- • Most population epidemiological studies have shown an inverse association between estimated protein intake and blood pressure. Total and plant proteins are consistently associated with lower blood pressure. Inverse associations of animal protein with blood pressure have also been shown.
- • A reduction in blood pressure with protein, in comparison with carbohydrate, is a consistent finding in controlled trials. Animal (including lean red meat and milk proteins) and plant (primarily soy) proteins appear to be similar with respect to effects on blood pressure.
- • It remains uncertain whether protein itself can lower blood pressure, or whether the benefits are due to partial replacement of some other factor in the diet, such as refined carbohydrate. Potential mechanisms remain speculative.
- • The implications of the available research for ‘healthy eating’ advice remains uncertain: the long-term effect of a dietary approach to increase protein intake, which is widely applicable to the population, needs to be evaluated, as does the influence of refined carbohydrate on blood pressure.
It was initially thought that a higher intake of animal protein specifically, and total protein intakes more generally, are detrimental for blood pressure. This idea derived from studies showing that vegetarian diets, which are lower in total protein and lack meat protein, can lower blood pressure.1 However, several studies have investigated whether type of dietary protein (animal vs plant) could explain the lower blood pressure with vegetarian diets.2–5 In addition, more recently, several studies have shown that non-vegetarian diets rich in vegetables, fruits and legumes can also lower blood pressure.6 This paper takes the position that it is possible that higher protein intakes could actually benefit blood pressure.
The relationship between protein intake and blood pressure has been investigated in many population studies.7–9 In cross-sectional and longitudinal epidemiological studies, the estimated protein intake has generally been inversely associated with blood pressure. Higher total and plant protein intake is consistently associated with lower blood pressure. Higher animal protein intake has also been associated with lower blood pressure in several epidemiological studies. This paper reviews research addressing the links between dietary components, notably protein and carbohydrate, with blood pressure.
TOTAL PROTEIN INTAKE
Most cross-sectional population studies have shown an inverse association between estimated protein intake and blood pressure.7–12 For example, the INTERSALT study, which involved over 10 000 men and women, reported an inverse relationship between urinary nitrogen and urea—as indicators of total protein intake—and blood pressure. Estimates from the present study indicate that an increase of 37 g/day of protein would lead to falls in population mean systolic and diastolic blood pressures of ∼3 and ∼2.5 mmHg, respectively.10 The estimated fall in blood pressure in individuals with elevated blood pressure may be greater. An increase in protein intake of 37 g/day represents a modest shift in protein intake from 30% below the population mean to 30% above the mean. To date, there have been more than 25 reported cross-sectional studies, with approximately two-thirds reporting an inverse relationship of protein intake with blood pressure and most of the remaining studies reporting no association.7,8
Fewer prospective studies have reported on the relationship between dietary protein and blood pressure or hypertension. Of these studies, two have found an inverse association, three have found no significant association, and one has found an inverse association for plant protein but a positive association for animal protein.8
PLANT VERSUS ANIMAL PROTEIN
Total and plant proteins are consistently associated with lower blood pressure. Where animal and plant proteins have been analysed separately, some cross-sectional studies suggest stronger associations for plant protein.12–14 In addition, in the Western Electric study, a prospective study of more than 1700 men with nine-year follow up, an inverse association was found for plant protein, but a positive association was found for animal protein.15 These results may not be surprising because many dietary sources of plant protein also contain other factors, such as fibre, folate and phytochemicals, which could contribute to lower blood pressure. In addition, plant and animal protein intakes may be associated with differences in dietary patterns and lifestyle factors.16,17
In contrast, results of several cross-sectional studies suggest an inverse association of animal protein with bloodpressure. Many of the large populations studied, where total protein intake has been inversely associated with blood pressure, eat predominantly animal (meat) protein.10,11 In addition, results of several studies, primarily in Chinese and Japanese populations, suggest that animal protein specifically is associated with lower blood pressure.18–21
RANDOMISED CONTROLLED TRIALS
Protein versus carbohydrate
A reduction in blood pressure with protein, in comparison with carbohydrate, is a consistent finding in controlled trials.22–27 Three studies have now compared plant (soy) protein with carbohydrate.22–24 In a randomised controlled parallel-designed trial, 36 treated hypertensive individuals followed a low-protein (∼12% of energy), high-carbohydrate (∼60% of energy) diet. Against this background, plant protein (66 g of soy protein), compared with carbohydrate (66 g of maltodextrin), resulted in a reduction in 24-hour ambulatory systolic and diastolic blood pressures by 5.9 and 2.6 mmHg.22 Washburn et al.23 found falls in clinic diastolic blood pressure in a randomised controlled crossover trial that compared 20 g/day of soy protein with 20 g/day of carbohydrate. He et al.24 found significantly lower systolic and diastolic blood pressure, by 4.3 and 2.8 mmHg respectively, with 26 g/day of soy protein in comparison with wheat carbohydrate.
Delbridge et al.,25 in a 12-month study comparing a high-protein with a high-carbohydrate diet, found a 6.6 mmHg lower systolic blood pressure in those randomised to high protein. Appel et al.26 showed that partial substitution of carbohydrate in a modified DASH (dietary approaches to stop hypertension) diet with approximately 55 g/day of protein from primarily plant-based sources resulted in a further 1.4 mmHg lower systolic blood pressure. This plant-based approach may be an ideal approach to increasing protein in the diet because of the accompanying increase in other potentially beneficial dietary factors. However, this is likely to be difficult to achieve within populations who consume primarily animal-based protein. In Australia, any advice to partially substitute carbohydrate-rich foods for higher-protein low-fat foods is likely to result in increases in animal protein intake.
The impact on blood pressure of an increase in animal protein at the expense of carbohydrate has also been investigated. In a controlled trial in 60 hypertensive individuals, the effect of an increase in protein intake from lean red meat on blood pressure was investigated. Partial substitution of carbohydrate (primarily starch) for protein (∼36 g/day) in isoenergetic diets for eight weeks resulted in a relative reduction in clinic and 24-hour ambulatory systolic blood pressure by 5.2 and 4.0 mmHg, respectively.27
In these studies, the increase in protein intake has ranged from 20 to 66 g/day. None of the diets would be regarded as very high protein. Because of differences in study design, it is difficult to ascertain whether there is a dose–response. Changes in body weight are unlikely to be an important confounding factor. In studies that used a dietary approach to alter protein and carbohydrate intakes,22,25 as opposed to studies using supplements, changes in sodium intake may have contributed to some of the blood pressure lowering observed. However, differences in sodium intake are unlikely to account fully for observed differences in blood pressure.
The potential role of refined carbohydrate
All these intervention studies have compared protein with carbohydrate. The possible role of the refined carbohydrate on the observed blood pressure differences remains uncertain. The population data on carbohydrate and blood pressure are limited and inconsistent,11–13 although there are data to indicate that wholegrain-derived carbohydrate may lower blood pressure.28–31 Studies comparing high-carbohydrate diets with higher-monounsaturated-fat diets are consistent with detrimental effects of refined carbohydrates.26,32,33 Appel et al.26 found that partial substitution of carbohydrate with either protein or monounsaturated fat resulted in lower blood pressure of a similar magnitude. This finding needs confirmation. Shah et al.32 found that a high-carbohydrate diet resulted in higher blood pressure in type 2 diabetic patients in comparison with a high-monounsaturated-fat diet. A recent meta-analysis of controlled trials to assess the effect of high-carbohydrate in comparison with high-monounsaturated-fat diets on blood pressure found that diets rich in carbohydrate were associated with higher systolic (2.6 mmHg) and diastolic (1.8 mmHg) blood pressures.33 In contrast, a high-carbohydrate low-glycaemic-index diet was found to reduce diastolic blood pressure in comparison with a high-monounsaturated-fat diet.34 The studies that have compared the effects on blood pressure of high- and low-glycaemic-index diets suggest that glycaemic index is not a major determinant of blood pressure.35,36 These studies were neither designed nor powered to address this question. Thus, it is premature to make any conclusions regarding glycaemic index and blood pressure. However, it appears that differences in carbohydrate intake may have contributed to the observed effects on blood pressure in studies to have compared protein with carbohydrate.
Plant versus animal protein
Several studies investigated whether type of dietary protein could explain blood pressure lowering with vegetarian diets.2–5 These studies consistently found that animal and plant proteins did not differ with respect to effects on blood pressure. For example, Prescott et al.5 conducted a 12-week trial comparing diets matched in protein from meat and non-meat sources and found no difference in blood pressure. More recently, researchers have been interested in the effects of soy protein on blood pressure. Several studies have compared soy protein with other sources of protein, primarily casein or milk proteins. These studies have been reviewed by Sacks et al.37 Most studies found no effect on blood pressure, and the weighted average change was −1 mmHg in systolic blood pressure. Thus, there is little evidence for differential effects on blood pressure of different sources of protein.
It remains uncertain whether protein per se can lower blood pressure, or whether the benefits are due to partial replacement of some other factor in the diet, such as refined carbohydrate. Thus, the potential mechanisms remain speculative. Amino acid composition of proteins may be relevant: several amino acids have been reported to affect blood pressure either in human studies or in animal experiments.38 Arginine, with the potential to act via nitric oxide,39 has been the amino acid most widely investigated in relation to blood pressure and endothelial function. Taurine has also been reported to lower blood pressure in animals40 and to be inversely related to blood pressure in population studies.41 A non-specific dose-dependent diuretic effect of amino acids may also contribute to the blood pressure-lowering effect.42
At present, information about the effects of specific amino acids on blood pressure is inadequate to allow any conclusion about the need to consider amino acid content of proteins. The INTERSALT data10 have been gathered worldwide from a range of populations with considerable variation in the type of protein consumed. If dietary protein can lower blood pressure, then this data source would suggest that the amino acid composition of the protein and source of protein are not critical determinants.
The results of cross-sectional and prospective epidemiological studies and randomised controlled trails support the proposal that higher protein intakes from plant and animal sources can reduce blood pressure. Results of the controlled trials suggest that for individuals with above optimal blood pressure, the partial replacement of refined-carbohydrate-rich foods with high-protein low-fat foods will reduce blood pressure. If there are benefits on blood pressure of increased protein intake, then a similar message is likely to be conveyed to this population: ‘to partially replace refined carbohydrate-rich foods with a variety of high-protein low-fat foods’. However, it remains uncertain whether this would be the most appropriate advice. This is largely because it is uncertain whether the protein per se lowers blood pressure, and/or whether refined carbohydrate raises blood pressure.