Effects of low energy availability and high-impact exercise on physiological markers of bone and vascular health

Athletes often expend large amounts of energy during exercise and some, particularly endurance-based athletes, are under pressure to maintain a low body weight. This places many athletes at risk of low energy availability (LEA), which has been associated with impaired bone health, menstrual function, and vascular function in female athletes and exercisers. There is a clear gap in the literature regarding non-pharmaceutical therapeutic interventions to protect bone health in females at risk of LEA. A review of the existing literature (Chapter 4) identified structural bone decrements that are consistent across studies in female athletes with long-term LEA. It was also shown in Chapter 4 that high-impact exercise interventions can improve bone structure in an energy efficient manner, but there was no research regarding the effects of high-impact exercise during LEA in young healthy populations. This was explored in Chapters 5 and 7. Performing a brief high-impact jumping intervention twice daily mitigated the effects of three days of LEA on bone resorption, but not formation, in active, regularly menstruating young females (Chapter 7). Bone stress injury incidence was elevated in competitive female distance runners reporting <9 menses over the previous 12 months (indicative of LEA), but only in those that did not regularly participate in high-impact plyometric style training (Chapter 5).

The effects of LEA on vascular function have only been investigated in cross-sectional studies, which compared females with amenorrhoea (secondary to LEA) to healthy counterparts. The effects of LEA per se, independent of estrogen deficiency, cannot be elucidated using this model and it is unclear when vascular effects begin to manifest. It was shown in Chapter 8 that resting regional blood flow was supressed during three days of LEA, and blood flow recovery from peak to resting during reactive hyperaemia was slowed, in active, regularly menstruating young females. Participants were estrogen replete and estrogen concentrations remained stable during LEA.

In conclusion, findings from Chapters 4, 5 and 7 show that short and long-term LEA exerts negative effects on bone and should be avoided. If bouts of LEA are unavoidable, high-impact jumping exercise can help to mitigate some of the negative effects on bone and may minimise the risk of bone stress injury. However, controlled prospective research is needed to investigate the benefits of high-impact exercise during longer-term LEA. Vascular effects of LEA manifest far more quickly than previously known, and independent of estrogen deficiency. This may limit exercise performance and recovery in active females that regularly experience bouts of LEA and could increase the risk of cardiovascular health complications.