Teenage Weight and Lifelong Bones: What an Israeli Study of 1.1 Million People Shows

A large Israeli study explains why being thin at 16-19 can lead to osteoporosis at 40-60 years old - and what to do about it now.

Osteoporosis is when the bone becomes porous and brittle. A person can break a wrist, a hip or a vertebra literally "out of the blue". The disease entails pain, operations, loss of independence and huge expenses. Therefore, the question "what to do to keep bones strong" is not about old age, but about investing in youth.

A giant Israeli study is about these investments: 1,083,491 people whose height and weight were measured at 16-19 years old, and then tracked for decades to see who would develop osteoporosis in adulthood. Plus, three-quarters of the participants also had an “adult” weight mark - it was possible to see the trajectories: who remained thin, who gained weight to the norm, who became obese, and who lost weight.

The main conclusion is extremely practical: teenage thinness is a strong predictor of future osteoporosis, especially if the thinness persists. If the teenager “comes out” of a weight deficit to a normal weight, the risk drops significantly.

Background of the study

Osteoporosis develops over decades and is largely determined by the peak bone mass a person achieved in childhood and adolescence. The lower the peak, the higher the risk of bone porosity and fractures in adulthood. This is why the period of late puberty to early adulthood is considered a “critical window” for future skeletal health. Despite this, there are few data directly linking body mass index (BMI) in adolescence and long-term risk of osteoporosis in the large general population; even fewer consider further changes in weight on the threshold of adulthood.

Biologically, the relationship between BMI and bone tissue is clear: mechanical load and hormonal environment stimulate mineralization in people with higher body mass, while mass deficit is associated with lower bone density. Observational studies in adolescents have shown a positive (sometimes saturable) relationship between BMI and bone mineral density, but in adults the picture is more complex: in women, excess weight is often associated with a lower risk of osteoporosis, while in men such a “protective” effect may not be present; in addition, obesity increases the risk of some fractures and carries its own metabolic risks. Given the heterogeneous results, it is especially important to understand where the risk lies in lean people and how it changes with the weight trend from adolescence to adulthood.

Methodologically, most large cohorts measure BMI in adulthood, making it difficult to disentangle the contribution of early (adolescent) BMI from subsequent changes. What is needed are large longitudinal datasets in which body weight is objectively measured at ages 16–19 and osteoporosis diagnoses are recorded in reliable registries many years later, taking into account covariates and baseline health. This is precisely the gap addressed by a new paper in JAMA Network Open: the authors used a national Israeli adolescent medical examination (fitness for military service) database and linked it to the osteoporosis registry of a major insurance system, additionally accounting for BMI trends across the transition to adulthood.

This design allows us to answer two fundamental questions: (1) how does adolescent BMI relate to osteoporosis risk decades later, independent of sociodemographic and health factors; and (2) what happens to risk in those who remain underweight versus those who gain weight to normal in early adulthood. Additionally, the study provides a chance to see sex differences and to test whether the supposed “protection” of higher BMI is preserved in women and absent in men, when the starting point is precisely adolescent weight and its subsequent dynamics.

How it was studied

• Who was included. All Israelis who underwent a standard medical examination before service (1967-2019). The baseline is height/weight measurement, BMI calculation, plus socio-demographics and a general “medical passport”.
• How osteoporosis was calculated. Any of three criteria were met: DXA T-score ≤ -2.5 (bone density), characteristic osteoporotic fracture (vertebra, radius, humerus, hip) or ≥2 purchases of antiosteoporotic drugs.
• Who was excluded. People with diseases that in themselves "drop" the bone (oncology, severe endocrine, etc.) - so as not to confuse cause and effect.
• About weight trajectories. For 74%, an "adult" BMI was found (on average, at 30-35 years), which allows us to understand whether a person has remained thin, has returned to normal, or, on the contrary, has become obese.

What happened?

1) The lower the BMI at 16-19 years, the higher the future risk of osteoporosis

• In women with extreme thinness (<3rd percentile), the risk was almost 2 times higher (HR ~1.88), while in overweight women it was below average (HR ~0.83).
• In men, thinness also increased the risk (HR ~1.82), but obesity did not provide convincing protection (HR ~1.14, statistically insignificant).

2) It’s not just the start that matters, but also the path to adulthood

• The highest risk is among those who remain thin into adulthood.
• If a teenager was thin, but by the age of 30-40 reached a normal BMI, the risk decreased, although it remained higher than average (in women, HR ~1.34).
• If people went from being thin to being obese, the risk for women became close to the reference value (HR ~1.02).
• Losing weight in adulthood and going into a mass deficit increased the risk even for those who started out normal.

Why Bones 'Remember' Teen Weight

• "Peak Bone Mass" - Your Bone Capital

By the age of 18-20, we have gained ~90% of our "bone capital", adding a little bit more until the end of the second decade. What is strengthened in puberty is what we live on. If during this period the bones do not receive load and nutrition, they are formed thinner and less dense - a long-term minus.

• Mechanical signal is the main "anabolic" of bone

Bone "loves" body weight, muscles and impact/power loads (jump rope, jumps, running accelerations, playing with a ball, deadlifts/squats/lunges). Chronic thinness = little mechanical stimulus → less bone formation.

• Puberty hormones

Lack of energy and weight deficit often delay puberty (in girls - late/rare menstruation, in boys - delayed puberty). Namely, sex hormones "cement" the skeleton in youth.

• Why does "plus weight" protect women, but not men?

In women, adipose tissue enhances estrogen supply (including aromatization), which supports bone. In men, obesity, especially visceral, is often accompanied by low testosterone (hypogonadism) - this harms bone and neutralizes the mechanical advantages of mass. Hence the gender differences.

Important: this is not an argument to "keep obesity for the sake of bones." The total harm of obesity (heart, blood vessels, metabolism, cancer risks) is much greater.

What to do with it now?

If a teenager/young adult is underweight

• The goal is to gently reach a healthy BMI (and not “keep cutting” for the sake of sports or aesthetics).
• Diet:
  • Protein: 1.2-1.6 g/kg/day;
  • Calcium: adolescents 1000-1300 mg/day (dairy/alternatives, leafy greens, mineral waters, fortified foods);
  • Vitamin D - by region and analysis (supplementation is often needed);
  • Energy adequacy: don’t “live on salad.”
• Loads: 2-3 times/week strength + 2-3 times/week impact/jumping (skipping rope 5-10 min, ball games, sprints, step aerobics).
• RED-S/ED screening: girls have a cycle (amenorrhea/oligo), everyone has obsessive calorie control, training "through fatigue", lack of energy. If you suspect - see a doctor and nutritionist.
• When to think about DXA: severe/long-term thinness, fractures “out of the blue”, cycle disorders, severe energy deficiency.

If the weight is normal

• We preserve: protein, calcium/D, strength + impact loads.
• For endurance sports (ballet, gymnastics, long distance running) - monitor energy balance and the menstrual cycle in girls.

If you are overweight/obese

• The goal is a healthy BMI range, but weight loss should be gradual, with an emphasis on strength training and protein to avoid losing muscle mass and bone density.
• Men with abdominal obesity and symptoms of low testosterone should discuss testing with their doctor.

Mini-myth busting

"Drink milk - and everything will be fine"
Calcium is important, but without protein, vitamin D and exercise, the bone will not "absorb" the resource. It is always a combination of factors.

"The thinner you are, the healthier your joints and bones"
For bones, chronic thinness is a minus: little mechanical stimulus and often hormonal disruptions.

"If obesity sometimes "protects" bone, let it stay
." No. The overall harm of obesity outweighs the potential "bone benefit." The goal is a healthy range.

Why Parents, Coaches, and Doctors Should Care

The window of opportunity is puberty and early adulthood. If we miss these years, we will no longer be able to "build up** bone capital" to the ideal, we can only slow down the losses. This means that osteoporosis prevention is:

• talk about diet without shame and moralizing;
• correct training plans in schools/sections (strength and jumping are a must);
• attention to signs of RED-S and eating disorders;
• a balanced approach to weight loss in aesthetic/weight sports.

And if it’s already 30-40?

Even if you were thin as a teenager, reaching a normal BMI plus strength/jump training reduces the risk of the "stay thin" scenario. If you need to lose weight, do it slowly, with protein and strength training, otherwise you can lose bone along with fat.

Important Disclaimers

The research is huge and high-quality, but:

• there was no data on nutrition/physical activity/genetics/medications;
• BMI is a crude metric (does not distinguish between fat/muscle and fat distribution);
• There may be a “detection bias”: thin people may be more likely to be referred for DXA.

However, the sample size, thoughtful filtering of comorbidities, and analysis of weight trajectories make the findings compelling.

Conclusion

• Teenage thinness is a persistent signal of future osteoporosis, especially if it is prolonged.
• Reaching a normal weight in early adulthood significantly improves the prognosis.
• In women, a higher BMI often supports bone, but in men there is no such “insurance” - and obesity brings its own risks.
• The best strategy is a healthy BMI, strength + jumping exercises and adequate nutrition from 12-25 years (and then - maintenance).

Source: Simchoni M, Landau R, Derazne E, Pinhas-Hamiel O, Nakhleh A, Goldshtein I, Tsur AM, Afek A, Chodick G, Tripto-Shkolnik L, Twig G. Adolescent Body Mass Index, Weight Trajectories to Adulthood, and Osteoporosis Risk. JAMA Network Open. 2025;8(8):e2525079. doi:10.1001/jamanetworkopen.2025.25079.