A study published in the European Journal of Applied Physiology in 2026 indicates that a high protein diet may not adequately protect bone health during severe caloric restriction. The research found that reducing calories dramatically led to decreased blood markers linked to bone formation, regardless of maintained protein levels. This challenges the belief that prioritizing protein can mitigate all negative effects of dieting.

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Researchers involved 19 healthy, physically active adults—nine women and ten men—in three different five-day dietary conditions. One diet provided adequate energy, while the second sharply restricted energy intake across all macronutrients. The third condition mirrored the energy restriction but preserved protein levels comparable to the adequate-energy diet.

After five days, both restricted diets showed reduced levels of P1NP, a marker associated with bone formation. Notably, the protein-preserved condition also indicated increased levels of CTX-1, which signifies bone breakdown, during recovery from exercise. The researchers noted that maintaining protein intake did not prevent these negative effects.

The study suggests that emphasizing protein may overlook the significance of carbohydrates in the diet. When protein was maintained within a caloric deficit, carbohydrate availability diminished further, leading to more pronounced disturbances in bone metabolism and inflammatory markers compared to adequate energy intake. The authors indicated that the findings might highlight issues with reduced carbohydrate intake rather than increased protein levels.

However, the comparisons between the two restricted diets were largely inconclusive, meaning the study cannot definitively say that high protein intake led to worse outcomes.

These findings are particularly relevant for active adults aiming to lose fat while preserving training and long-term health. While the study assessed blood biomarkers rather than actual bone density loss or fractures, its small sample size and brief duration limit conclusions about permanent skeletal damage. Still, the results pose important questions regarding whether maintaining protein intake is sufficient when caloric intake is insufficient to support exercise and recovery.

This article was originally published by Men's Journal on October 12, 2026.