Low energy availability: the impact on an athlete’s bone health

Many practitioners and athletes are unaware that bone is more than a stagnant structure. It’s a metabolically active tissue that requires mechanical loading as a signal for bone formation, and optimal energy and nutrient intake to support bone health.

This topic was eloquently discussed in a recent narrative review, and I explore part of it in this blog article.

Bone is continuously broken down (bone resorption) and remade (bone formation) every day – this dynamic process is responsible for determining our bone mineral density, and needs to be balanced to stop our bone health becoming compromised.

The interesting thing is that bone mineral density changes across our lifecycle. During childhood and adolescence, we achieve higher bone accumulation. As we move from adulthood into older age, we actually lose more bone every year. There also seems to be a difference between sexes: men reach a higher peak bone mass, while women’s age-related bone loss is augmented, especially when levels of oestrogen decline in the post-menopausal phase, increasing the risk of developing osteopenia and/or osteoporosis.

With these physiological changes, and the fact that athletes have nutrient demands greater than their non-athletic counterparts, it’s important that athletes maximise and preserve their bone health. Athletes who participate in aesthetic or weight-restricted sports are often more prone to sacrificing their bone health to achieve optimal performance because of lower energy availabilities. There also seem to be sport-based differences: road cyclists and swimmers exhibit lower bone mineral densities than athletes participating in power-based sports due to the non-weight bearing aspect. Loading the bone is the main consideration for optimal bone health, so for athletes participating in non-weight bearing sports or those prone to lower energy availability, it becomes necessary to incorporate some weight-bearing training to stimulate the bone.

Nutrition also plays a vital role, and becomes more challenging for athletic populations. Calcium, protein, magnesium, phosphorus, vitamin D, potassium, and fluoride are all key to supporting bone formation, while an athlete’s diet should also provide optimal amounts of silicon, manganese, copper, boron, iron, zinc, vitamin A, vitamin K, vitamin C and B vitamins to support other aspects of bone metabolism. Note I use the word ‘optimal’ here. General dietary guidelines usually focus on preventing nutrient deficiencies – for athletes, we need to focus on optimal health for better performance.

While all these nutrients are important, energy availability is the number one consideration. Low energy availability puts an athlete at a greater risk of acute bone injuries (stress fractures), as evidenced by an altered bone metabolism – which makes sense since the body prioritises other essential bodily functions over bone health. It also has longer-term implications such as osteopenia and/or osteoporosis, which athletes rarely think about given that these conditions are mostly associated with elderly women.

Although guidelines suggest an energy availability of 45 kcal/kg of lean body mass/day to optimise bone health, some argue this is an unrealistic target for many athletes due to a mismatch between their energy intake and their energy expenditure from training.

This mismatch can occur voluntarily from planned caloric restriction or planned high-energy expenditures in training, which is often seen in Relative Energy Deficiency in Sport (RED-S), but some athletes also consider caloric restriction necessary to push for the endurance phenotype.

However, this mismatch can also occur involuntarily. Take elite ultra-endurance athletes (event duration of at least 6h) for example – they can induce an energy deficit of up to 7000 kcal per day from the gruelling energy they expend (2)! No wonder some athletes struggle to offset this deficit, especially since they also have limited time for refuelling. Is there a lower threshold (less than 45 kcal/kg lean body mass/day) that does not adversely affect bone formation? And can an athlete benefit from restricting calories to drive the endurance phenotype without sacrificing their bone health? These interesting questions have been raised in the narrative review.

Naturally, the lower the energy availability, the lower the intake of all the other essential nutrients for bone health mentioned above. The next question that arises is whether the lack of certain nutrients could have an additional adverse effect on bone health? For example, low iron availability has been shown to affect thyroid health and dysregulate the growth hormone/insulin-like factor-1 axis, which is crucial for bone formation (3) – a great example of how all our body systems are interlinked.

So, it is vital as practitioners, that we ensure our athletes are eating optimally to support their bone health from an energetic perspective. This, in itself, is challenging since estimating energy intake and energy expenditures has its limitations. But if we can apply these estimation methods, use trial and error, bring in our intuition, really observe and listen to our athletes, I believe we’ll be on the right path to ensure optimal bone health.

References:

  1. Sale C & Elliot-Sale KJ (2019). Nutrition and athlete bone health. Sports Med. [Epub ahead of print]
  2. Nikolaidis PT et al (2018). Nutrition in ultra-endurance: state of the art. Nutrients. 10(12):1995.
  3. Petkus DL et al (2017). The unexplored crossroads of the female athlete triad and iron deficiency: a narrative review. Sports Med. 47(9):1721-1737.