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Background Elevated blood pressure is a major risk factor for cardiovascular diseases. Diet and lifestyle have a substantial impact on blood pressure, but the role of protein intake is not yet clear. This thesis focuses on total dietary protein, types of protein (i.e. plant and animal), protein from specific sources (i.e. dairy, meat, and grain), and specific amino acids in relation to blood pressure levels and incident hypertension. Methods The associations of dietary protein, protein types, and protein from specific sources with population blood pressure levels were cross sectionally examined in 20,820 Dutch adults aged 25 to 65 y (MORGEN Study). The relation with risk of hypertension was examined in 3,588 of these adults with 15 years of follow-up (Doetinchem Study) and in 2,241 older Dutch adults (≥55y) with 6 years of follow-up (Rotterdam Study). In the latter cohort we also examined the relation of specific amino acids (i.e. glutamic acid, arginine, lysine, cysteine, tyrosine, and essential amino acids) with blood pressure levels and risk of hypertension. As an ancillary Study, a fully controlled randomized cross over trial with different protein-rich diets was conducted to obtain objective biomarkers for dietary protein types that may be used in future epidemiological studies. Finally, we performed several meta-analyses to summarize our findings for dietary protein and protein types in relation to blood pressure and incident hypertension, combined with data from the literature. Results The epidemiological studies presented in this thesis and a meta analysis of observational studies showed no associations of total protein and animal protein with blood pressure or incident hypertension. A meta analysis of 14 randomized controlled trials, however, showed a pooled blood pressure effect of protein supplementation (weighed mean contrast in intake of 41 g/d) of 2.1 mmHg systolic (95%-CI: 2.9 to 1.4) when compared to carbohydrate intake. In the epidemiological studies in this thesis plant protein was significantly inversely associated to blood pressure levels ( 1.8/ 1.0 mmHg with 14 grams higher energy adjusted intake), but not with incident hypertension (all HR per SD ~1.00). Meta-analyses of cross sectional studies showed a small differential association of plant and animal protein with blood pressure (-0.52 mmHg per SD of dietary plant protein versus +0.03 mmHg per SD of animal protein), but this association was not present in meta-analyses of prospective studies and trials. The epidemiological analyses on meat protein and dairy protein in this thesis revealed no consistent associations with blood pressure or incident hypertension. Grain protein was inversely associated with diastolic (but not systolic) blood pressure, and with borderline significant lower risk of hypertension in a general Dutch population (HR: 0.75, 95% CI: 0.73 to 1.00), but this association was absent in older adults. No associations with blood pressure or incident hypertension were found for amino acid intakes. Finally, we identified a combination of 3 urinary amino acids as a potential biomarker for meat protein intake and a combination of 7 plasma amino acids as a potential biomarker for grain protein intake Conclusion Results from this thesis suggest a small beneficial effect of protein on blood pressure if consumed instead of carbohydrates. Plant protein, e.g. from grain, may be more beneficial to blood pressure than animal protein but data are too limited to draw firm conclusions. After validation, future epidemiological studies could make use of biomarkers as more robust estimates for protein from specific sources and amino acid intakes. Randomized controlled trials are warranted to examine the blood pressure effect of specific types of protein, reflecting habitual intakes in western societies, compared to different types of carbohydrate. At present, a prudent diet for the prevention of hypertension with adequate amounts of dietary protein, preferable from plant sources, is recommended. |
