BMI Misleads on Fracture Risk: The Reality of Body Fat and Muscle in Aromatase Inhibitor Users
For bone health, it is generally said that "being a little overweight is protective" — extra weight helps through both mechanical loading and estrogen produced by fat tissue. But in women taking an aromatase inhibitor (AI) for breast cancer, this rule breaks down.
This study showed something striking: in these women, what determines fracture risk is not weight (BMI), but body fat percentage. When body fat was above 40,8% , vertebral fracture risk doubled — and, more importantly, in women who appeared to be "normal weight" (BMI <25), this risk was more than sixfold. In other words, BMI misses a large proportion of the highest-risk women. There is good news too: muscle is protective.
AI and bone: Aromatase inhibitors suppress estrogen; because estrogen protects bone, AI therapy increases bone loss and fracture risk — this is a known adverse effect.
BMI's blind spot: BMI looks only at height and weight; it does not know how much of the body is fat and how much is muscle . Two women can have the same "normal" BMI, but one may have high fat/low muscle ("normal-weight obesity" or sarcopenic obesity), while the other may be muscular. These are very different for bone — and the scale does not show that.
769 postmenopausal women with a median age of 63 were assessed with DEXA at baseline and at months 18–24–30 during AI therapy. During a median follow-up of 46 months, vertebral fracture progression occurred in 69 patients (9%). After adjustment for age, previous fractures, bone density and BMI:
| Factor (adjusted) | Vertebral fracture risk | Direction |
|---|---|---|
| Body fat >40,8% (all patients) | HR 2,00 (p<0,001) | ▲ increases risk (2-fold) |
| Excess body fat — in those with BMI <25 | HR 6,63 (p<0,001) | ▲▲ very marked increase |
| Arm and leg (appendicular) muscle mass ↑ | HR 0,38 (p<0,001) | ▼ protective |
| Bone mineral density (BMD) ↑ | HR 0,79 (p=0,01) | ▼ protective |
The most important message of this study is that many of the highest-risk women may be women who "do not look overweight" . A woman whose weight appears normal on the scale but who has high body fat and little muscle — she stays below the BMI radar, yet her fracture risk is actually high (HR 6,63 with BMI <25). The authors therefore recommend adding body composition measurement (such as DEXA) to fracture risk assessment — and AI patients already undergo DEXA for bone density.
The second message is even more encouraging: muscle protects. As muscle mass in the arms and legs increases, fracture risk decreases markedly (HR 0,38) — making exercise (particularly resistance/strength training) a concrete preventive measure.
Retrospective and single-center. It shows an association , not definitive cause and effect; residual confounding may remain.
Use of bone medication was high, so the number of fractures was lower than expected — meaning the actual effect may be even more pronounced, or the picture may differ somewhat.
Visceral (internal organ) fat was not measured. Nevertheless, the findings suggest that, regardless of fat distribution, excess fat — during AI therapy, when estrogen is suppressed — may harm bone health. In short: a strong, logical signal, but it should be strengthened with prospective studies.
- Do not look only at weight/BMI: assess body composition (fat and muscle) — the DEXA scan performed for AI therapy can already provide this information.
- Build muscle: regular resistance/strength exercise and adequate protein both preserve muscle and may reduce fracture risk.
- Protect bone: adequate vitamin D/calcium and — when indicated — bone-protective drugs recommended by the physician.
- Do not overlook excess fat: saying "I am normal weight" may not mean low risk.
- "Being a little overweight protects bone."
- "My BMI is normal, so my fracture risk is low."
- "Losing weight is always good."
- "Fracture risk is measured only by bone density."
- In AI users, excess fat increases risk instead.
- A normal BMI can hide high fat/low muscle; risk may still be high.
- What matters is reducing fat and preserving muscle; muscle mass is protective.
- Fat percentage and muscle mass are also independent markers — bone density alone is insufficient.
Postmenopausal women taking AIs for breast cancer are also a very large group in Türkiye, and bone density monitoring (DEXA) is already routine in these patients. Since the same DEXA devices can also measure body composition (fat percentage, lean/muscle mass), this information can be obtained without additional cost at most centers.
Practical implication: when patients start an AI, assessing bone health not only through BMD and weight but together with body fat and muscle mass is sensible, as is planning exercise (particularly strength training), nutrition and bone-protective treatment when needed as a whole. Decisions should be individualized through cooperation with the oncology team and, when necessary, endocrinology/physical medicine.
This news is intended to empower, not frighten. An aromatase inhibitor can weaken bones; this study shows a better way than the scale to predict risk and — best of all — that you have a lever at your disposal : muscle.
What you can do: ask your physician for a body composition assessment (it may be available from your existing DEXA), regularly do strength/resistance exercise , get adequate protein–vitamin D–calcium and take recommended bone medications consistently. Rather than being reassured by "my weight is normal," discuss your muscle and fat too. Set your program together with your oncologist.
Women taking aromatase inhibitors are a very large group, and fractures seriously impair quality of life. The reversal in this group of the generalization "BMI/being overweight protects bone," and the fact that risk is primarily determined by body composition , provide a practical insight that could change practice.
769 patients, serial DEXA measurements, adjustment for numerous variables (age, previous fracture, BMD, BMI). The findings are consistent and biologically plausible: fat harms, muscle protects. The most striking point is that one third of those with excess fat had BMI <30 — making BMI's blind spot concrete.
Retrospective, single-center; it shows an association, not causation. The fracture count remained low because use of bone-protective drugs was high. Visceral fat was not measured. Prospective validation and intervention (exercise) studies are needed.
Do not limit bone monitoring in AI patients to BMD and weight; also read body fat and muscle mass from DEXA. Plan strength exercise, adequate protein and vitamin D/calcium, and bone-protective treatment when indicated together. Do not let "normal BMI" provide false reassurance.
Does reducing body fat and increasing muscle actually prevent fractures (intervention study)? What is the role of visceral fat? Which threshold/measurement is the best marker? And does this finding also apply to estrogen-suppressing situations other than AI therapy?
- Schivardi G et al. Adiposity, Body Composition, and Vertebral Fracture in Postmenopausal Women Receiving Aromatase Inhibitors. JAMA Netw Open. 2026;9(7):e2626450. doi:10.1001/jamanetworkopen.2026.26450. (Retrospective, n=769; body fat >40,8% → HR 2,00; HR 6,63 in the BMI<25 subgroup; appendicular muscle HR 0,38; BMD HR 0,79.)
- Aromatase inhibitors and bone: bone loss and fracture risk due to estrogen suppression (general oncology knowledge).
- Limitations of BMI in reflecting body composition; the concepts of "normal-weight obesity" and sarcopenic obesity (background).
- Relationship of muscle mass/exercise to bone health; body composition measurement with DEXA (background).
Editor's note: This content is for general information, not medical advice. The findings presented are based on a retrospective, single-center study; it shows an association between body fat/muscle mass and fracture risk, not proof of cause and effect. Decisions on exercise, nutrition, bone-protective treatment and screening should be individualized based on the person's general condition, fracture risk and the treatment team's assessment. Consult your physician before starting any exercise or medication program.
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