Bone density is a lagging indicator of your energy budget

You finish a long run and the thought of eating feels like a chore. You grab a coffee, maybe a small protein bar, because the internet said so. You feel a virtuous sense of control. The numbers on your watch look great, your mileage is up, and yet you feel fragile, brittle. Later, you’re sitting at your desk, wrapped in a jumper even though it’s mild outside, and you can’t shake a deep, cellular cold. You get another stress fracture, your third in two years, and you blame your running shoes, the pavement, your genetics. You tell your doctor you’re ‘fit’ and they nod along, never asking if your period has gone missing, never connecting the dots between your relentless training schedule and the fact your body is slowly, methodically disassembling itself.

Low energy availability triggers a systemic shutdown

Your body runs on a strict energy budget, a principle known as energy availability. This is the amount of energy left over for basic physiological functions after the cost of your training has been deducted from your food intake. When the energy coming in consistently falls short of the energy going out, your body doesn't applaud your discipline. It initiates a series of emergency protocols known as Relative Energy Deficiency in Sport, or RED-S.

Think of your body as a company facing a severe budget shortfall. The executive board (your hypothalamus, a region of your brain) sees that expenses (exercise) are far exceeding revenue (food). Its job is to keep the lights on—to preserve brain function, circulation, and breathing at all costs. It doesn't shut down these core operations. Instead, it makes ruthless cuts to departments considered long-term investments or non-essential luxuries. Research and development is frozen. The marketing budget is slashed. All expansion projects are halted.

In your body, this translates to shutting down the systems that aren't required for immediate, minute-to-minute survival. Your reproductive axis is one of the first to go offline. Your thyroid function slows, dialling down your entire metabolic rate and making you feel perpetually cold. Your immune system is downgraded, leaving you susceptible to every cold that goes around the office. And the department of long-term maintenance—bone remodelling—is quietly defunded. For years, I interpreted this kind of systemic shutdown as a failure of will. I was wrong. It is the body’s cold, logical calculation for survival, a rational response to an impossible budget.

Related anchors: perimenopause anchor · skin anchor · performance anchor

Hormonal suppression halts bone repair

Healthy bone is a dynamic tissue in a constant state of flux, a process called remodelling. Imagine a meticulous workshop responsible for maintaining a critical piece of infrastructure. One team of specialised cells, the osteoclasts (your demolition crew), is responsible for disassembly. They move through the bone, identifying and clearing away old, worn-out tissue. Following behind them is the assembly team, the osteoblasts (your builders), who lay down new, strong bone matrix in its place. In a healthy system, these two teams work in perfect balance, ensuring the structure is constantly repaired and reinforced.

This entire operation is directed by your hormones. They are the work orders. When your brain senses a persistent energy deficit, it throttles back the hypothalamic-pituitary-gonadal (HPG) axis, the command chain that controls reproduction. The signal to produce sex hormones goes quiet. For women, estrogen levels plummet. For men, testosterone drops.

These hormones are the primary authorisation slip for the osteoblasts. Without sufficient estrogen or testosterone, the work order to build new bone never arrives. The assembly team is sent home. The demolition crew, however, continues its work, as its activity is less dependent on these specific signals. The result is an uncoupled process: breakdown continues while building has stopped. It’s like a business where the demolition department gets a blank cheque while the construction department is furloughed. Day by day, more tissue is removed than is replaced, and the integrity of your bones silently erodes. I see this pattern constantly in high-achieving clients: the body’s accounts are quietly being drained, but the external performance looks fine, right up until the moment something breaks.

Chronic cortisol accelerates bone degradation

As if halting all repair work wasn't enough, a chronic energy deficit is also registered by your body as a persistent, low-grade threat. To your nervous system, a chronic energy deficit is indistinguishable from any other persistent threat, like a famine or a siege. The response is the same: flood the system with stress hormones to survive the immediate crisis, even at the cost of long-term integrity. Your adrenal glands respond by releasing cortisol. In acute situations, cortisol is a powerful ally, liberating stored energy to help you fight or flee.

When the 'threat' is a permanent gap between your training load and your food intake, cortisol stays chronically elevated. This has a directly toxic effect on your bones. High cortisol levels send a powerful 'stop' signal to the osteoblasts, your assembly team, further inhibiting their function. It can even trigger their programmed cell death (apoptosis). At the same time, cortisol appears to boost the activity and lifespan of the osteoclasts, the disassembly team.

You are now facing a triple assault on your skeleton. The primary 'build' signal (estrogen/testosterone) is offline. A loud 'stop building' signal (cortisol) is blaring constantly. And that same signal is encouraging the demolition crew to work overtime. The part I find most galling is that this entire cascade is often framed as 'discipline' or 'toughness' by the very cultures that create it. The weakness was not an accident; it was accumulating for months, a predictable consequence of an unbalanced energy equation.

Common Questions

But I'm not underweight. Can I still have RED-S?

Yes. RED-S is determined by energy availability, which is the mismatch between your energy intake and your exercise expenditure. It is not defined by body weight or BMI. You can be at a weight that is considered normal or healthy and still be in a severe energy deficit, experiencing all the physiological consequences, including bone loss and hormonal disruption. Your weight is not a reliable indicator of your energy status.

Isn't losing my period a sign that I'm training hard enough?

This is a dangerous myth that persists in athletic culture. Amenorrhea (the absence of menstruation for three or more months) is a primary diagnostic signal that your body has shut down your reproductive system to conserve energy. This is a serious medical sign with significant consequences for your bone density, cardiovascular health, and overall endocrine function. It is a signal of physiological distress, not peak fitness.

Can I fix my bone density with calcium and vitamin D supplements?

Supplements are insufficient to solve a systemic energy crisis. While calcium and vitamin D are essential raw materials for bone health, they are like a pallet of bricks delivered to a job site with no workers. Without sufficient energy to fuel the process and the correct hormonal signals (like estrogen) to authorise the work, your body cannot use those materials to build new bone. The first and most critical step is restoring energy availability.

Closing

Restoring your energy availability is the most fundamental act of taking your performance, and your long-term health, seriously.

• The Athlete Performance Anchor is my 1:1 container for building a fuelling and recovery strategy that matches your training load and protects your health.
• The RED-S Recovery Care Package provides a set of clear, actionable protocols to begin closing your energy gap today.
• Download the free nervous-system journal prompts to start tracking your body’s daily signals of capacity and load.

TL;DR

Bone density loss in athletes stems from an energy deficit, not a calcium deficiency. When your energy intake doesn't meet the demands of your training, your body enters a state of Relative Energy Deficiency in Sport (RED-S). It responds by shutting down 'non-essential' functions, including the hormonal pathways that regulate bone repair. Without the 'build' signal from estrogen or testosterone, bone breakdown outpaces formation. Chronic stress from the deficit further accelerates this loss, leading to weakness and fractures. The only solution is to fuel for the work required.

Sources

• Mountjoy, M., Sundgot-Borgen, J. K., Burke, L. M., et al. (2018). IOC consensus statement on relative energy deficiency in sport (RED-S): 2018 update. British Journal of Sports Medicine, 52(11), 687–697.
• De Souza, M. J., Nattiv, A., Joy, E., et al. (2014). 2014 Female Athlete Triad Coalition Consensus Statement on treatment and return to play of the female athlete triad. British Journal of Sports Medicine, 48(4), 289.
• Sale, C., & Elliott-Sale, K. J. (2019). Nutrition and athlete bone health. Sports Medicine, 49(Suppl 2), 139–151.
• Heikura, I. A., Uusitalo, A. L. T., Stellingwerff, T., et al. (2018). Low energy availability is difficult to assess but outcomes have large impact on bone injury rates in elite distance athletes. International Journal of Sport Nutrition and Exercise Metabolism, 28(4), 403-411.