Why High Protein Diets Cause Bone Loss

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Author: Leo Akin
June, 2015

Introduction

One third of human bone mass is made up of protein. Therefore, many believe that if protein is an important composition of the bone then increased protein intake should make the bone stronger and prevent osteoporosis. However, just like the similar calcium argument, this protein-for-osteoporosis theory is wrong. It took multiple large and long studies investigating the relationship between diet and the risk of osteoporosis before researchers were able to conclude that high protein diets are strongly linked to increased frequency of bone fractures later in life.

This is a rather curious finding because proteins not only make up a large portion of bone mass, they also promote the absorption of calcium (the most abundant mineral in the bone). But the evidence is clear that increasing your protein intake raises the risks of osteoporotic bone fractures. So, how can proteins be harmful to bone health? The answer lies in the nature of protein metabolism.

The Acid Load of Proteins

High protein diet represents a considerable acid load. When proteins are split into amino acids, they form acidic metabolites that end up in the blood. Therefore, the increased protein metabolism prompted by high protein diets increase blood acidity. Proteins can be rated by their acid loads and this classification corresponds to the degree to which they raise blood acid level. Meats usually have an acid load (PRAL or potential renal acid load) of 9.5 while fish are rated 8.0. In addition, milk and dairy products have a broad range of acid load values but they can be rated as high as 23.On the other hand, plant-based proteins usually have low acid loads with vegetables ranked at 2.8 and fruits rated at 3.1.

Blood acid-base balance needs to be tightly controlled. Therefore, when proteins raise blood acidity, the body quickly recruits alkalizing agents to neutralize the acidic metabolites of proteins. Unfortunately, the most important alkalizing agent in the body is calcium and because the bone is the largest calcium store in the body, the body strips calcium from there. Therefore, long-term consumption of high protein diets will slowly but surely deplete the bones of calcium.

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Will Taking Calcium Supplements Help?

Some health experts have suggested that combining calcium supplements with high protein diet may stop this progressive bone loss. However, every study investigating such combinations found that bone mineral density continued to fall despite the increased calcium intake. This is a further confirmation that calcium supplementation ironically does precious little for bone health and does not lower the risk of osteoporotic bone fractures. Among dietary proteins, meat is the most closely studied animal protein in osteoporosis research. While there is no consensus regarding the extent to which meat consumption affects bone health, most health experts agree that meat and animal proteins encourage bone loss and are only best consumed when combined with fruits and vegetables.

Why Plant Protein Is Better Than Animal Protein

Although all proteins increase blood acidity, plant proteins represent significantly lower acid loads than animal proteins. Studies show that plant proteins have lower acid loads because they contain fewer sulfur-containing amino acids than animal proteins. The sulfur parts of amino acids such as methionine and cysteine readily form acidic metabolites. Besides the difference in amino acid compositions of plant and animal proteins, plants also have lesser impact on blood acidity because of their high composition of alkalizing minerals.

Vegetables and fruits are especially rich in potassium and magnesium. These alkalizing minerals can be used to neutralize the acidic metabolites of proteins in the blood. Therefore, they prevent the body from sacrificing the calcium in bones. The protection provided by potassium and magnesium against bone loss has been demonstrated repeatedly. By giving common potassium and magnesium supplements without lowering protein intake, researchers found that these alkalizing agents protected calcium and maintained bone mineral density. Epidemiological studies also confirmed that plant proteins were better than animal proteins for bone health. Such studies established that the prevalence of osteoporosis among vegans, who abstained from meat and other animal proteins, was lower than for the general population.

Other epidemiological studies also confirmed that the incidence of osteoporosis was lower in countries known for their vegetable diets than in those with high meat and dairy consumption. Lastly, a 2001 study published in The American Journal of Clinical Nutrition compared the protein intakes of over 1,000 postmenopausal women with the X-ray results of their bone masses. The results of the study showed that the women who ate more vegetables than animal proteins had lower risks of femur and hip bone fractures than those who ate a lot of animal proteins and little plant proteins.

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