What’s better for fat loss, Time Restricted Feeding or Flexible Dieting?

Study Reviewed:

The Effects of a Macronutrient-Based Diet and Time-Restricted Feeding (16:8) on Body Composition in Physically Active Individuals —

A 14-Week Randomized Controlled Trial

Time restricted feeding (also referred to as Intermittent Fasting) has become a very popular method of fat loss in the fitness and health industry. As its name suggests time restricted eating involves taking in no calories for a certain period of the day and being allowed another window of time each day to eat. And it seems it has become popular for good reason, it works!

Of course, there's nothing magical about time restricted feeding and fat loss. Any form of dieting that reduces caloric intake (whether it be from time restricted feeding or from any other dieting strategy) will cause fat loss. There tend to be no restrictions to what you are allowed to eat on a time restricted feeding method. Simply a time limit. This is what makes time restricted feeding so appealing to so many.

There are many different types of time restricted feeding. The most common is the 16:8 method, in which you have 16 hours of the day that you are not allowed to eat and an 8-hour window to eat each day. OMAD or One Meal A Day, which typically consists of 22-23 hours of no intake and 1-2 hours to eat. Alternate Day or the 5:2 method, in which 5 days a week you would eat and 2 days you would eat nothing at all. These days would be nonsequential. There are others but those are the more popular methods.

The study watched men and women between the ages of 20 and 40 years old, with a BMI average of 26. All the participants were physically active prior to the study. Engaging in 2 resistance training days and 2 cardio days per week. Study length was 16 weeks. Participants were divided into two groups, Flexible Dieting and Time Restricted Feeding.

During the first 2 weeks (referred to as the familiarization phase) the subjects learned how to document their food intake into a food database diary. Even though the subjects were not specifically instructed to decrease their calories during the initial 2 weeks of the study, it was quite evident that they did as both groups lost body weight during this time.

The following 8 weeks they were guided by a nutrition coach. The nutrition coaches provided instructions to the subjects in each diet group and were available to answer any questions that the subjects had. The last 6 weeks there was no instruction or guidance from the nutrition coach.

The flexible dieting group was given instruction to eat 80% “unprocessed foods” and 20% could come from “processed food”. Think of “unprocessed” foods as the types of food that most people would classify as healthy (whole foods, fruits, vegetables, and foods with higher amounts of micronutrients); and the “processed” foods as the types of foods that most people would classify as unhealthy (high sugar/high fat snacks, foods with little to no micronutrients present).

The subjects in the TRF group followed a 16:8 method of time restricted feeding (subjects were instructed to fast for 16 hours each day, and only consume food during the 8-hour meal period. During the 16-hour fasting period, only calorie-free drinks (coffee, tea, zero-calorie soft drinks) and water was permitted.

The subjects were allowed to eat between the hours of noon and 8pm each day. They were allowed to eat ad libitum (as much or as often as necessary or desired) during the 8-hour meal period. During the 8-hour eating window, there was no specific number of meals to be eaten or calories to consume. The investigators did not give the subjects in this group specific protein intake recommendations. Rather, the researchers gave the subjects a general nutrient distribution of carbohydrates (to be comprised of about 40-65% carbohydrates, 20-35% protein, and 20-35% from fats).

The subjects engaged in resistance training two days per week and aerobic-based exercise two days per week. The researchers measured changes in muscle mass and body fat mass at the beginning of the study, after the 8-week diet with coaching support, and at the end of the study (the last 6 weeks) when there was no coaching support offered and no specific diet instructions.

What were the results?

As we look at the results, remember that the only differences between the two groups in this study was the diets that they were placed on. The Flexible Dieting group was told to reduce their calories by 500 kcals per day, to get at least 1.4 grams of protein/kg body weight, and to get most of their calories from unprocessed foods. The Time Restricted group was not told to reduce their calories per se, but to limit the hours in the day when they were allowed to consume food. In this way, by limiting the eating window, the authors assumed that the subjects would naturally enter into a caloric deficit during the 8-week diet phase.

Average intake for the flexible dieting group was 1,736 kcal with 1.4g/kg protein, 2.7g/kg carbohydrates, 0.7g/kg fats. And 1,801 kcal with 1.3g/kg protein, 2.6g/kg carbohydrates, .8g/kg fats.

Both the FD and TRF groups lost a significant amount of bodyweight, fat mass, and body fat percentage during the first 10 weeks of the study. There were no significant differences between the two groups for any of these measures. There were no significant differences over time in the loss of muscle mass for either group. Both groups lost a little bit of muscle mass, but not enough to reach the level of statistical significance.

Total Weight Lost:

Flexible Dieting: Bodyweight= -8.8lbs, Bodyweight percentage= -5.4%, Fat mass= -6.4lbs, Body fat Percent= -2.5%

Time Restricted Eating: Bodyweight= -8.4lbs, Bodyweight percent= -4.8%, Fat mass= -7.5lbs, Body fat percent= -3.5%

What happened during the last 6 weeks of the study intervention when the subjects received no specific dieting instructions or coaching support?

Nothing. During the last 6 weeks of the study, the subjects maintained their bodyweight and body fat losses (they did not gain back any of the body weight or body fat). Also, their muscle mass levels were maintained (did not gain or lose additional muscle mass) during the last 6 weeks of the study.

So, which one is going to be better for your fat loss goals?

My opinion is that one diet is not superior to the other in terms of FAT LOSS (I do have some issues with time restricted feeding if one's goal is to maximize muscle mass). 

My approach would be to make sure that regardless of the diet method chosen (time restricted feeding or flexible dieting), that protein intake would be relatively high for both options and whichever method a client is better able to follow and adhere to would be the diet that I would recommend for them.