Correspondence

Dietary Patterns and Blood Pressure

N Engl J Med 1997; 337:636-638August 28, 1997

Article

To the Editor:

In 1982 we provided evidence that a diet low in dairy products may increase the risk of hypertension.1 In 1984, using data from the First National Health and Nutrition Examination Survey, we reported that in the United States a diet low in calcium and potassium resulted in a dose-related increase in systolic blood pressure, the likelihood of hypertension, or both.2 The latter report documented that a low intake of dairy products, followed by a similarly low consumption of fruit and vegetable juice, was the dietary pattern most predictive of hypertension. We further showed that this benefit of consuming dairy products, fruits, and vegetables did not depend on age, race, sex, or weight.

Appel et al. (April 17 issue)3 compellingly confirmed the validity of our observational data. The provision of recommended amounts of low-fat dairy products as well as fruits and vegetables to the participants in the Dietary Approaches to Stop Hypertension (DASH) trial reduced blood pressure, as we had predicted.2 This scientific agreement between properly executed observational studies and a well-controlled clinical trial gives strong evidence that adequate mineral intake profoundly benefits our society in terms of improved blood-pressure control. As the DASH authors postulated, there would probably be a reduction in associated cardiovascular events if we focused on the critical importance of correcting the inadequate mineral and fiber content of the American diet. This nutritional public health measure has already been accepted for the prevention of osteoporosis and cancer.

David A. McCarron, M.D.
Oregon Health Sciences University, Portland, OR 97201-2940

3 References

1
McCarron DA, Morris CD, Cole C. Dietary calcium in human hypertension. Science 1982;217:267-269
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2
McCarron DA, Morris CD, Henry HJ, Stanton JL. Blood pressure and nutrient intake in the United States. Science 1984;224:1392-1398
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3
Appel LJ, Moore TJ, Obarzanek E, et al. A clinical trial of the effects of dietary patterns on blood pressure. N Engl J Med 1997;336:1117-1124
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To the Editor:

Appel and colleagues report an unexpected observation — a prominent reduction in the urinary excretion of calcium (by a mean [±SD] of 47±6 mg per 24 hours) in the subjects given the experimental diet rich in fruits and vegetables.1 They suggest that the high-fiber diet reduces the absorption of calcium, which is reflected in the lower degree of calciuria.

The observation that the fruits-and-vegetables diet reduces calciuria can be accounted for by an alternative hypothesis — that the effect is due to a reduction in the dietary acid load. If the control diet in the study resembles the average American diet, it probably contains a large amount of acid in the form of phosphates and sulfates; it is an “acid–ash” diet. To maintain the normal pH of the internal environment, the influx of this dietary acid is buffered by the bone mineral compartment.2 This relation of dietary acid load and bone metabolism is reflected in a strong correlation between the urinary excretion of net free acid and the urinary excretion of calcium.3 A dietary change that reduces the acid load reduces the net amount of free acid excreted and the associated amount of calcium lost in the urine.

Breslau et al.4 compared a diet containing 75 g of animal protein per day with an equivalent vegetarian diet. They kept the dietary content of calcium, phosphorus, sodium, and potassium constant in the two study groups. The gastrointestinal absorption of calcium was not affected by the different regimens. The vegetarian diet, as compared with the acidogenic animal-protein diet, caused a significant increase in the urinary pH and significant decreases in the net acid excretion as well as in calciuria. The urinary calcium excretion decreased by 47 mg per day (103±15 vs. 150±13).

In a different approach to the same subject, Sebastian et al.5 showed that adding potassium bicarbonate to a high-protein diet significantly reduced the net excretion of acid and the calciuria, but it did not affect the gastrointestinal absorption of calcium, so the calcium balance became substantially less negative.

The similarity between the studies by Appel et al. and Breslau et al. with regard to the change in calciuria is provocative. It is quite likely that the fruits-and-vegetables diet in the study by Appel et al. provided a reduced acid load relative to the control diet and reduced the calciuria similarly.

Uriel S. Barzel, M.D.
Montefiore Medical Center, Bronx, NY 10467

5 References

1
Appel LJ, Moore TJ, Obarzanek E, et al. A clinical trial of the effects of dietary patterns on blood pressure. N Engl J Med 1997;336:1117-1124
Full Text | Web of Science | Medline

2
Green J, Kleeman CR. Role of bone in regulation of systemic acid-base balance. Kidney 1991;39:9-26
CrossRef

3
Barzel US. The skeleton as an ion exchange system: implications for the role of acid-base imbalance in the genesis of osteoporosis. J Bone Miner Res 1995;10:1431-1436
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4
Breslau NA, Brinkley L, Hill KD, Pak CY. Relationship of animal protein-rich diet to kidney stone formation and calcium metabolism. J Clin Endocrinol Metab 1988;66:140-146
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5
Sebastian A, Harris ST, Ottaway JH, Todd KM, Morris RC Jr. Improved mineral balance and skeletal metabolism in postmenopausal women treated with potassium bicarbonate. N Engl J Med 1994;330:1776-1781
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To the Editor:

Appel et al. are to be commended for their study. However, given that only 5.7 percent of the 8813 persons initially screened started the run-in phase, we wonder about the authors' conclusion that their results should apply broadly to the U.S. population. We would have liked to see why 8311 persons apparently failed to qualify for the study. It may also be important to note that the subjects were reimbursed and given other incentives to promote their adherence to the diet. In real life, compliance with dietary regimens poses a problem, with dropout rates ranging from 50 to 70 percent in one to two years.1 Although there is certainly no harm in recommending a low-fat lactovegetarian diet to the general population, further studies based on intention-to-treat analyses of the efficacy and cost-benefit aspects of similar dietary recommendations among unselected patients with hypertension may prove useful.

Arya M. Sharma, M.D.
Ulrike Schorr, D.Sc.
Free University of Berlin, 12200 Berlin, Germany

1 References

1
Dunbar J. Practical aspects of dietary management of hypertension: compliance. Can J Physiol Pharmacol 1986;64:831-835
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Author/Editor Response

The authors reply:

To the Editor: McCarron suggests that the favorable effects of the combination diet in the DASH study resulted from its high mineral and fiber content, provided in the context of a diet rich in fruits, vegetables, and low-fat dairy products. Besides these nutrients and foods, there were several other aspects of the combination diet that may have reduced blood pressure, in particular the lower levels of saturated fat, total fat, and cholesterol and the higher level of protein. There were also differences in other nutrients; for example, the fruits-and-vegetables diet and the combination diet had a higher carotenoid and folate content. In view of the numerous differences among the diets, we emphasize, as we did in the Discussion section of our article, that the DASH trial was not designed to identify the specific nutrients and foods responsible for the observed reductions in blood pressure.1 Inferences about the effects of specific nutrients and foods are speculative and depend more on the interpretation of data from other studies than on the results of the DASH trial itself.

Barzel provides an intriguing alternative hypothesis to explain the pattern of calciuria observed in the DASH trial. If his hypothesis is true, then an additional benefit of the combination diet may be the maintenance of the bone mineral compartment, because less of it will be required to buffer excess acid. Unfortunately, the trial did not measure the acid load of the diets.

Sharma and Schorr point out that a large number of subjects who expressed preliminary interest in DASH did not ultimately enroll in the trial. Among the subjects for whom there was information on the reason for not enrolling, 14 percent were not interested in the trial and 70 percent were ineligible, primarily because their blood pressure was too low or too high; this is to be expected, however, because the blood-pressure eligibility criteria would be met by approximately 40 percent of adults in the United States.2 Sharma and Schorr also mention that the participants in our feeding study were given incentives to promote adherence to the regimen. We concur that additional studies of free-living persons who select their own food would yield valuable information on the acceptability and effectiveness of the DASH combination diet and on compliance with it.

Finally, the term “low-fat, lactovegetarian diet” used by Sharma and Schorr does not describe the combination diet accurately. Approximately 26 percent of its kilocalories were derived from fat, and it included meat (beef, pork, poultry, and fish).

Lawrence J. Appel, M.D., M.P.H.
Johns Hopkins University, Baltimore, MD 21205-2223

Thomas J. Moore, M.D.
Brigham and Women's Hospital, Boston, MA 02115

Eva Obarzanek, Ph.D.
National Heart, Lung, and Blood Institute, Bethesda, MD 20892

for the DASH Collaborative Research Group

2 References

1
Sacks FM, Obarzanek E, Windhauser MM, et al. Rationale and design of the Dietary Approaches to Stop Hypertension trial (DASH): a multicenter controlled-feeding study of dietary patterns to lower blood pressure. Ann Epidemiol 1995;5:108-118
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2
Burt VL, Whelton P, Roccella EJ, et al. Prevalence of hypertension in the US adult population: results from the Third National Health and Nutrition Examination Survey, 1988-1991. Hypertension 1995;25:305-313
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