Beyond Bodyweight: In RED‑S but not underweight

Seeing signs of RED-S but not underweight?

When it comes to weight loss, the science seems simple: Eating more calories than we expend = weight gain Eating less calories than we expend = weight loss

But our bodies aren’t quite so simple. We’ve spent millions of years evolving into finely-tuned machines that know how to protect us from unwanted weight loss. Why? Because, back in the day when our ancestors faced food scarcity and predators, preserving energy was a matter of life or death. And, while this might not be a problem anymore, our bodies have adapted to help us maintain homeostasis regardless, via a few main mechanisms.

When faced with a prolonged calorie deficit, various hormones begin to slow down the metabolism to preserve energy for essential functions (e.g. movement - to out-run those lions). Since our metabolic rate (how many calories you burn at rest) represents roughly 60% of our total daily energy expenditure (depending on various factors e.g. body weight/composition, age, gender and genetics), slowing it down in an attempt to minimise energy expenditure serves to prevent weight loss. Often, various lifestyle factors come into play as well – even if they aren’t always obvious. While you can distinguish between escaping a lion’s lunch and the stress of an upcoming exam or a training session, your body can’t, and the stress response is the same. The result is an increase in hormones such as cortisol, adrenaline and insulin which, when produced in abundance, contribute to fat storage. As explained by this research, “Increasing levels of activity may bring diminishing returns in energy expenditure because of compensatory responses. People who compensate more may be more likely to accumulate body fat. Alternatively, the process might occur within individuals: as we get fatter, our body might compensate more strongly for the calories burned during activity, making losing fat progressively more difficult”. 

At first, the body’s reaction to these stressors may be subtle. Hormones and the metabolism take time to adapt, yet typically, the more extreme the energy deficit, the more significant the response. While every individual is different, research examining the effect of a calorie deficit on all body tissue has shown how individuals with more body fat tend to experience slower adaptations compared to those with less body fat, whose “adaptive thermogenesis” tends to be more extreme. Ultimately, if your body doesn’t deem itself to be in a safe environment (e.g. in a state of RED-S), it will do whatever it takes to preserve precious energy. 

As explained by this article on the topic of eating disorders, “Too often, we don’t worry about someone’s relationship with food and body unless they’re visibly emaciated, or they’ve lost a significant amount of weight in a short time”. This often leads to overlooking the crucial bottom line when it comes to RED-S: low energy availability can occur at any size. 

Take a deeper dive into the research below ↓

References: 

https://pubmed.ncbi.nlm.nih.gov/27136388/  

https://pubmed.ncbi.nlm.nih.gov/23404923/ 

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3673773/ 

https://pubmed.ncbi.nlm.nih.gov/34054716/