Increased protein intake linked to stronger bones, but the source might matter Original paper

Protein intake plays an essential role in bone health via several different mechanisms, and it may also be protective against osteoporosis and osteopenia — especially for older adults at a greater risk for these conditions. Some observational studies have found that protein intake is associated with a higher bone mineral density (BMD) and reduced hip fracture risk.^[1] Interestingly, one study found that animal protein, but not plant protein, was associated with higher BMD in women.^[2] Thus, the effects of protein source on BMD warrant further investigation.

Despite these associations, there has been little longitudinal research on the direct effect of protein intake on BMD, and more evidence is required to confirm the somewhat conflicting observational data. Does increasing protein intake lead to stronger bones?

This study integrated data from 4 studies that each investigated the association between protein intake and BMD and the effect of a protein-based intervention (with or without exercise) on BMD. A total of 1570 older adult participants were included (median age of 71; 56% women, 44% men) who were classified as healthy, at risk of malnutrition, or frail/prefrail.

The following interventions were used in the 4 included studies:

• NU-AGE: A 1-year intervention with a Mediterranean-style diet designed to meet older adult nutrition needs
• ProMO: 12 weeks of oral nutrition supplementation including 24 grams of casein protein (intervention 1) or 22 grams of whey and casein protein (1:1 ratio) and 7 grams of free branched-chain amino acids (intervention 2)
• ProMuscle: 24 weeks of protein supplementation, resistance exercise, or protein + resistance exercise (30 grams per day for all interventions given as a milk protein concentrate)
• PiP (ProMuscle in Practice): 24 weeks of increased protein intake (a minimum of 25 grams per meal) + resistance exercise

The primary study outcomes were total BMD and spine BMD. Cross-sectional analyses were conducted to determine the association of the participants’ baseline protein intake with BMD. Longitudinal analyses were conducted to determine the impact of increased protein (with or without exercise) for 12–24 weeks on BMD.

After adjusting for variables such as age, sex, physical activity, smoking, alcohol, and calcium and vitamin D intake, total protein and animal protein intake were both associated with a higher total body and spine BMD, whereas plant protein was associated with a lower total body and spine BMD.

Total protein intake was more strongly associated with spine BMD in women, whereas plant protein intake had a stronger negative association with spine BMD in men.

In the longitudinal analyses, 12–24 weeks of increased protein intake, with or without resistance exercise, did not improve total or spine BMD; the results were similar when women and men were analyzed separately.

On average, the participants in this study had normal BMDs for their age, only 12% had a diagnosis of osteoporosis, and only 1.8%–4.3% experienced a hip fracture or other fracture in the past 12 months.

As such, it may be that protein and exercise interventions have little effect on increasing BMD in adults with normal BMD; instead, they may only be effective in adults at a higher risk for conditions related to bone health.