University of Western Australia, School of Medicine and Pharmacology, Royal Perth Hospital, Perth, Australia
Correspondence to Dr Jonathan Hodgson, UWA School of Medicine & Pharmacology, GPO Box X2213, Perth, WA 6847, Australia Tel: +61 8 9224 0267; fax: +61 8 9224 0246; e-mail: Jonathan.Hodgson@.uwa.edu.au
Hypertension is the leading risk factor for total mortality worldwide. It currently affects approximately one quarter and is projected to affect one third of world's population, about 1.5 billion people, within 20 years . Diet and lifestyle modification are the primary means to tackle the problem. Several factors are regarded as having proven benefit, including: engaging in regular moderate physical activity, maintaining a healthy body weight, limiting alcohol consumption, reducing salt (sodium) intake, maintaining an adequate intake of potassium and consuming a diet rich in vegetables, legumes, fruit and low fat dairy products, and low in saturated fat. There is suggestive, but less conclusive, evidence for benefits of other factors including: increasing magnesium and calcium intakes, increasing dietary fibre, increasing fish (long-chain ω-3 fatty acid) intake, maintaining an adequate vitamin D status and increasing protein intake. The effects of carbohydrate, generally, and starch, specifically, on blood pressure are unclear.
Dietary starch and blood pressure
Starch provides a major contribution to total energy intake in most human diets. Thus, understanding the influence of starch on blood pressure is important. Dietary starch is increasingly perceived as potentially detrimental to blood pressure. This is partly due to limited data from epidemiological studies  and randomized controlled trials [3,4].
There is a shortage of data from population studies on the relationship of dietary starch with blood pressure. A positive association was observed in more than 11 000 middle-aged American men from the Multiple Risk Factor Intervention Trial (MRFIT) . Data from smaller population studies have suggested an inverse association [5,6]. In the current issue of the Journal of Hypertension, Brown et al.  present the results of a cross-sectional analysis of the relationship between starch intake and blood pressure in the International population study on macronutrients and blood pressure (INTERMAP). A major strength of this study is the inclusion of over 4500 middle-aged men and women from diverse populations (from the People's Republic of China, Japan, the United Kingdom and the United States of America). Unlike MRFIT, INTERMAP suggests that starch intake is not directly related to blood pressure. If anything, the relationship was weakly inverse.
The reasons for the apparent discrepancy in findings between MRFIT  and INTERMAP  are not obvious. Although the studies were similar in a range of aspects, several study differences were outlined as possible explanation . A population difference in diet, including the food sources of starch, is another possible explanation. This would seem unlikely, given that a subgroup analysis of the American men with higher coronary heart disease risk in INTERMAP – a similar population to the men included in MRFIT – did not find evidence for a direct relationship of starch with blood pressure . However, there is evidence that a direct association between starch and vegetable protein intakes may partly account for the observed inverse associations of starch with blood pressure in population studies [5–7].
Randomized controlled trials
Several randomized controlled trials have assessed the effects on blood pressure of protein  or monounsaturated fats , in comparison to carbohydrate. A beneficial effect on blood pressure with protein compared with carbohydrate is a consistent finding . This has been largely attributed to a blood pressure lowering effect of protein rather than to a blood pressure raising effect of carbohydrate, a conclusion supported by population data [8,9]. Substitution of monounsaturated fat for carbohydrate has also been associated with lower blood pressure, but the differences in blood pressure appear to be quite small .
It is possible that differences in carbohydrate intake, and perhaps starch were at least partly responsible for the observed differences in blood pressure in these studies. However, it is difficult to dissect the contributions of each macronutrient. In addition, the source of carbohydrate and amount of starch used in these trials has varied widely. This may be an important factor influencing effects on blood pressure.
Carbohydrate and blood pressure
Different carbohydrates have different physiological effects. It is, therefore, likely that dietary carbohydrates will differ in their impact on blood pressure. Carbohydrate-rich foods may contain sugars, starch, resistant starch and soluble and insoluble fibre, may range in glycaemic index, may be primarily whole grain or refined and may be an important dietary source of protein, fat and a range of micronutrients. These factors could influence the effect of carbohydrate-rich foods on blood pressure. Structural differences in starch may also be important. There is an attempt to account for many of these factors in multivariate analyses when attempting to ascertain the relationship of starch with blood pressure in population studies. However, residual confounding may be a problem. Furthermore, any impact of starch on blood pressure in the randomized controlled trials is difficult to quantify because of differences in source of carbohydrate and the amount of starch used.
Data from INTERMAP , which included populations with diverse dietary patterns, suggest that dietary starch does not raise blood pressure. There remains little evidence from population studies or randomized controlled trials that dietary starch causes raised blood pressure. However, the food source of starch may be an important determinant of effect.
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