Scientists have found that animal protein consumption positively correlates with bone density in older adults. Things become more complicated regarding plant protein .
Too little or too much?
Just how much protein people should consume has been a point of contention. Decades-old research, performed mainly in underdeveloped communities, shows that protein deficiency can be harmful . Since then, the prevalence of this problem has greatly diminished, and the opposite, protein overconsumption, has come into the spotlight.
A widely cited study from 2014 showed that low protein intake is associated with a major reduction in cancer and overall mortality in people younger than 65. However, the opposite was true for older people . This led the authors to suggest that “low protein intake during middle age followed by moderate protein consumption in old subjects may optimize healthspan and longevity.” The study also showed that the increase in mortality only correlated with animal protein intake.
Research in animals seems to support this hypothesis. Protein restriction has emerged as one of the few proven methods of extending lifespan in various model organisms, including rodents . Clinical studies in humans are scarce, but one from last year showed that protein restriction can alleviate metabolic dysfunction .
Animal vs plant protein
Even if high protein intake might be overall unhealthy, protein is known to be important for bone health, which deteriorates with age, leading to osteoporosis and frailty. This new study looked for a connection between protein intake and bone density in elderly people by analyzing results of four earlier trials.
First, the scientists performed a cross-sectional analysis of the studies’ population: they analyzed baseline values of protein intake and bone mass density (BMD) for all 1570 participants. Participants had a median age of 71, and 56% were female. In their analysis, the researchers adjusted for several confounding factors: age, sex, physical activity level, smoking status, and alcohol intake.
Both total protein intake and animal protein intake were associated with higher total body and spine BMD, while plant protein intake showed a negative correlation. Interestingly, a stronger association was detected between total protein intake and spine BMD in females than in males, but the opposite was true for plant protein intake, which seemed to be more detrimental for spine BMD in males than in females.
In a subgroup analysis of participants with an adequate calcium intake and serum vitamin D levels, the association between total protein intake and BMD became stronger. According to the researchers, this could mean that animal protein improves bone density independently of calcium and vitamin D, as it might be more digestible and have a more complete amino acid profile (some plant-based foods, such as quinoa, also contain all essential amino acids).
However, the negative association between BMD and plant protein intake became insignificant after adjusting for calcium and vitamin D. This, in turn, could mean that plant protein isn’t inherently harmful for bone health, but its consumption negatively correlates with calcium and vitamin D intake.
Little effect from short-term interventions
The researchers then performed longitudinal analyses of the interventions used in the earlier trials. The largest of the four, NU-AGE, did not involve a ‘true’ protein intervention. Instead, its participants were asked to adhere to the Mediterranean diet. Consequently, it was excluded from the longitudinal analysis, which left the researchers with just 340 participants that did receive various protein interventions. However, none of those interventions, which spanned between 12 and 24 weeks in length, significantly improved BMD.
In one trial, after 24 weeks of protein supplementation plus resistance exercise, a noticeable but statistically insignificant improvement was observed. The researchers predict that “a larger sample size or a longer exposure to a protein and exercise intervention would increase the magnitude of effect”, but this, of course, requires proof.
In conclusion, we found an association between higher total and animal protein intake with higher total body and spine BMD. In contrast, higher plant protein intake was associated with a lower total body and spine BMD. Research is warranted to investigate further the added value of dietary protein alongside calcium and vitamin D for BMD improvement, especially in osteopenic or osteoporotic individuals. Furthermore, more research on the impact of a plant-based diet on bone health is needed.
While the design of this study has many limitations, it expands our limited knowledge of the relationship between protein intake and bone density in older adults. The current notion that older people should increase their protein intake (plus, probably, physical activity) seems to hold, but questions arise about the contribution of plant protein, which should be resolved by future, more rigorous, studies.
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 Groenendijk, I., Grootswagers, P., Santoro, A., Franceschi, C., Bazzocchi, A., Meunier, N., … & De Groot, L. (2022, September). PROTEIN INTAKE AND BONE MINERAL DENSITY: CROSS-SECTIONAL RELATIONSHIP AND LONGITUDINAL EFFECTS OF PROTEIN SUPPLEMENTATION IN A COMBINED COHORT OF FRAIL, MALNOURISHED AND HEALTHY OLDER ADULTS. In AGING CLINICAL AND EXPERIMENTAL RESEARCH (Vol. 34, No. SUPPL 1, pp. S270-S271).
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 Levine, M. E., Suarez, J. A., Brandhorst, S., Balasubramanian, P., Cheng, C. W., Madia, F., … & Longo, V. D. (2014). Low protein intake is associated with a major reduction in IGF-1, cancer, and overall mortality in the 65 and younger but not older population. Cell metabolism, 19(3), 407-417.
 Mirzaei, H., Raynes, R., & Longo, V. D. (2016). The Conserved Role for Protein Restriction During Aging and Disease. Current opinion in clinical nutrition and metabolic care, 19(1), 74.
 Ferraz-Bannitz, R., Beraldo, R. A., Peluso, A. A., Dall, M., Babaei, P., Foglietti, R. C., … & Foss-Freitas, M. C. (2022). Dietary protein restriction improves metabolic dysfunction in patients with metabolic syndrome in a randomized, controlled trial. Nutrients, 14(13), 2670.