In a comment to last week's post Choose Vegetable Calcium Over Animal Calcium Helena sought Dr. Fuhrman's thoughts about Gabe Mirkin's opinion on animal protein and calcium absorption. Here's what Dr. Fuhrman had to say:
Gabe Mirkin, M.D.: Studies done many years ago suggested that eating a lot of protein increases calcium loss in the urine and therefore it was thought that eating protein weakens bones by taking calcium out of them. However, recent studies show that eating protein increases calcium absorption so the extra calcium in the urine comes from increased absorption, not from being take out of bones. Reports in the American Journal of Clinical Nutrition (Volume 78, Issue 3, 2003) show that eating plenty of protein and lots of foods from plants helps to keep bones strong. Most scientists now feel that a very low-protein diet can cause osteoporosis, while a moderately high-protein diet may help to prevent it.
Dr. Mirkin is not giving enough information to understand the entire story. Let's review some of the evidence from the studies in question and come to some recommendations that are more specific. Even though excessive consumption of animal protein over many years does encourage bone disease and bone loss and a higher consumption of vegetable protein over animal protein is conducive to less osteoporosis it is also true that too little protein in later life (after age 70) when digestive efficiency declines could lead to less calcium absorption, muscle wasting and bone thinning. Studies suggest that both too much protein (animal protein) and too little protein are unfavorable to bone mass. Therefore, it may be advisable as we age to assure adequate protein intake and pay more attention to it especially if we find a decrease in weight and muscle mass with later life aging. Let's review the following relevant studies:
Rapuri PB ; Gallagher JC ; Haynatzka V. Protein intake: effects on bone mineral density and the rate of bone loss in elderly women. Am J Clin Nutr. 2003; 77(6):1517-25.
This study investigates the associations of dietary protein intake with baseline bone mineral density (BMD) and the rate of bone loss over 3 y in postmenopausal elderly women. It finds " no association seen between protein intake and the rate of bone loss (in a three-year period). The study did note that all these elderly women did not eat much protein in general and the highest range of protein consumption was only about 70 grams a day. Elderly women with low protein intake at baseline (before the study began were noted to have lower bone mineral density, likely because of a little less muscle mass), because muscle mass and strength is linked to bone mass. This study does not tell us much more than logic and common sense would. Protein digestive efficiency declines in the elderly and getting adequate protein is necessary for adequate muscle and bone mass in later life.
Sellmeyer DE ; Stone KL ; Sebastian A ; Cummings SR. A high ratio of dietary animal to vegetable protein increases the rate of bone loss and the risk of fracture in postmenopausal women. Study of Osteoporotic Fractures Research Group. Am J Clin Nutr. 2001; 73(1):118-22.
This study followed over 1000 elderly women over a 7 - 10 year period. The study scientists concluded that animal foods provide predominantly acid precursors, whereas protein in vegetable foods is accompanied by base precursors not found in animal foods. Imbalance between dietary acid and base precursors leads to a chronic net dietary acid load that may have adverse consequences on bone. The study found increase bone loss and risk of hip fracture in those with a higher ratio of animal protein to vegetable protein. The study scientists concluded that an increase in vegetable protein and a decrease in animal protein may decrease the risk of hip fracture in the elderly. This study illustrates the importance of getting the majority of calories (and protein) from plant sources and cautions that protein from animal food sources should be a minor contributor to total protein requirements for maximizing bone health. Even if some animal protein is added to a diet, it should not be the major source of protein, green vegetables, beans, nuts and seeds, should be the major source of protein and animal source a minor source.
Devine A ; Dick IM ; Islam AF ; Dhaliwal SS ; Prince RL Protein consumption is an important predictor of lower limb bone mass in elderly women. Am J Clin Nutr. 2005; 81(6):1423-8.
This study compared protein intake with bone mineral density in the heel in 75 year old women and illustrated at this later age the higher range of protein intake (above 80 grams a day) had better bone mineral density. They did not follow hip fracture rates. This study shows that as we get older it is important to assure adequate intake of protein (as well as other nutrients).
Kerstetter JE ; O'Brien KO ; Insogna KL. Dietary protein, calcium metabolism, and skeletal homeostasis revisited. Am J Clin Nutr. 2003; 78(3 Suppl):584S-592S.
This study showed reduction in calcium absorption and very low protein intakes below .8 mg per kg, and support the other evidence that elderly women should strive to maintain their total protein intake above a gram per kilogram of body weight.
- It is important to keep exercising to maintain muscularity in our later years.
- To maximize mineral absorption and bone health, supplements of D and many other nutrients are advisable and become increasingly important as we age as digestive efficiency may decrease.
- Vegetable protein sources, beans, edamame, nuts, seeds, and green vegetables are the most favorable sources of protein for long life and bone health. Adequate protein intake is important and vegetarian diets should be designed so that adequate protein intake is consumed. Vegetarian diets where the vast majority of calories come from grains and roots such as rice and potato are not ideal for long-term health.
- Attention to protein intake with supplements or some animal products may be a useful to maintain peek muscle and bone mass to prevent the occurrence of increasing frailty, common in the elderly, but for reasons of cardiovascular health, diets should be designed so that animal products are used sparingly and not the major source of protein in the diet.