Intake of dairy products or calcium and blood pressure

In this issue of the journal, Ruidavets et al. [1] describe the relationship between the intake of dairy products and/or calcium and blood pressure among middle-aged males, as evaluated over 3 consecutive days. Multiple other outcome variables, such as body weight, body mass index, dyslipidemia and diabetes were taken into account. In addition, contributing factors such as smoking, physical activity, marital or socio-economic status, drug treatment for hypertension and residence in a large city were considered. After extensive and careful data collection followed by statistical analysis to eliminate the confounding effects of other variables, the authors concluded that higher intakes of dairy products or calcium were independently associated with a lower systolic pressure and the combination of high dairy product intake and high calcium intake produced an even stronger association with lower systolic pressure. These associations were even stronger when patients on hypertensive medication were excluded. There were some lesser associations between high dairy and/or calcium intake and decreased diastolic pressure, which were significant in some cases. These findings are generally consistent with other observational studies and with randomized trials in which diets were modified to contain low-fat dairy products, amongst others [2–6]. The benefits from the ingestion of dairy products were usually assumed to derive from their high calcium and low sodium content [5]. However, Ruidavets et al. [1] suggest that the effect of an increased intake of dairy products may have effects independent of their high calcium content and even their lower sodium content. The mechanisms underlying the effects of a higher dietary intake of dairy products and calcium on systolic blood pressure are also unclear. The lowering of systolic pressure was enhanced when high calcium intake was combined with a high dairy product intake. The nature of this interaction, additive or otherwise, is unclear from their study. There are limitations to the conclusions that can be drawn from the study by Ruidavets et al. [1]. The long-term consequences of an association between a high dairy product and calcium intake and lower systolic blood pressure for cardiovascular morbidity and mortality are not clarified. Whether the incidences of hemorrhagic stroke or other outcomes will be lowered is uncertain. A follow-up of these patients over a period of 3–5 years could reveal whether these results can be translated into decreased morbidity and mortality. However, the strong evidence of an association of higher systolic blood pressure with an adverse cardiovascular outcome in the study by Ruidavets et al. [1] suggests that the use of diets high in dairy products and calcium is to be recommended, and certainly not to be discouraged. In the DASH (Dietary Approaches to Stop Hypertension) trials [7], low-fat dairy products were used to induce lowered blood pressure. Although Ruidavets et al. [1] did not specify the ingestion of low-fat dairy products, the recommendation of the use of low-fat dairy products still remains until further study. Ruidavets et al. [1] noted that higher intakes of dairy products or calcium were associated with more healthy profiles and behaviors, such as more physically active individuals, fewer smokers, and less alcohol consumption. These differences did not explain the relationship between diet and blood pressure. However, they point to a limitation of this type of study: outcomes from sampling over a brief time period could be affected by the previous life histories of these patients. For example, some individuals could have quit smoking recently or changed their exercise habits or diet. The implicit and untested assumption is that the findings for the 3-day period reflect the persistent lifestyle of the individuals studied. However, because of the large sample size, the careful randomization of individuals and the use of multivariate regression analysis to adjust confounding variables, it appears likely that the findings do demonstrate the effects of the levels of dairy food and calcium intake on systolic blood pressure. Intervention studies with a higher intake of low-fat dairy food and calcium support this conclusion [6,7]. The failure to find as large and as significant differences associated with higher intakes of dairy foods and calcium in diastolic compared to systolic blood pressure is puzzling. There are reports [8,9] that elevations in systolic blood pressure and/or pulse pressure are most closely associated with an increased risk of hemorrhagic stroke, but some studies suggest a more general increase in the risk of adverse cardiovascular events [10]. In future, it is hoped that there will be a follow-up of these individuals to discover if better outcomes in terms of cardiovascular adverse events, hemorrhagic or ischemic stroke, myocardial infarction and ischemic disease are related to these dietary components. In summary, the study of Ruidavets et al. [1] represents a new approach to defining relationships between diet and blood pressure, and adds to the evidence indicating that dairy and calcium intake in the diet help to maintain normal systolic blood pressures. The limitations of the study derive from it being an observational rather than an interventional study. Additional follow-up of these patients might add to the significance of these findings for cardiovascular health.

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