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How does dietary protein intake affect bone health?

Bone health is becoming a popular topic of interest given the increasing age of our population and increased prevalence of osteoporosis and sarcopenia. Diets high in protein have often been discouraged in individuals with poor bone health due to the associated increases in urinary calcium and acid production. So, if we are concerned with bone health, should we be increasing or decreasing our dietary protein intake?

It is well known that a daily supply of protein (~1.2 g per kg per day) is required for bone metabolism and normal bone maintenance. However, high protein intakes in Western diets are all too common and are cited as a potential risk factor for osteoporosis. In addition, research generally agrees that lower protein intakes (0.8 g per kg body weight) cause a release of calcium from bone and a reduction in bone maintenance.

So what about the type of protein ingested? IGF-1 is a hormone that plays an important role in bone metabolism. A diet containing predominantly animal protein is associated with higher levels of IGF-1 and therefore, increased bone mineralization. Soy, on the other hand, has been linked with lower levels of IGF-1.

To make matters even more complicated, the acid-base balance of our diet also contributes to the relationship between protein intake and bone health. The western diet is predominantly abundant in acid-forming foods such as meat, fish, eggs and cereal. Alkaline foods include vegetables and other plant-based foods. Therefore, the association made between high protein intakes and poor bone health may in fact be a reflection of inadequate intake of fruits and vegetables and not a high protein diet per se.

During weight loss, a high protein diet is known to preserve bone mineral more effectively than a diet low in protein. Protein intake increases the risk of increasing urinary calcium loss but it is not clear whether this results in a negative calcium balance. Dairy intake, or calcium intake to be specific, may also play an important role in bone health, particularly during weight loss.

What remains clear is that we need to look at the bigger picture and not just protein intake alone. The protein source, calcium intake, calorie restriction and pH of the diet all heavily influence bone health. High protein diets are associated with greater bone mass and fewer fractures when calcium intake is adequate. Certainly, we should spend more time focussing on our intake of fresh fruit and vegetables and calcium rather than our protein intake alone.