Eating too little may impair muscle building among trained female athletes

Diet and lifestyle 12. okt 2023 3 min PhD student Mikkel Oxfeldt Written by Kristian Sjøgren

New research shows that female athletes experience negative effects if they restrict their diet when they train intensively. A researcher says that both females who exercise casually and elite female athletes will benefit more from exercise if their energy intake matches their energy expenditure – rather than keeping an eye on the bathroom scale.

Elite sports have always strongly focused on weight, with coaches often trying to get athletes to shed a few kilos to be lean for competition.

The effort to lose weight often requires reducing energy intake, but according to a new study, this can adversely affect training results.

If the benefit from the training declines, whether the athlete loses or gains a kilo or two may not matter.

“We want to dispel the notion that we should always lose weight. Our results suggest that both casually active females and elite female athletes will benefit more from consuming sufficient energy in relation to their training and getting the full benefit rather than compromising the body’s response,” explains researcher and project leader Mikkel Oxfeldt, PhD student, Department of Public Health, Aarhus University.

The research has been published in the Journal of Physiology.

Difficult to balance energy intake and expenditure

In sports, it has been well known for many years that elite female athletes can experience irregular menstruation, eating disorders and low bone mass, all of which can be linked to exercise and very strict diets.

This can have major effects in both the short and long term.

The researchers wanted to investigate how the imbalance between energy intake and energy expenditure affects female athletes.

Everyone has a basic daily energy requirement to keep their body functioning. Training adds to this and thus increases the daily energy requirement.

Exercising a lot but not increasing dietary energy intake produces an energy deficit, and the study focuses on what this does to the body.

“If you exercise a lot, keeping track of your energy intake and whether this matches the requirement can be difficult. Some athletes consciously choose to reduce energy intake themselves, whereas others are pressured to do so. A third group does this unconsciously, because they may increase their training volume over time without changing their energy intake,” says Mikkel Oxfeldt.

30 females in a well-controlled study

The researchers recruited 30 females 18–30 years old.

The females were healthy and fit and performed regular exercise at least four times per week. They did not use hormonal contraception and were excluded if they had an eating disorder or irregular menstrual cycles.

On day 1, immediately after their menstrual cycle started, the researchers collected blood samples and took muscle biopsies from the non-dominant leg. The females also consumed doubly labelled water enriched with a trace material, which enabled the researchers to measure the build-up of new muscle by determining how many trace elements were incorporated into the females’ muscle tissue over time.

The females then trained for 5 days according to a standardised, supervised combined resistance and cardiovascular exercise training programme. They were also assigned a diet that exactly matched their daily energy needs based on the weight of their muscles and organs.

After the 5 days, the researchers took another blood sample and muscle biopsy, and the females were divided into two groups. One group continued to eat the amount of food that matched their energy expenditure, and the other halved their energy intake .

The females continued the exercise programme for another 10 days with the differentiated energy intake.

After the 10 days, the researchers again took blood samples and muscle biopsies.

Low energy intake impairs muscle building

The results show very clearly that physically active females who do not match their energy intake to their requirements do not derive full benefit from their training.

By comparing the quantities of trace elements integrated into the muscles of the females during the experiment, the researchers found that females who balanced energy intake and expenditure during the training period had better muscle building.

Conversely, the females with reduced energy intake developed significantly less muscle mass during the 10-day period compared with the 5-day period when their food intake was not reduced.

The results applied both to the part of the muscles called the contractile tissue (muscle that performs the work) and the sarcoplasmic fraction, the part of the muscles including enzymes, cell nuclei and mitochondria.

“Intuitively, it makes good sense that you get less benefit from exercise when you do not eat enough. However, short-term studies have found that both strength training and a higher relative protein intake protect the muscles when you consume insufficient energy. Our study shows that, despite carrying out strength training and having a high-protein diet, measurement methods of the highest quality show that trained females are not protected against these effects,” explains Mikkel Oxfeldt.

Body turns on energy-saving mode

The researchers also found that the group of females with low energy intake had much lower concentrations of metabolic hormones, which indicates that the body is starting to scale back energy-intensive processes.

“The body is smart and knows that it cannot continue to burn off energy forever. It therefore settles into a state similar to the battery-saving mode on a mobile phone, burning less energy, which also affects muscle building after exercise,” says Mikkel Oxfeldt.

He elaborates that, although reducing energy intake by 50% may seem extreme, this is not unusual for people who exercise a lot. Not increasing your energy intake produces a deficit, and the kilos lost may seem to be a benefit but also may result in less optimal training outcomes.

“It is important not to compromise the response from training, and we clearly found that insufficient energy intake reduces the yield of training. The body will probably react more positively if you continue to train strenuously but consume energy that matches expenditure,” concludes Mikkel Oxfeldt.

Low energy availability reduces myofibrillar and sarcoplasmic muscle protein synthesis in trained females” has been published in the Journal of Physiology. The study received funding through the Novo Nordisk Foundation awarding a grant of DKK 100 million to Team Danmark with the aim of strengthening Denmark’s competitiveness within international elite sports and of creating new knowledge on physical activity that can also benefit the general public. Several authors are employed at Steno Diabetes Center Aarhus, Aarhus University Hospital, which also receives funding from the Novo Nordisk Foundation.

Mikkel Oxfeldt is a PhD student at Aarhus University, Department of Public Health, investigating the impact of endogenous and exogenous female sex hor...

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