Here's a number that should stop every serious cyclist mid-scroll. A systematic review examining 31 separate studies found that 84% of competitive cyclists met criteria for osteopenia or osteoporosis. Not recreational riders. Not people who cycle occasionally instead of exercising properly. Competitive cyclists — lean, fit, high-VO2max athletes who train more hours a week than almost anyone else in endurance sport — and 84% of them have bones that are, by clinical definition, weaker than they should be.
Controlled for age and body weight, cyclists are seven times more likely than runners to have osteopenia of the spine. Seven times. That's not a subtle difference buried in a footnote. That's one of the starkest divergences you'll find between two endurance sports that, on the surface, look like they should produce similarly fit, similarly healthy athletes.
We touched on the basics of this in an earlier piece on cycling and bone density. This is the full version — the mechanism, the science, and exactly what runners and cyclists both need to do about it.
Why cycling is uniquely bad for bone density
Three factors compound to make cycling almost a perfect storm for bone loss, and understanding each one matters because the fix is different depending on which piece of the puzzle you're addressing.
Non-impact. Cycling generates no ground reaction forces. Your feet never strike anything — they're clipped to pedals moving in smooth circles, and the forces involved are rotational and continuous rather than the sharp, repeated impacts of a foot striking the ground. Bone responds to exactly this kind of impact loading. Take it away and, per Wolff's law, the body has no signal telling it to maintain density. It's not that cycling actively destroys bone. It's that cycling gives bone no reason to keep what it has.
Non-weight-bearing. On top of removing impact, cycling removes even the baseline stimulus of gravity acting on your own bodyweight. You're seated. The saddle and the bars are supporting a significant portion of your mass. Compare that to standing, walking, or running, where your skeleton is constantly bearing and redistributing your full bodyweight with every step. Cycling strips out both the impact and the weight-bearing stimulus at once — a double removal that few other forms of exercise manage.
Calcium losses through sweat. This one surprises most cyclists. Endurance cycling, especially in heat, produces large volumes of acidic sweat, and that sweat carries measurable amounts of calcium out of the body. Over a long ride, or a training block full of long rides, this adds up. If dietary calcium intake doesn't keep pace with what's being lost, the body has one place left to source calcium from: bone. It's a slow, quiet drain that compounds the other two mechanical problems.
Put those three together — no impact, no weight-bearing, and an ongoing calcium leak — and you have a sport that is structurally set up to work against bone density, no matter how fit or lean the athlete riding is.
Wolff's law, explained plainly
You'll see "Wolff's law" referenced constantly in this research, so it's worth understanding in plain terms rather than as a name you nod along to.
Bone is not inert scaffolding. It's living tissue that remodels itself continuously, throughout your life. Two types of cells drive this. Osteoblasts build new bone where the skeleton is under load and needs reinforcing. Osteoclasts break down bone tissue where the load isn't there to justify maintaining it. Wolff's law, named after the 19th-century German surgeon Julius Wolff, is the observation that bone adapts its structure to the mechanical demands placed on it — more load, more density; less load, less density.
Impact loading — the kind you get from running, jumping, or any activity where your foot strikes the ground — sends a strong, unambiguous mechanical signal through the skeleton. That signal triggers osteogenesis, the formation of new bone. It's the same principle behind why astronauts lose bone density in zero gravity even while exercising intensely on resistance machines: without impact and without a gravitational load to work against, the skeleton reads the environment as "bone density isn't needed here" and adjusts accordingly.
Cycling, for all its cardiovascular demands, sends almost none of that signal. You can be extraordinarily fit, with a resting heart rate in the 40s and a power-to-weight ratio that would make most runners envious, and still have a skeleton that's quietly losing ground because the specific stimulus it needs — impact — simply isn't part of the sport.
Why running is protective
Running solves exactly the problem cycling creates, and the mechanism is direct. Each running stride generates ground reaction forces of roughly 2.5 to 3 times bodyweight. Those forces travel up through the tibia, the femur, the pelvis, and into the spine — precisely the sites where the cycling research shows the biggest deficits. That repeated, substantial mechanical loading is exactly the stimulus that triggers osteogenesis.
This is the reason runners consistently show higher bone mineral density than cyclists at matched ages, matched training volumes, and matched body composition. It isn't that running makes you generally healthier in some vague sense that happens to include bone. It's that running applies a specific, well-understood mechanical signal that cycling structurally cannot provide, no matter how many hours you put in on the bike.
It's worth being precise here: this isn't an argument that running is a "better" sport than cycling, or that cyclists are doing something wrong by cycling. It's that the two sports are mechanically different in a way that has a direct, measurable consequence for one specific tissue, and that consequence is worth knowing about rather than discovering at 55 when a DEXA scan or, worse, a fracture, delivers the news.
The runner's specific concern: what happens if you stop running
This matters more for our audience than the general "cyclists have weak bones" headline, because a lot of you are runners who are adding cycling, not lifelong cyclists wondering if they should take up running.
Here's the honest answer. If you reduce your running significantly, or stop altogether, in favour of cycling, bone density starts declining within 6 to 12 months without that weight-bearing stimulus. The bone density you built over years of running is not a permanent asset sitting in a vault. It requires ongoing maintenance, the same way cardiovascular fitness or muscular strength does. Stop the stimulus and the adaptation reverses, just on a slower timescale than fitness does.
This is not a reason to avoid cycling, and it's certainly not a reason to keep running through an injury that cycling could otherwise let you recover from — cycling for injured runners covers that trade-off in detail. It's a reason to be deliberate about what you keep in the programme rather than letting running quietly disappear as cycling volume creeps up.
How much impact loading you actually need
This is the number that should replace worry with a plan. Research suggests that 2 to 3 sessions per week of weight-bearing exercise — running, brisk walking, jumping-based work, or resistance training under load — is sufficient to maintain bone density in most people.
That's a low bar, and it's good news for anyone trying to balance both sports. A runner who adds substantial cycling volume but keeps 2 to 3 running sessions a week is not meaningfully increasing their bone density risk. The problem is concentrated specifically in athletes who replace running with cycling entirely — the runner who gets excited about a new bike, or picks up cycling to manage an injury, and simply never goes back to running once the volume shifts.
If you're building a week that mixes both sports, the practical target is simple: protect 2-3 running sessions, however short, and build your cycling volume around them rather than instead of them. The hybrid athlete who runs 2-3 days and cycles 2-3 days is, from a bone health perspective, in the best position of anyone in this conversation — getting the impact stimulus running provides and the low-impact aerobic volume cycling adds, without the downside of either sport in isolation.
DEXA scans: know your numbers
Given how symptomless bone density loss is — there's no ache, no twinge, no warning sign until a stress fracture or a break happens — a DEXA scan is the only way to actually know where you stand rather than guess.
Every cyclist over 40 should get a baseline DEXA scan, and repeat it every two years to track the trend rather than relying on a single snapshot. Pay specific attention to your T-scores for the lumbar spine and femoral neck — these are consistently the sites most affected in the cycling research, and they're the numbers a clinician will use to classify you as normal, osteopenic, or osteoporotic. A DEXA scan is quick, involves a low dose of radiation, and is worth the modest cost for the clarity it provides. If you're a serious amateur cyclist putting in 8-plus hours a week on the bike with minimal running or resistance work, this is not an optional extra. It's a baseline you should have.
Calcium and vitamin D: the nutritional side
The mechanical loading argument is the headline, but nutrition plays a supporting role that's worth getting right, particularly given the sweat losses cycling causes.
Daily targets for endurance athletes sit around 1,000 to 1,200mg of calcium and 2,000 to 4,000 IU of vitamin D. Timing matters more than most people assume — the body absorbs calcium more efficiently in smaller doses spread across the day than in one large dose taken all at once, so spreading intake across meals beats a single evening supplement. Food sources — dairy, fortified plant milks, tinned fish with bones, leafy greens — should do most of the work, with supplements filling genuine gaps rather than replacing diet entirely. Vitamin D is trickier to get from food alone, and most cyclists training indoors through winter or based somewhere with limited sun will benefit from supplementation, ideally checked against an actual blood test rather than guessed at.
None of this replaces the mechanical loading argument — you cannot supplement your way out of a non-impact sport's effect on bone density. But it removes one of the three compounding factors, and it's the easiest of the three to fix.
Common misconceptions worth clearing up
A few myths circulate around this topic and they're worth addressing directly, because they lead people to either dismiss the risk or panic about it unnecessarily.
"I lift weights, so my bones are fine." Resistance training helps, but not all resistance training produces the same osteogenic signal. Controlled, joint-friendly loaded movements and bodyweight-based impact work — step-ups, light jumping drills, resistance band work — can contribute meaningfully to bone stimulus without the injury risk that heavy compound barbell lifting carries for a 35-55-year-old frame that's spent years in a cycling position. You don't need to load a bar heavily to get a bone benefit; consistent, moderate loading done well beats occasional heavy loading done badly.
"I'm strong on the bike, so my bones must be strong too." Muscular strength and bone density are related but not the same thing, and cycling is a good example of where they diverge sharply. You can have powerful, well-conditioned leg muscles from years of pedalling and still have a spine that scores in the osteopenic range on a DEXA scan, because the muscles were built under load patterns that never involved impact.
"This is only a problem for elite or professional cyclists." The 84% figure comes from competitive cyclists, but the mechanism — no impact, no weight-bearing, calcium losses — applies to any cyclist logging significant volume, professional or not. A serious amateur riding 10-plus hours a week with no other weight-bearing activity is working under the same mechanical conditions as the athletes in the research, even without the results or the contract.
"It's too late, the damage is done." Bone remodels continuously throughout life, which cuts both ways — density can decline, but it can also improve. Studies on formerly cycling-only athletes who added running or resistance work show measurable bone density improvements within 6-12 months of consistent impact loading. This isn't a one-way ratchet. The stimulus works whenever you apply it, not just if you started young.
What this means for women specifically
The research on cycling and bone density applies to both sexes, but the stakes are higher for women, and it's worth being direct about that rather than treating this as a gender-neutral footnote. Women reach peak bone mass earlier than men and experience a steeper decline in bone density after menopause, driven by the drop in oestrogen, which plays a direct role in regulating bone remodelling. A woman who spends her thirties and forties cycling exclusively, without impact loading, is building her pre-menopausal bone bank on a foundation that's already working against her compared to a male training partner doing the same sport.
This doesn't mean women should avoid cycling — it means the case for keeping running, walking, or resistance work in the programme is stronger, not weaker, for female athletes. It also strengthens the case for an earlier baseline DEXA scan. Where 40 is a reasonable starting point for men, women with a history of disordered eating, amenorrhea, or low energy availability at any point in their athletic history should discuss an earlier scan with their doctor, since these factors independently suppress bone density and compound the cycling-specific mechanism described above.
The practical takeaway
Strip away the science and the message is simple. Cycling and running are complementary for bone health in a way that almost no other pairing of endurance sports is. Pure cyclists, especially those training 8-plus hours a week with little else, need impact loading from somewhere — running, if the joints allow it, or a structured jumping and resistance programme if they don't. Pure runners who are shifting toward cycling need to be deliberate about keeping some running in the week, not letting it quietly vanish as bike volume grows.
The hybrid athlete — 2 to 3 running sessions, 2 to 3 cycling sessions, calcium and vitamin D intake dialled in, a DEXA scan on the calendar if they're over 40 — is in the best position of anyone in this conversation, no caveats. Not despite doing both sports, but because of it.
If you're building a training week that balances running and cycling properly — sequencing, volume, how to protect the sessions that matter for both bone health and performance — that's exactly the kind of detail we work through with members. Come find us at Roadman Cycling on Skool. You're not done yet.