What is the impact of intermittent fasting on athletic performance? The answer is not simple

Intermittent fasting is gaining more and more popularity and is also gaining popularity among athletes.

It consists of depriving oneself of food for more or less prolonged periods. Outside of these times, there are no restrictions on the types and amount of food eaten. There are several variations of intermittent fasting including, but not limited to, alternate fasting (every other day), modified fasting (reducing calorie intake on two non-consecutive days a week) and time-restricted eating (for example , fasting from 6 p.m. to 10 a.m.).

What are the effects on athletic performance? What are the benefits, practicalities and risks to consider?

I am a dietitian-nutritionist, holder of a doctorate in nutrition from Laval University, and currently a postdoctoral fellow at the University of Quebec at Chicoutimi. This article was written in collaboration with Geneviève Masson, sports nutritionist, who advises high-level athletes in Canadian Sport Institute Pacific and teach at Langara College of Vancouver.

Read more: Is intermittent fasting for you? Here’s what the science says

Variable effects on athletic performance

During physical activity, the body mainly uses its stores of carbohydrates, called glycogen, as a source of energy. In a fasting situation, glycogen stores decrease rapidly and the body increases the use of lipids in order to meet its energy needs.

The practice of intermittent fasting has been associated a decrease in fat mass and the maintenance of lean mass in athletes. However, these changes do not always correspond to an improvement in sports performance, as evidenced by the contradictory results of several studies.

Several studies report aerobic capacity, measured by a VO test₂ max, unchanged after intermittent fasting in elite cyclists and runners of bottom and of middle distance well trained. At driven runnersthere was no effect on running time (10 km), level of perceived exertion and heart rate.

In addition, perceived fatigue and muscle pain were increased in trained cyclists during Ramadan, which may be partly due to dehydration since fluids are also restricted during this period, when nothing can be consumed from sunrise to sunset.

power sports

In the context of fasting, low glycogen (carbohydrate) stores can limit the performance of repeated intense efforts. Moreover, there was a decrease in the speed of repeated sprints in active adults after fasting for 14 hours a day for three consecutive days.

Power and anaerobic capacity, assessed using the Wingate test (stationary bicycle test) was reduced after ten days of intermittent fasting in active studentswhile it was increased after four weeks in another study also in active students.

strength training

Of the men and women following a weight training program had similar gains in muscle mass and strength when they practiced intermittent fasting compared to a control diet. There was no significant difference in muscle power between the active men who practiced, or not, intermittent fasting. However, one study reported an increase in muscle strength and endurance in young active adults after eight weeks of resistance training combined with intermittent fasting.

Thus, the results vary greatly from one study to another and are influenced by several factors, including the type of fast and its duration, the level of the athletes, the type of sport, etc. In addition, very few studies have been performed in women andno control group in most studies cannot isolate the effect of intermittent fasting.

For the moment, it is therefore not possible to rule on the effectiveness of intermittent fasting in improving parameters related to sports performance.

Eating before and after training

The athlete who wishes to resort to intermittent fasting must take into account several practical considerations. Is his training schedule compatible with this dietary approach? For example, does the period during which he is allowed to eat allow him to eat enough before physical exercise or to recover adequately after training?

And, not insignificant aspect, what about the food quality, knowing that a adequate protein intake is essential for the recovery and maintenance of lean mass in order to limit the negative impacts on performance.

Questioning the impacts — and the reasons — of fasting

Intermittent fasting could induce an energy deficit that is too great for some athletes with high energy needs, including endurance sports athletes (running, cycling, cross-country skiing, triathlon, etc.) due to the volume of training. They may then suffer from relative energy deficit in sportwhich is a syndrome that notably affects hormone secretion, immunity, sleep and protein synthesis. If the deficit is prolonged, adverse consequences on athletic performance ensue.

It is also essential to question what motivates the adoption of a dietary practice as strict as intermittent fasting. Some people do it for religious considerations like Ramadan. Others are motivated to adopt weight control behaviors in the hope of achieving an “ideal” body according to socio-cultural norms.

A recent study demonstrated that practicing intermittent fasting over the past 12 months was significantly associated with behaviors associated with eating disorders (overeating, compulsive exercise, vomiting, and laxative use). Although this study cannot determine whether it is fasting that causes eating disorders or if it is eating disorders that lead to fasting, it does highlight an associated risk of this practice.

Finally, we must also question the impact of intermittent fasting on social interactions. The fasting schedule might limit participation in social activities involving food. What is the risk of negatively influencing the eating behaviors of other family members, especially children or adolescents who see their parents refrain from eating and skip meals?

Is this a good idea or a false good idea?

With such contradictory scientific data, it is not possible at this time to comment on the effects of intermittent fasting on sports performance.

Further studies are needed before this practice can be recommended, especially for seasoned athletes. Moreover, the potential negative effects on other aspects of health, including eating behaviors and social interactions, are not negligible.