Intermittent fasting alone does not boost weight loss and could cause loss of muscle, according to a new study

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  • Fasting for 16 hours a day may not make a significant difference for weight loss or other health measures such as blood sugar control, according to a new study. 
  • Researchers found no difference in weight loss or metabolic health between people who fasted and a control group during a 12 week study, when both group consumed roughly the same amount of calories. 
  • That suggests fasting may work only if it helps people cut calories.
  • The study also found fasting could lead to a significant loss of muscle mass, leading to weakness and risk of weight regain. 
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Intermittent fasting — or, only eating within a specific window of time — is an increasingly popular way to lose weight, control blood sugar, and even potentially reduce the risk of illness. 

But fasting alone may not confer any particular benefits compared to normal eating patterns, according to a study published today in JAMA Internal Medicine.

Researchers led by Dr. Ethan Weiss, cardiologist at the University of California, San Francisco, looked at 116 adults randomly assigned to either a fasting group or a control group over a 12 week period. The fasting group was instructed to limit all calorie intake to between 12 pm and 8 pm each day, while the control group ate three meals a day and were allowed snacks. 

By the end of the 12-week study, there were no significant differences between the two groups, researchers found, either in terms of weight loss or other markers of metabolic health such as fat mass, cholesterol, or blood sugar control. 

Both groups had lost a small amount of weight and while the fasting group lost slightly more weight on average, the difference was not enough to be statistically significant, the study found. The fasting group also lost slightly more lean mass (in the form of muscle, not fat) than the control group. 

Other research has found fasting boosts weight loss, but the key could be calorie reduction

The results of this study are in contrast with previous research findings that fasting can boost weight loss. However, a key difference in this study was that participants didn't have to adhere to any calorie restrictions, exercise goals, or specific macronutrient intake (of carbs, fat, or protein).

While participants didn't record their daily calorie intake, researchers estimated that both groups consumed roughly the same amount of daily calories throughout the study. 

Previous research that links fasting to better weight loss also found that participants cut as many as 500 calories a day, which could explain the difference. 

That suggests that fasting could have benefits for weight loss if it helps people reduce their calorie intake without tracking or restricting certain foods. The results of this study, however, suggest that fasting by itself, without calorie reductions, does not have any particular benefits for weight loss. 

Longer fasts could lead to metabolic benefits, but also muscle loss

Researchers also theorized that the 8-hour fasting window was too long to see some of the metabolic benefits, such as blood sugar control, in other studies. A small study from January, for instance, found that a 6-hour eating window was linked to weight loss and cardiovascular benefits. 

However, there could be a tradeoff to fasting — loss of muscle mass, which over time is linked weight regain as well as side effects such as weakness. That may be because in addition to typically lowering calorie intake, fasting is often linked to lower protein consumption, unless participants are given a specific daily goal for protein consumption. 

This study found fasting participants loss slightly more lean mass than the control group, and in fact 65% of their total weight loss was muscle mass. That's significantly more than in a typical calorie-restricted diet, in which muscle mass accounts for 20-30% of weight loss. 

That suggests fasting may not be worth the downsides of hunger and muscle loss, according to some experts, but more research is needed to better understand how fasting affects lean muscle mass. 

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