  • Exercise is also promoted.


  • You could have a lack of energy, especially if you are following the 5:2 format and consuming only one-quarter of your daily recommended calories.
  • Symptoms such as lack of concentration, hunger, feeling cold and constipation have been reported.
  • The concept of healthy eating really focuses on creating healthy daily eating habits, which are not encouraged on this sort of eating plan.
  • Intermittent fasting is inappropriate for certain folks, including those who are prone to low blood sugar or are on beta-blockers or diabetic medication. It also shouldn’t be followed by pregnant or breastfeeding women, children and anyone with an eating disorder.

The Research

A 2011 study published in the International Journal of Obesity examined the effects of intermittent fasting on weight loss and metabolic disease. In this six- month randomized control study, 107 overweight and obese women either had a general calorie restriction (25% below their calorie needs) or followed a 5:2 form of intermittent fasting (where eating days were calorie controlled in order to maintain their current weight). At six months, the weight loss was comparable between the two groups.

Where significant differences appeared was with the percentage of body weight lost. Some 34% of the intermittent fasters lost 10% or more of their body weight in contrast to 22% of calorie restrictors. Although weight loss did occur in both groups, it’s tough to know if the weight loss occurred because of the time people were eating or because the total amount of calories consumed was less.

Animal studies also show weight loss and health benefits. A 2017 study published in Ageing Research Reviews found that rodents who were intermittent fasting successfully lost weight and had a reduction in symptoms for insulin resistance and cardiovascular disease. A 2016 study published in Metabolism found that rodents who followed the time-restricted form of intermittent fasting had improved insulin resistance and increased their use of fat for fuel.

Just this August, three separate studies on mice and on people in the journal Cell found inflammation decreased when they were put on variations of an intermittent fasting diet.

The Latest Promising Study

A July 2019 study published in Obesity examined the effects of early time restriction (eating only between 8 a.m. and 2 p.m.) on energy metabolism in 11 overweight adults. The adults ate for four days on the early time-restricted feeding schedule or a control schedule (eating from 8am to 8pm). After four days, the participants were swapped to the other group (a random, crossover design).

The same amounts and types of foods were consumed on both schedules. Each meal was composed of 50% carbohydrates, 35% fat and 15% protein. Researchers measured the metabolism of participants (in a respiratory chamber), appetite levels throughout the day and hunger hormones morning and night.

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Even though participants all ate the same things – one group just ate during a shorter period of time – only the early eaters lost weight. The results showed that compared to eating 12 hours in the day, the early time-restricted eating facilitated weight loss primarily by decreasing appetite. The study concluded that this early eating pattern may also increase fat being burned.

Taylor C. Wallace, principal and CEO of Think Healthy Group, Inc. and adjunct professor in the Department of Nutrition and Food Studies at George Mason University, weighed in. “The data suggest meal-timing can facilitate weight loss by suppressing appetite. Other data clearly show that consuming breakfast (particularly a high-protein breakfast) may suppress help keep one satiated throughout the day.”

This study gives better insight into why consuming breakfast is so important, he notes. But Wallace, who was not involved in the study, adds, “This is a very well-done study but should not be taken as definitive due to the small number of participants. Larger clinical studies are needed to confirm these effects.”

So Does When You Eat Matter More Than What You Eat?

The latest study shows that there may be some benefit to eating in the morning. As for intermittent fasting in general, it’s still not understood if the benefits seen in studies occur because you’re cutting calories or because you’re not eating for a period of time. As a registered dietitian, I always recommend eating within one hour of waking up and making sure that you consume a variety of foods that your body needs (like whole grains, lean protein, dairy, fruits and vegetables). I’m sure we’ll be seeing more research on this topic coming soon.

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