Is Fasting While Doing Sports a Good Idea?

22nd November 2019
Working out without eating can either compromise or improve your activity performance. What does it depend on?

Doing sports after fasting is a subject that’s gained a lot of popularity in the last few years. It’s more proof that there is a multitude of trends within the fitness world. Is it a beneficial habit? Is it dangerous to fast and practice sports at the same time?

Fasting before doing sports: the basics

Intermittent fasting is also known as ‘sporadic fasting’, ‘flexible fasting’, ‘fasting’, ’16/8 diet’, or ‘5:2 method’. It’s a nutritional protocol that a lot of athletes follow. It’s not just another diet similar to keto, which is also very trendy right now.

Basically, it consists of distributing your food intake in a particular way throughout the day, independently of the diet that you’re following. This system limits the hours of the day in which you can eat.

The period in which you can eat is known as a ‘feeding window‘. Depending on how restrictive you want to be, this feeding window can go on for various amounts of time.

There are several possibilities, such as fasting for 16 hours and eating for eight; fasting for 20 hours and eating for four, or even not eating for 24 hours and then eating freely during the next 24-hours period. Keep in mind that the goal of this strategy is not compensating for excesses; it’s not a re-feed.

Shouldn’t I eat every few hours?

There’s a generalized belief that fasting periods -even short ones- can cause your metabolism to slow down. This affirmation isn’t right. As a matter of fact, short periods of fasting can actually boost your metabolism.

The illustration of a clock printed on a plate to simbolize intermittent fasting

The benefits of fasting

It’s very important to clarify that most of the results we’re about to explain are mostly observed in animal studies. As for their application in humans, we need more data to start talking about real benefits.

In the first place, it could lower the risk of cardiovascular diseases, diabetes, chronic illnesses, blood pressure, and triglycerides.

In relation to diabetes, some people believe that fasting can increase insulin sensibility. This is a very important point for people with type 2 diabetes and those with elevated blood sugar levels.

However, you can also achieve this improvement with a hypocaloric diet and physical exercise, which is why the effect of intermittent fasting is quite small.

Likewise, fasting would also help to increase the production of the human growth hormone. But this increase is irrelevant, as it doesn’t provide any additional benefits.

Other possible effects are:

  • It can promote greater oxidation of fats.
  • It can improve brain health.
  • Intermittent fasting may decrease the levels of inflammatory markers such as homocysteine and C-reactive protein.
  • It could promote autophagy.
  • A better sensation of appetite. However, there’s a great interpersonal variety within this point.
  • Increased life expectancy, just with other nutritional protocols, such as a simple caloric restriction.

The problems of fasting

The problem of fasting comes when we do it for extended periods of time. If the restriction period is short, the metabolic adaptations reverse as soon as we go back to a normal diet.

On the contrary, if the time period is longer, it can create complications in our body. If our body doesn’t get food continuously, it goes into ‘survival mode’. Our metabolism slows down and it starts to use the proteins in our muscles as an energy source.

A person in front of an empty plate to simbolize long periods of fasting before doing sports

Some of the adaptations that would occur if the fast is aggressive are:

  • Muscle loss.
  • Decrease of leptin levels and increase of ghrelin levels. These hormones regulate the feeling of satiety.
  • An increased sensibility to insulin in adipose cells, which entails a greater accumulation of fats.
  • Lower metabolic rate.
  • Testosterone decrease.
  • Lower activity levels in the thyroid gland.

Summary and latest studies

The first meta-analysis (a synthesis of several scientific studies about the subject) regarding intermittent fasting became available last February. The study concludes that, without there being too much evidence to make specific recommendations, people who are overweight or suffer from obesity can actually use this method to lose weight.

We can’t put aside the fact that this meta-analysis extracts only short term conclusions since there are no long term publications.

Therefore, while we can say that some effects on humans do apply to humans, we can’t consider this method as something revolutionary. The greatest advantage happens on a satiety level, and on a psychological level in definition and hypocaloric stages.

Eating more food in a single sitting can have very positive psychological effects. On top of that, forgetting about the five or six meals per day rule can be liberating for a lot of people. It can even help them to stay on track with their diets.

  • Harris L, Hamilton S et al. 2018. Intermittent fasting interventions for treatment of overweight and obesity in adults: a systematic review and meta-analysis. JBI database of systematic reviews and implementation reports, 16(2), 507-547
  • Davis CS, Clarke RE et al. 2016. Intermittent energy restriction and weight loss: a systematic review. European journal of clinical nutrition, 70(3), 292