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U.S. Department of Agriculture, Agricultural Research Service, Grand Forks Human Nutrition Research Center, Grand Forks, ND 58202-9034
Dear Editor,
We appreciate the thoughtful remarks by Dr. Sebastian and also Drs. Remer and Manz about our recent article in The Journal of Nutrition (1). Their misgivings about the characteristics of the diets tested in some ways parallel our own concerns that broad conclusions about the adverse effects of high meat diets on calcium homeostasis are too often based upon short-term studies with purified proteins that do not test meat at all. Our objective was to perform a practical test of the effects of typical high and low meat diets on calcium homeostasis.
We replaced meat fat with vegetable fat and meat protein with low phytate complex carbohydrates as practical isoenergetic substitutions, without an attempt to maximize the difference in renal net acid excretion (RNAE). We agree that our diets, with potential renal acid loads (PRAL) of 60 versus 30 mEq/d, were both net acid producing. However, calcium retention from these two diets was similar and our extensive assessment of bone biomarkers did not indicate an adverse effect on calcium homeostasis. Dr. Sebastian’s suggestion that calcium homeostasis would be improved by substituting fruits and vegetables for either meat or grains needs to be tested within the context of a practical diet. This could present a challenge because isoenergetic replacement of the 49 g/d of meat protein with fruits and vegetables would result in a much greater total food volume than with high starch foods (500 g carrots, 8 tomatoes or 400 g of orange sections versus 3 slices of bread or 180 g of cooked rice), possibly influencing long-term acceptability of the diet, and applicability of the findings. Dr. Sebastian’s suggestion of substituting energy-dense nutrient poor fats and sugars for meat has been tested and did not affect urinary calcium loss (2). Substitution of soy protein for meat also did not affect urinary calcium excretion or whole body calcium retention (3).
Drs. Remer and Manz suggest that a PRAL of 70–80 mEq/d (equivalent to RNAE of 100 mEq/d) is required for hypercalciuria and that the protein content of our high meat diet (117 g total protein, 297 g meat) was insufficient for such an effect. Although we agree that greater differences in PRAL can be achieved using purified amino acids or proteins, or possibly by following high protein weight loss plans or using protein supplements, we also maintain that 297 versus 45 g/d of lean meat in diets with 20 versus 12% of energy as protein is a practical comparison of extremes in meat intake for postmenopausal women.
Our statistical analysis (1) does not support the suggestion by Drs. Remer and Manz that urinary potassium (and therefore dietary potassium) changed with time. To maintain a constant diet composition, the purchase and preparation of the weighed food items were carefully monitored, using the same production lots as much as possible. The diet-related differences in urinary sulfate and ammonium excretion were also unaffected by time, providing further evidence of dietary compliance. Unlike urinary potassium, RNAE did change over time during each dietary treatment, indicating that calculations of PRAL (4) may need to be modified to account for adaptation in absorption and excretion of acid- and base-forming elements.
Recently, the effect of dietary protein on calcium metabolism has been the subject of much scientific debate (5–7) and the comments by Drs. Sebastian, Remer and Manz highlight the need for further research in this area. We agree that one cannot make a "generic" comparison of high versus low meat diets, and that interpretation of the results is limited by the test conditions. Accordingly, conclusions about the calciuric effects of meat protein should be based upon practical comparisons using meat and common isoenergetic substitutions.
Manuscript received 10 July 2003.
LITERATURE CITED
1. Roughead, Z. K., Johnson, L. K., Lykken, G. I. & Hunt, J. R. (2003) Controlled high meat diets do not affect calcium retention or indices of bone status in healthy postmenopausal women. J. Nutr. 133:1020-1026.
2. Hunt, J. R., Gallagher, S. K., Johnson, L. K. & Lykken, G. I. (1995) High-versus low-meat diets: effects on zinc absorption, iron status, and calcium, copper, iron, magnesium, manganese, nitrogen, phosphorus, and zinc balance in postmenopausal women. Am. J. Clin. Nutr. 62:621-632.
3. Roughead, Z. K., Hunt, J. R., Lykken, G. I. & Johnson, L. K. (2003) Substituting soy protein for meat protein did not affect calcium (Ca) retention or biomarkers of bone and cardiovascular health in a controlled feeding study of healthy postmenopausal women. FASEB J. 17:A705.
4. Remer, T. & Manz, F. (1994) Estimation of the renal net acid excretion by adults consuming diets containing variable amounts of protein. Am. J. Clin. Nutr. 59:1356-1361.
5. Heaney, R. P. (2002) Protein and calcium: antagonists or synergists?. Am. J. Clin. Nutr. 75:609-610.
6. Massey, L. K. (2003) Dietary animal and plant protein and human bone health: a whole foods approach. J. Nutr. 133:862S-865S.
7. Roughead, Z. K. (2003) Is the interaction between dietary protein and calcium destructive or constructive for bone?. Summary. J. Nutr. 133:866S-869S.
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