Ever wondered why an astronaut needs to sit in a wheelchair when they come home? They lose significant amounts of muscle. But they also lose bone, and at the hip they lose it roughly 10 times faster than a post-menopausal woman does on Earth [1][2].
NASA tried for years to stop this loss, and they kept failing. But every failure added an important piece to their understanding, and finally they found an answer.
Now, chances are you're not going to space. But the same rule that governs an astronaut's skeleton governs yours. And the strategies NASA found to protect an astronaut's bones translate directly into protection for your bones.
Table of Contents
- Attempt One: Just Exercise More
- Attempt Two: Feed The Bone
- Attempt Three: The Crossroads
- The Answer: Go Heavy
- What It Means For Your Bones
- The Last Rung: The Drug From Your Kettle
- The Ladder, In Order
- References
Attempt One: Just Exercise More
They tried exercise first. On Earth this is settled: active people have stronger bones than people who sit still. And the astronauts were already exercising hard anyway, strapped to treadmills and bikes to protect their hearts and their muscles. So the thinking was simple. Keep them moving, and the bones should hold.
They didn't. Even crews putting in hours of exercise came home with their hips melting away at the same rate as everyone else, well over a percent a month [1].
Sit with that number for a second, because it's the whole reason this story exists. Hip bone density fell by 1.4 to 1.5% per month across four- to six-month flights [1]. On Earth, a woman going through menopause loses roughly 1% per year at the hip [2]. Same site, same measurement, and the astronauts were doing in a month what menopause does in more than a year.
So if cardio exercise on its own wasn't the answer, maybe the problem was supply. Maybe the bone was simply running out of the stuff it's built from.
Attempt Two: Feed The Bone
Bone isn't a dead scaffold. It's living tissue, rebuilt your entire life. There are cells called osteoclasts whose only job is to dissolve old bone away, and other cells called osteoblasts right behind them laying down fresh bone. A demolition crew and a construction crew, working inside you every single day.
So here's the logical thought. If bone is constantly being rebuilt, give it plenty of raw material. Calcium is literally what bone is made of. Vitamin D helps you absorb it. Top up the building blocks, and surely the construction crew can keep up. There was real optimism here. This felt obvious.
But when NASA tested this idea in space, it did nothing to stop the bone loss.
That matches what later studies found down here on Earth. A lot of people take calcium supplements believing it will improve their bone strength. But a 2017 analysis that pooled more than 51,000 people found that calcium, with or without vitamin D, wasn't linked to fewer broken hips, fewer spine fractures, or fewer fractures of any kind [3].
But more surprising than calcium supplements failing to strengthen bone is what they appear to do to blood vessels.
Two researchers in Auckland, New Zealand, where I did my medical training, Ian Reid and Mark Bolland, ran a trial giving older women calcium, fully expecting stronger bones and fewer fractures. Instead they noticed something they weren't even looking for: more heart attacks in the calcium group. Thirty-one events versus fourteen on placebo [4].
They spent years being told they were wrong, by parts of the supplement industry and by some of the bone-medicine field. And they held their ground. Reid put it simply: they hadn't set out to be myth-busters, the evidence just kept pointing the other way. Years later they won New Zealand's top science prize for that work [5].
Official US guidance eventually shifted too. The US Preventive Services Task Force now recommends against daily supplementation with 400 IU or less of vitamin D and 1000 mg or less of calcium for the primary prevention of fractures in community-dwelling postmenopausal women [6].
So let me be clear about what I'm saying and what I'm not. Calcium the mineral matters. You need it. But the evidence says you should get it from food rather than a pill: dairy, leafy greens, and fish. Whole foods beat supplements here.
One important caveat. If your doctor has specifically put you on calcium supplements, for a genuine deficiency or because you don't eat dairy, none of this is a reason to stop. That's a conversation to have with them, not a decision to make off the back of an article.
But step back and look at where NASA now was. Cardio exercise hadn't worked. Feeding the bone more material hadn't worked. And that points at something deeper: the astronauts' bones weren't starving for calcium. They were starving for something else entirely.
Attempt Three: The Crossroads
So they built a machine. If gravity was the missing ingredient, maybe you could fake it with resistance, and give the astronauts something to pull against. The early Space Station had exactly that: a resistance device that let them load up to about 300 pounds.
But even that barely moved the needle. When researchers looked at the data, the lighter machine came out no better than the cardio [7].
Sit in that moment for a second, because this was the low point. They'd tried cardio. They'd tried nutrition. They'd tried resistance. Every single thing you'd reach for if your bones were thinning on Earth, and the astronauts kept crumbling anyway. It genuinely looked like maybe you just couldn't stop it.
There was one idea left. And it was almost embarrassingly blunt. What if the machine simply wasn't heavy enough?
The Answer: Go Heavy
Here's the insight everyone had walked past. Bone doesn't rebuild itself because you feed it, and it doesn't rebuild because you move. It rebuilds when it's pushed on hard enough that it has no choice but to respond. The signal isn't the material. The signal is the force.
And nothing in orbit, not the treadmill, not the pills, not the light machine, was pushing hard enough to send a strong enough signal.
You can see this written into people's skeletons here on Earth. Take a professional tennis player. Their racket arm and their other arm belong to the same person. Same genes, same diet, same calcium, everything identical, except one arm has taken decades of hard load. So what does the bone look like? Cortical bone thickness on the playing side was greater by 34.9% in men and 28.4% in women compared with the non-playing arm [8].
Nearly 35% thicker bone, in the same body, built by nothing but load. And it lasts. Researchers scanned a former professional pitcher at 94 years old, 55 years after his last pitch, and his throwing arm was still built differently, retaining 14.3% greater bone cross-sectional area [9].
His skeleton remembered. And there's a timing lesson in it too. The effect was two to four times greater in players who started loading their bones before puberty than in those who started as adults [9]. That's worth knowing if you've got kids: the bone you build young is a deposit you keep drawing on for life, and it's much harder to top up later.
So NASA stopped being gentle. In 2008 they sent up a new machine, the Advanced Resistive Exercise Device, and instead of 300 pounds it let astronauts load up to 600. The space equivalent of a heavy barbell squat and deadlift. This was the last-ditch idea.
And for the first time, the bone loss stopped. Bone mineral density was unchanged from preflight only for the crew members using the heavier device. Pelvis, hip and lumbar spine were all preserved with the heavy machine and not with the lighter one [7].
Look at those numbers, because this is the whole hunt in one experiment. On the lighter machine the total hip still fell about 8%. On the heavy machine it barely moved, around 2%. The lumbar spine went from −4% to zero. Whole body went from −3% to zero. The heavy-lifting astronauts came home with essentially the bone they left with. Every site that could be measured moved the same way. It wasn't the calcium. It wasn't the cardio. It was heavy load.
One honest caveat: even the heavy lifting didn't fully protect the spongy honeycomb bone deep inside the hip. Resistive exercise prevented reductions in the hard outer cortical bone of the femoral neck, but it did not prevent declines in hip trabecular bone [10]. To lock that down too, NASA added a bone drug on top, and we'll come back to that drug at the end, because it has one of the strangest origins in medicine.
But the headline is simple, and it cost NASA billions of dollars and decades to learn. Bone responds to being loaded hard. So what does that mean for you, standing on Earth, with gravity for free?
What It Means For Your Bones
The whole lesson comes down to one word: load. Here's the ladder, from the easiest thing you can start today to the one that actually rebuilds bone.
Start easy. Hopping. It sounds too simple, but there's a clever experiment behind it. Researchers had women hop on one leg only, so the other leg acted as the built-in comparison inside the same body. Fifty hops a day, seven days a week, increased femoral-neck bone density by 1.8% in the exercised leg. Fewer than daily sessions produced no effect at all [11].
The hopped hip gained bone density that the resting hip didn't. But notice the dose. You had to do it every single day. Less than daily did nothing. This is a perfect exercise snack after an hour of computer work, and it's exactly what I do between patients at the clinic. I'll do a set of push-ups and some hopping.
The effect held after menopause too. A separate trial found femoral-neck bone density rose 0.81% in the hopping leg while falling 0.57% in the control leg [12].
While that's a great start, the effect size isn't large, and it didn't improve spine bone density. Pooled data on impact exercise showed no significant effect on lumbar-spine bone density in any subgroup [13]. So hopping is a free, easy first step. It is not a cure.
If you want the thing that rebuilds meaningful bone density, your own version of that heavy machine on the Space Station, it's progressive, heavy resistance training.
What about safety? Here's the part people find hard to believe. It's been tested in exactly the people who are usually told to be careful: older women who already had low bone density. High-intensity resistance and impact training increased lumbar-spine bone density by 2.9%, versus a 1.2% loss in the low-intensity control group, with one minor adverse event [14].
Look at what happened to the comparison group. They did the gentle, careful, low-intensity exercise that people with fragile bones are usually told to do. And they still lost bone at the spine and the hip. The heavy-lifting group gained it. There were no injuries aside from one woman with a minor lower back spasm.
Here's the actual protocol from that trial, and the final part is the most important. It ran for eight months, twice weekly, 30 minutes a session. Resistance exercises were deadlift, overhead press and back squat, performed in 5 sets of 5 repetitions at an intensity above 80–85% of one-rep max, with up to 2 warm-up sets of deadlifts at 50–70%. Impact loading was applied via jumping chin-ups with drop landings: grasp an overhead bar with shoulders and elbows flexed to 90 degrees, hands shoulder-width apart with an underhand grip, jump as high as possible while simultaneously pulling up with the arms, then at the peak of the jump drop to the floor, landing as heavily as is comfortable [14].
And the critical detail: every session was run in small groups of no more than eight participants per instructor, and each instructor was an exercise scientist and physiotherapist. These exercises were properly supervised. So for my patients at the clinic who are convinced by this data, I strongly encourage them to see a personal trainer initially, so the movements are taught correctly and done safely.
The Last Rung: The Drug From Your Kettle
And if you already have osteoporosis, or loading isn't enough on its own, there's one more rung. This is the drug NASA added on top for the astronauts, and its story is genuinely strange, because it didn't start in medicine. It started in plumbing. The earliest uses of these compounds were industrial: preventing scale and lime deposits in water pipes, boilers and detergents [15].
The same family of chemicals that stops chalky scale building up in your kettle turned out to do something similar in the body. It grips onto bone and slows down the crew that dissolves it away. To be precise, the drug isn't the identical molecule you'd drop in a boiler; it's a close, tuned-up cousin. But the lineage is real: from descaling agent to one of the most effective bone drugs ever made, the bisphosphonates.
And they work. Alendronate reduced the risk of vertebral fractures by roughly 47%, and hip fractures by around half, in women with existing fractures [16]. Put that next to the roughly 1% density bump you get from a calcium supplement and the gap is enormous.
As with any drug, there are potential side effects. The most important to know about is osteonecrosis of the jaw, a rare condition involving exposed, non-healing jawbone, pain and infection. This is why your doctor would make sure your dental hygiene is immaculate before prescribing this medication.
The Ladder, In Order
Calcium and vitamin D supplements haven't been shown to reduce fracture rates in the general community-dwelling population [3][6]. Calcium is best obtained from whole foods rather than supplements. For vitamin D, the recommended dietary allowance is 600 IU a day for adults up to 70, and 800 IU from 71 onwards, and those figures are set assuming minimal sun exposure [17].
As for exercise, hopping is a good start. But to build meaningful bone density, heavy resistance exercise done under proper supervision is what's required. And if that isn't enough, bisphosphonate medications can be considered with your doctor.
It took NASA decades and billions of dollars to learn a lesson your skeleton has been waiting to hear the whole time. Your bones don't respond to what you swallow. They respond to what you ask them to carry. And loading your bones and building muscle are really the same job.
References
1. https://pubmed.ncbi.nlm.nih.gov/15125798/
2. https://pubmed.ncbi.nlm.nih.gov/18160467/
3. https://pubmed.ncbi.nlm.nih.gov/29279934/
4. https://pubmed.ncbi.nlm.nih.gov/18198394/
5. https://www.rnz.co.nz/news/national/289423/myth-breaking-bone-scientists-take-top-prize
6. https://pubmed.ncbi.nlm.nih.gov/29677309/
7. https://onlinelibrary.wiley.com/doi/10.1002/jbmr.1647
8. https://pubmed.ncbi.nlm.nih.gov/845205/
9. https://pmc.ncbi.nlm.nih.gov/articles/PMC3986122/
10. https://pubmed.ncbi.nlm.nih.gov/23334732/
11. https://pubmed.ncbi.nlm.nih.gov/20004758/
12. https://onlinelibrary.wiley.com/doi/10.1002/jbmr.3867
13. https://pubmed.ncbi.nlm.nih.gov/37555459/
14. https://pubmed.ncbi.nlm.nih.gov/28975661/
15. https://www.sciencedirect.com/science/article/abs/pii/S8756328211000652
16. https://pubmed.ncbi.nlm.nih.gov/8950879/
17. https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/
