Last week, I touched on the negative impacts of RED-S on bone health (click here). This week, I’m going to highlight the impact of RED-S on metabolic rate. Resting metabolic rate (RMR) refers to the amount of calories the body is burning at complete rest. I often find that athletes forget that the body still requires a lot of calories, even if they were to lie in bed all day. This is because the body requires energy for essential functions that keep us alive, like breathing and blood circulation.
Athletes with RED-S often present with a suppressed RMR. This reduced RMR is why athletes can be eating in a calorie deficit but their weight remains stable. This is why the energy balance equation of “weight loss/weight gain = total calories consumed – total calories burned” is flawed. The number of calories consumed influences the number of calories burned. When too few calories are consumed, the body is able to reduce the number of calories it requires by reducing RMR.
Resting metabolic rate can be calculated using various equations that take into consideration things like body weight and age. The gold standard for calculating RMR is indirect calorimetry, which is done in a laboratory setting. Indirect calorimetry is able to determine the number of calories you burn by measuring your oxygen consumption and carbon dioxide production at rest after an overnight fast. The picture below is of me wearing a ventilated hood that would measure my oxygen consumption and carbon dioxide production to determine the number of calories I’m burning.
If the RMR measured from indirect calorimetry is lower than the RMR calculated through an equation this would indicate an athlete has a suppressed RMR. For instance, if indirect calorimetry measures an athlete’s RMR as 1200 calories, but the equation that used body weight and age indicates an athlete’s RMR should be 1500 calories, then this may indicate that RMR is suppressed. This may mean that the body is reducing the calories it requires in an attempt to conserve energy.
A downshift in metabolic rate is a sign that the body is not getting enough calories to keep it healthy as the body acts to conserve energy. Don’t fall for the lie that changes in body weight are as simple as reducing calories consumed and/or increasing the amount exercise. This is an oversimplified model that fails to consider the body’s ability to change its metabolic rate.