Fasting For Athletes: Dangerous or Good Idea?

You’ve probably heard all of these criticisms of intermittent fasting:

“You’ll lose muscle.”

“Your performance will suffer!”

“You’ll stress your adrenals!”

“It’s not practical.”

Until recently, it was hard to know exactly what the truth was due to a lack of research on fasting in active subjects.

Now, two studies have been published that test the effects of intermittent fasting in conjunction with a strength training program. We have real, raw data that shows what happens when you start skipping meals and limit your food intake to a shorter feeding period.

In case you’ve never heard about intermittent fasting before, let’s get definitions out of the way. There are two general approaches to fasting that have been tested in studies:

Alternate Day Fasting (ADF) is when you reduce your calorie intake, generally only eating one meal, on 1-3 days per week.

Intermittent Fasting (IF) is when you extend the overnight fast so that there is a shorter feeding period daily, ranging from 4 to 12 hours. IF is often called time-restricted feeding.

Why would anyone want to try fasting to begin with?

Fasting gained popularity due to health benefits observed in animal studies and as a solution to the obesity crisis. We know from animal studies that calorie restriction, which typically accompanies fasting, is associated with an increase in lifespan. Additionally, fasting allows for autophagy, which is basically a process of cellular cleanup in which damaged cells are repaired or eliminated.

In obese and overweight people, fasting can restore metabolic health, lower cholesterol and triglycerides, and result in greater fat loss than low-calorie dieting. For example, a review found that fasting results in a reduction of body fat ranging from 4 to 15 percent, with overweight subjects losing the most.

Which begs the question, is fasting appropriate for people who are not overweight? What happens to athletes who are training hard or lifting weights?

Researchers in Italy decided to find out, recruiting male bodybuilders who were steroid free. They were put into either a control group or an IF group that ate all their calories during an 8-hour eating window each day for 8 weeks.

Both groups were instructed to maintain their calorie intake. The IF participants were asked to eat between 1 and 9 pm (eating 3 meals at 1 pm, 4 pm, and 8 pm). They fasted from 9 pm to 1 pm the following day. The control group ate meals at 8 am, 1 pm, and 8 pm. No snacks were allowed for either group, but they did have 20 g of whey protein after each training session.

Workouts were performed 3 days a week, following a split format (chest, back, and legs) for 3 sets of 6-8 reps at 85-90 percent, and reps were performed to failure with 3 minutes rest between sets. All workouts were supervised and no additional exercise was performed.

Groups were matched for calories and macros. (The difference in calories was not considered statistically significant):

At baseline, the IF group ate 2,826 calories a day, of which 54 percent was carbohydrates, 24 percent was fat, and 22 percent was protein.

The control group ate 3007 calories a day, with 55 percent coming from carbs, 24 percent from fat, and 21 percent from protein.

By the end of the trial both groups had reduced calorie intake per day by about 90 calories. This wasn’t intentional and may have been a result of being in a study in which food intake was recorded.

Results showed the following

The IF group decreased body fat by 16.4 percent, losing 1.6 kg of fat. In contrast, the control group lost 2.8 percent body fat, decreasing by 0.3 kg. Both groups increased strength and muscle mass by similar amounts (about half a kg). Leg press strength increased significantly in both groups. Testosterone and IGF-1 decreased significantly in the fasting group with no change in the control group. Even so, this didn’t appear to have a negative effect because lean mass was maintained and strength increased with no difference between the two groups.

Insulin and blood glucose decreased significantly in the fasting group with substantial improvements in the HOMA insulin resistance test. Adiponectin, a hormone with anti-inflammatory effects, also increased significantly.

Resting energy expenditure was unchanged in either group, which is noteworthy because one concern when losing body fat is that metabolic rate typically drops, both because people are eating fewer calories and they are losing body weight. Researchers think two things prevented the drop in energy expenditure:

1. Subjects were training hard, triggering a small hypertrophic effect whereby they increased lean mass.
2. Calorie intake was maintained so that even though subjects spent a longer period than normal fasting each day, the body didn’t “feel” threatened by a lack of incoming energy, as appears to occur with the metabolic adaptation that coincides with low-calorie dieting.

Respiratory ratio dropped in the IF group to 0.81 with no change in the control group. A lower respiratory ratio indicates a greater degree of metabolic flexibility and easier ability to burn body fat for energy. Being metabolically flexible is a strong predictor that subjects will be able to maintain fat loss, whereas a higher respiratory ratio above 0.85 has been shown to be a predictor of future weight gain.

Overall, results in the IF group were envious

By constricting your eating window daily and training a very reasonable 3 days a week it’s possible to lose a significant amount of fat, improve metabolic health, and get stronger and more muscular.

The question is: Will everyone get similarly awesome results? A second new study sheds some light:

This study used active men who had not been training during the previous 3 months. They were put on an upper/lower body strength training program 3 days a week on which they ate normally. The other 4 days of the week, subjects were required to consume all of their calories during a 4-hour eating window. There was no limit on how much they could eat, only on when they could eat.

Results showed the following:

The IF group lost 0.5 kg of body fat, whereas the control group had a small increase in body fat of 0.4 kg. Lean mass stayed the same in the IF group, while the control group gained an impressive 2.3 kg of muscle. This difference suggests that the IF protocol may have resulted in a blunted muscle building response, possibly explained by a difference in protein intake.

Because the IF group limited their feeding window to only four hours on non-training days, they automatically reduced calorie intake by 650. On unrestricted days they averaged 2318 calories and on fasting days 1631 calories. Researchers were surprised that this drop in calories had no significant impact on body fat (the 0.5 kg loss was not considered statistically relevant).

The lower calorie intake also resulted in a reduced protein intake. Subjects who were fasting only consumed 1 g/kg of body weight of protein across all meals (fasting and non-fasting days), whereas the control group averaged 1.4 g/kg of protein. Current research indicates protein intake of 1.3-1.8 g/kg (or higher) is ideal for lean mass retention during fat loss.

In regards to performance changes in response to the training program, individuals who consumed more calories, carbs, and protein on fasting days had greater increases in maximal strength. This highlights the importance of nutrition in athletic adaptations and reinforces the wisdom to avoid low-calorie dieting.

Comparing the first 8-hour IF study with the second 4-hour protocol reveals some key take aways:

A properly designed strength training program makes everything better.

Based on the information provided in the two studies, it appears that the protocol used in the 8-hour IF study used higher intensity ranges. This, combined with the fact that those subjects had a larger feeding window and higher calorie intake may have resulted in greater improvements in body composition than was observed in the 4-hour IF study.

Getting sufficient protein is a top priority

When trying to produce body composition changes form fasting. On the higher end of the 1.3-1.8 g/kg of protein range is recommended.

Consuming protein around the training session is necessary

The best results likely coming from drinking a liquid protein shake containing at least 20 grams of protein after workouts. More research is needed but based on the current evidence, longer feeding windows in the 8- to 12-hour range are recommended in active individuals.

Combining heavy strength training with IF that includes a relatively high protein intake

This may produce significant reductions in body fat without cutting calories. This may be an effective alternative to low calorie diets for countering obesity.

IF should be considered if your goal is to improve insulin sensitivity

...and blood sugar tolerance. It is also a viable choice for lowering inflammation.

More research is needed but based on current evidence, the decrease in testosterone and IGF-1 that occurs with a reduced feeding window doesn’t have negative effects on athletic performance making it an option for athletes who need to improve metabolic health or reduce body fat.

The 4-hour IF study found that although the protocol was initially difficult, subjects adapted and it got easier as the study progressed. Some subjects reported that fasting was only difficult when multiple fasting days in a row were utilized, making it reasonable to cycle between fasting and non-fasting days, especially in the initial stages of metabolic adaptation.

There was a wide range of individual responses to the protocols

This indicates that it’s important to experiment. If you find an IF protocol isn’t working for your, either due to low energy, preoccupation with food, insomnia, or anxiety, try extending your eating window. Alternate day fasting may also be a better option.

In the mainstream, low-calorie dieting and aerobic cardio are recommended for fat loss. The 8-hour IF study shows that significant fat loss can occur without either if heavy strength training is combined with an 8-hour eating window.