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Old Thu, Aug-01-02, 09:17
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Voyajer Voyajer is offline
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Default Protein Lowers Risk of Heart Disease--But because of fat-fear they don't recommend it

American Journal of Clinical Nutrition, Vol. 70, No. 2, 221-227, August 1999

Dietary protein and risk of ischemic heart disease in women
Frank B Hu, Meir J Stampfer, JoAnn E Manson, Eric Rimm, Graham A Colditz, Frank E Speizer, Charles H Hennekens and Walter C Willett

Background: Ingestion of animal protein raises serum cholesterol in some experimental models but not in others, and ecologic studies have suggested a positive association between animal protein intake and risk of ischemic heart disease. Prospective data on the relation of protein intake to risk of ischemic heart disease are sparse.

Objective: The objective was to examine the relation between protein intake and risk of ischemic heart disease.

Design: The study was a prospective cohort study.

Results: We examined the association between dietary protein intake and incidence of ischemic heart disease in a cohort of 80082 women aged 34–59 y and without a previous diagnosis of ischemic heart disease, stroke, cancer, hypercholesterolemia, or diabetes in 1980. Intakes of protein and other nutrients were assessed with validated dietary questionnaires. We documented 939 major instances of ischemic heart disease during 14 y of follow-up. After age, smoking, total energy intake, percentages of energy from specific types of fat, and other ischemic heart disease risk factors were controlled for, high protein intakes were associated with a low risk of ischemic heart disease; when extreme quintiles of total protein intake were compared, the relative risk was 0.74 (95% CI: 0.59, 0.94). Both animal and vegetable proteins contributed to the lower risk. This inverse association was similar in women with low- or high-fat diets.

Conclusions: Our data do not support the hypothesis that a high protein intake increases the risk of ischemic heart disease. In contrast, our findings suggest that replacing carbohydrates with protein may be associated with a lower risk of ischemic heart disease. Because a high dietary protein intake is often accompanied by increases in saturated fat and cholesterol intakes, application of these findings to public dietary advice should be cautious.

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Rebuttal

American Journal of Clinical Nutrition, Vol. 71, No. 3, 849-850, March 2000
Letters to the Editor

Animal protein and ischemic heart disease
T Colin Campbell
Cornell University, Division of Nutritional Sciences, N204 MVR, Ithaca, NY 14053, E-mail: tcc1~cornell.edu

Dear Sir:

The important paper by Hu et al (1) on findings from the Nurses' Health Study (NHS) deserves comment, especially now that these findings have been publicized in the popular media (Raloff J. High-protein diets may help hearts. Science News 1999;156:86). In my opinion, several highly questionable interpretations of the data have been made. The study by Hu et al purports to show, in contrast with previous evidence, that increased consumption of animal protein is associated with decreased risk of ischemic heart disease (IHD), although this observation is only marginally significant.

The importance of this paper can hardly be overemphasized. It not only reports evidence mostly contrary to the existing literature, but also reveals an important shortcoming of this widely reported study, both in the experimental design and in the method of analysis. Dietary protein (as quintile means) ranged from 14.7% to 24.0% of energy and an overwhelming 80% of this protein was from animal sources; these findings suggest the consumption of a virtually carnivorous diet. This becomes even more evident in view of the relatively high intake of total fat (36–41% of energy, as quintile means) and the very low intake of fruit and vegetables and dietary fiber of the study cohort. Even the intake of protein in the lowest quintile was 50% more than recommended (3), perhaps close to 90–100% more than the requirement when the unusually high concentration of high-efficiency animal protein and the statistical construction of the recommended dietary allowances are taken into account.

This dietary experience contrasts sharply with the findings of the original international correlation studies (4, 5) that showed impressive associations between selected dietary factors and chronic degenerative diseases. The contrast between the diets of the cohorts in the international correlation studies and in the NHS can be illustrated by comparing the respective protein-fat associations in these cohorts. The correlation of total fat with animal protein in the international correlation study diets was 90–95%, whereas in the NHS dietary range, the correlation was small and nonsignificant (15%). Furthermore, both animal fat and animal protein, but not plant fat, were found to be tightly associated with breast cancer in the international correlation studies.

These earlier findings from the international correlation studies suggest that the incidence of chronic degenerative disease was associated with animal-based food consumption over a broad range of intakes—from very low amounts—at the expense of plant-based food consumption, and not necessarily with the consumption of any particular nutrient or nutrient group. The NHS dietary experience, in contrast, differs because of the uniformly high consumption of animal-based foods, thus severely limiting a meaningful investigation of the comprehensive effects either of this food group or of individual foods and related nutrients within this group. As for virtually every other study of Western subjects, the NHS therefore does not permit a discriminating analysis of the diet-disease associations originally observed in the international correlation studies; the necessary range of intake of these foods and their respective nutrients is missing. Not only is the detection of meaningful disease-related associations for individual foods and their indicator nutrients compromised, but the prospect of making paradoxical observations is increased and the investigation of the more comprehensive dietary effects is ignored.

-----------------------------------

ARGUMENT


American Journal of Clinical Nutrition, Vol. 71, No. 3, 850-851, March 2000

Letters to the Editor

Reply to TC Campbell
Frank B Hu and Walter Willett

Dear Sir:

Although dietary protein has been the focus of controversy regarding several popular diets, scientific data on the effects of protein intake on the development of chronic disease are limited. International studies suggest a positive correlation between animal protein intake and ischemic heart disease rates across countries (1). Therefore, we tested this hypothesis directly in the Nurses' Health Study (2). By analyzing repeated measures of dietary data over 14 y of follow-up, we firmly rejected the hypothesis that high protein intakes increase the risk of ischemic heart disease. In contrast, our data suggest a modest inverse association for both animal and vegetable protein intake. This finding is compatible with results of metabolic studies indicating improved blood lipid profiles when protein replaces carbohydrates (3). In the conclusion, we cautioned the application of these findings to public dietary advice because a high dietary protein intake is often accompanied by high saturated fat and cholesterol intakes.

Campbell questioned the validity of our findings because they contradict the results of international correlation studies on animal product consumption and disease rates. However, international correlations such as those cited by Campbell are intractably confounded by other dietary and lifestyle factors associated with economic affluence in different countries; differences in physical activity, body fat, and smoking are particularly important. Also, the food disappearance data used in most of the calculations may be more indicative of food wastage within a country than actual consumption. Correlational studies conducted within a country can usually provide more credible data than international comparisons because of relatively homogeneous populations and the possibility of collecting data on potential confounding variables at individual levels. A survey of 65 counties in rural China, however, did not find a clear association between animal product consumption and risk of heart disease or major cancers (4).

Prospective cohort studies of individuals, in which diet is assessed before the occurrence of disease, are typically considered to be the strongest nonrandomized design because it is possible to control for other known risk factors. One common misperception is that the dietary experience of a single population in a typical prospective cohort study is too homogeneous to detect associations with disease risk. In the Nurses' Health Study, however, we identified several important dietary factors for risk of ischemic heart disease, including trans fatty acids (5), the ratio of polyunsaturated to saturated fat (5), -linolenic acid (6), cereal fiber (7), nuts (8), whole-grain products (9), and fruit and vegetables (Kumudi unpublished observations, 2000).

Because high protein intakes are associated with other dietary variables and lifestyle factors, we conducted careful statistical analyses to adjust for these variables. However, the multivariate relative risks were similar to the age-adjusted ones, suggesting that confounding by other dietary variables and lifestyle factors was likely to be minor. Also, in stratified analyses according to levels of smoking and exercise and intakes of dietary fat and fiber, the modest inverse association with dietary protein persisted. Campbell suggests that the independent effects of various nutrients cannot be teased out because of their high correlations. This assertion is not substantiated by our analyses showing opposite associations with risk of coronary heart disease for different types of dietary fat that are intercorrelated (5). The large sample size, the long follow-up, and the multiple dietary measurements made in the Nurses' Health Study provide high power to examine independent effects of many individual nutrients. Although we agree that overall dietary patterns are also important in determining disease risk (10), we believe that identification of associations with individual nutrients should be the first step because it is the specific compounds or groups of compounds that are fundamentally related to the pathophysiology of the disease. Specific components of diet can be modified, and individuals and the food industry are actively doing so. Understanding the health effects of specific dietary changes, which Campbell refers to as "reductionism," is therefore an important undertaking.
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