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Deep Dive: Comparing different protein sources' impact on bone turnover

Eating more plants can be healthy but may negatively impact bone health due to lower protein, calcium, and vitamin D intake. This trial examined how the same amount of protein from animal or plant sources affected bone turnover.

Study under review: Partial Replacement of Animal Proteins with Plant Proteins for 12 Weeks Accelerates Bone Turnover Among Healthy Adults: A Randomized Clinical Trial

Introduction

Nutrition plays an important role in the development and maintenance of bone tissue. Bone undergoes continuous remodeling, meaning that it is constantly broken down and rebuilt, so an adequate supply of raw materials, or nutrients, are needed to support bone health. The amount of these nutrients provided to the body depends on diet and age.

Bone mass peaks around 25–30 years of age[1] and stays about the same for the next 10–20 years, then gradually declines. These changes are very slow, with people over 50 losing 0.5–1% of bone mineral density per year[2].

Researchers use biomarkers that change more rapidly than these slow changes in bone mass to help predict long term changes in bone mass. A few of these markers include parathyroid hormone (PTH), serum intact procollagen type 1 amino-terminal propeptide (S-iP1NP), and serum collagen type 1 cross-linked C-terminal telopeptide (S-CTX). These, in combination with urinary calcium (U-Ca) and urinary phosphate (U-Pi) levels, can be used to estimate bone resorption and formation rates. See Figure 2 and the What Were the Findings? section for more on these markers.

Micronutrients also play a role in bone formation and development. For example, insufficient vitamin D status can lead to osteoporosis and fractures[3]. Vitamin D is available through the diet in foods like fatty fish, but is mostly produced by the body after sunlight exposure. Calcium is also required to improve or maintain bone mineral density. Calcium is a major component of bone[4] that provides skeleton strength and structure, and bone serves as a reservoir for the body’s calcium pool. Calcium intake affects bone health by increasing bone mineral density, which can serve as a surrogate endpoint for fracture risk. Furthermore, increasing calcium intake through the diet or by taking supplements provides a small, non-progressive benefit[5] in bone mineral density, but may increase the risk of coronary heart disease[6]. Lastly, phosphorus is important for bone health, while excessive dietary phosphorus has been associated with adverse effects on bone[7] and mineral metabolism.

Some evidence suggests that plant-based diets result in lower bone mineral density[8] and a higher risk of fractures. Plant-based diets can lead to low intakes of protein, vitamin D, and calcium, which translates to a greater risk of low bone mineral density[9]. On the other hand, plant-based diets provide benefits that might protect against cardiovascular diseases and possibly type 2 diabetes[10]. There is also some evidence that a high ratio of animal to vegetable protein can increase the rate of bone loss[11]. In terms of total protein, some experts think protein levels above the current RDA may be beneficial for reducing bone loss and hip fracture risk[12]. The goal of the study under review was to help determine how animal and plant-based diets compare in terms of their effects on bone health.

Observational studies aren’t very helpful for comparing the effects of diets on bone health,, since they’re likely to be confounded by a major variable: protein intake levels. Since one of the major differences between animal and plant-based diets is the average overall protein intake, with people consuming an animal diet generally consuming more protein, studies looking at people eating plant-rich diets without controlling for overall protein consumption could produce misleading results.

Protein provides the structural matrix of bone, influences IGF-1 levels, and can play a role in calcium kinetics. Overall, the net effect of protein on bone seems to depend on both the amount and type of protein[13]. This is why dietary choices may have downstream effects on bone health. An important component of the current study was to standardize participant protein intake, so that any differences found could be ascribed to the type of protein, not overall intake.

The current study is one of the first to put animal-based diets and plant-based diets head to head in healthy adults to see how they compare in bone turnover and mineral metabolism. There was even a third group in the study, which consumed a half plant-based and half animal-based diet to allow researchers to explore possible dose-response effects.

Macronutrients and micronutrients play an important role in bone formation and resorption. However evidence is mixed on whether animal-based diets or plant-based diets are better for bone health. The current study compares the effects of the two, along with a 50/50 group, on bone biomarkers for remodeling.

What was studied?

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What were the findings?

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The bigger picture

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