Bone density is a lagging indicator of your energy budget

I used to think stress fractures in runners were mostly about shoes. I'd blame bad luck, bad genetics or bad form. I've changed my mind. Usually it's the brain, doing its job perfectly. Your brain doesn't speak English; it speaks energy. When it senses a chronic energy shortfall, it registers a threat—a famine, a siege—and starts making ruthless decisions to keep you alive. Bone density is one of the first things it sacrifices.

You finish a long run and the thought of eating feels like a chore. 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 tell yourself you’re just tired, but you feel wired, buzzing with an anxiety that has no name. You wake up at 3am every night, heart racing, even though you’re exhausted. Your body feels brittle, disconnected. These are not random glitches. These are data points. They are the early warning signals of a system being quietly disassembled to pay an energy bill that’s gone unpaid for too long.

Common Questions

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

Yes. Relative Energy Deficiency in Sport (RED-S) is determined by the gap between your energy intake and your training expenditure. It is a state of physiological function, not a number on a scale. You can be at a weight considered normal or healthy and still be in a severe energy deficit, with all the downstream consequences for your hormonal health and bone density.

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

A missing period is a fire alarm, not a badge of honor. Amenorrhea is a primary diagnostic signal that your body has shut down your reproductive system to conserve energy. It is a sign of profound physiological distress with serious consequences for your bones, your cardiovascular system, and your brain. It means your body is choosing survival over function.

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

Supplements cannot fix a systemic energy crisis. While calcium and vitamin D are essential raw materials for bone, they are useless without enough energy to run the biological machinery and the correct hormonal signals to authorize the work. First, you have to restore energy availability. Then the body has the capacity to use the materials you give it.

Your brain’s fuse box

Your body runs on a strict energy budget. The technical term is energy availability: the amount of energy left for basic function after the cost of exercise is subtracted from your food intake. When that budget is consistently in the red, your hypothalamus—the brain region that acts as your central command—doesn't panic. It gets strategic.

Think of it like a physiological fuse box. Faced with an overwhelming electrical load from your training, it doesn’t let the whole system short out. It trips the breakers to circuits it deems non-essential for immediate survival. Power gets cut to reproduction. It gets cut to thyroid function, dialing down your metabolic thermostat and leaving you perpetually cold. And the power gets cut to the department of long-term maintenance: bone. This is the logic of nervous system regulation—prioritizing survival above all else.

The signal goes dead

Healthy bone is a dynamic tissue, constantly being refreshed. Cells called osteoclasts clear away old, tired bone. Following behind, cells called osteoblasts lay down new, strong matrix. In a regulated system, they work in a balanced partnership, directed by your hormones.

Estrogen and testosterone are the primary work orders for the osteoblasts—the 'build' signal. When your hypothalamus trips the fuse on the energy budget, it throttles the entire hormonal chain that leads to their production. Levels plummet. The work order to build new bone never arrives. The osteoblasts are idled. The osteoclasts, however, keep clearing out old tissue. Breakdown continues while building has stopped. Day by day, your bones get weaker. I see this constantly in my high-achieving clients: the performance looks fine from the outside, right up until the moment something snaps.

Your body does not admire your training discipline. It sees an energy crisis and starts liquidating assets.

The cortisol effect

As if halting all new construction wasn't enough, your nervous system registers a chronic energy deficit as a persistent, low-grade threat. The adrenal glands respond by releasing cortisol. In short bursts, cortisol is an ally, liberating stored energy for a fight or a flight. But when it’s chronically elevated, it becomes corrosive.

High cortisol levels send a powerful 'stop' signal to the osteoblasts, further inhibiting their function. It can even trigger their death. At the same time, cortisol appears to boost the activity of the osteoclasts, the disassembly team. So now you have a triple assault. The main 'build' signal is offline. A loud 'stop building' signal is blaring constantly. And that same signal is encouraging the demolition crew to work overtime. Your brain, faced with what it perceives as a famine, couldn't care less about your race time next spring. It just wants you to survive the next five minutes, even if it has to pawn the furniture to do it.

What to do this week

1. Eat within 30–60 minutes of finishing your workout. Don't wait. Your body needs the signal that the famine is over, immediately. Aim for a 3:1 or 4:1 ratio of carbohydrates to protein. This is non-negotiable.
2. Add 300 calories. Starting today, add 300 calories of nutrient-dense food to your daily intake. A handful of nuts and a piece of fruit. A full-fat yogurt. An extra serving of sweet potato with dinner. Don't overthink it. Just add it.
3. Take a real rest day. No "active recovery" run. No "light" spin class. A day where your only goal is to move less. Lie on the sofa. Read a book. Let your system exit its threat state.
4. Track your signals. Instead of mileage, track how you feel. Are you cold? Are you sleeping through the night? Note it down in the Journal. This is the data that matters most right now.

Where this fits in the Kokorology system

This entire pattern is a breakdown in the Regulation layer of the Kokorology system. The body, perceiving a threat, down-regulates non-essential systems to conserve capacity. The solution isn't to "push through," but to restore safety at a physiological level by closing the energy gap. Specific protocols for this are inside the Anchors library, especially for those navigating perimenopause, where bone density is already a critical focus. Once regulation is restored, Kokorology can rebuild Performance on a solid foundation.

Closing

Your bone density is a lagging indicator of your energy budget. By the time a scan shows a problem, the deficit has been running for months or years. The work is to fix the systemic energy crisis that is causing the body to sacrifice it.

• Work with me inside the Performance Anchor to build a fuelling strategy that matches your real-world load.
• Start today with the 7-day guided on-ramp inside The Reset to begin closing your energy gap.
• Get the free guide to 5 Resets for a Dysregulated Nervous System to start tracking your body’s daily signals.

TL;DR

Bone density loss in athletes is a symptom of an energy crisis, not a calcium deficiency. When your training outpaces your fuelling, your brain perceives a threat and enters a state of Relative Energy Deficiency in Sport (RED-S). It responds by shutting down hormonal pathways, like estrogen and testosterone production, that are essential for bone repair. Without the "build" signal, bone breakdown continues while formation stops. Chronic cortisol from the stress of the deficit accelerates this loss. The solution is physiological, not supplemental: fuel for the work required.

Sources

• Mountjoy, M., et al. (2018). IOC consensus statement on relative energy deficiency in sport (RED-S): 2018 update. British Journal of Sports Medicine.
• De Souza, M. J., 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.
• Sale, C., & Elliott-Sale, K. J. (2019). Nutrition and athlete bone health. Sports Medicine.
• Heikura, I. A., 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.