Many Western diets provide more protein than the body needs, causing excess nitrogen to be excreted as urea in urine. The excess nitrogen has been linked in some studies with reduced kidney function in old age. Some, but not all studies have found that when people have impaired kidney function, restricting dietary intake of protein slows the rate of decline of kidney function.768
Excessive protein intake also can increase excretion of calcium, and some evidence has linked high-protein diets with osteoporosis,769 particularly regarding animal protein.770 On the other hand, some protein is needed for bone formation. A double-blind study showed that elderly people whose diets provided slightly less than the recommended amount of protein suffered less bone loss if they consumed an additional 20 grams of protein per day.771 A doctor can help people assess their protein intake and needs.
Amino acids include several different nutrients, each of which has the potential for side effects. Look up the unique side effects for each and discuss the potential benefits and risks with your doctor or pharmacist:
- Essential: isoleucine, leucine, lysine, methionine, phenylalanine, valine
- Semi-essential: histidine
- Nonessential: arginine, alanine, cysteine, glutamine, glutamic acid, glycine, tyrosine