Prevalence of the Main Causes of CKD (Hypertension, Type 2 Diabetes and MetS) in Vegetarians
Vegetarianism and Blood Pressure
Numerous cross-sectional studies have found that, in industrialized countries, after adjustments for age, sex and body weight, blood pressure (BP) was lower among vegetarians than non-vegetarians. In observational studies, vegetarians also experience a blunted increase in BP with age. In black as well as white subjects of the Adventist Health Study-2, two different analyses showed that the vegetarian Adventists had lower systolic and diastolic BP than their omnivore counterparts.[1,14] A recent meta-analysis, including seven controlled trials (311 participants) and 32 observational studies (21 604 participants), found that when omnivores are switched to a vegetarian diet, the mean systolic BP is reduced by 4.8 mmHg and the mean diastolic BP by 2.2 mmHg in the controlled trials, and the mean systolic BP is reduced by 6.9 mmHg and the mean diastolic BP by 4.7 mmHg in observational studies.
Furthermore, vegetarian diets have beneficial effects on weight reduction, and low rates of overweight and obesity in vegetarians have been confirmed in different series. In a study including >90 000 Seventh-day Adventists, the body mass index (BMI) was 2.5 kg/m2 lower in vegetarians compared with non-vegetarians. In an analysis of three prospective cohort studies including >120 000 men and women, investigating the relationship between lifestyle factors and weight changes at 4-year intervals, consumption of plant-based foods was inversely associated with weight gain. In a more recent meta-analysis, including 15 intervention trials, prescription of a vegetarian diet of >4-weeks duration, without energy intake limitations, was associated with a mean weight change of −3.4 kg (P < 0.001).
These results may account for the lower BP in vegetarians, since numerous observational studies and clinical trials have shown that BP is directly associated with weight. Dietary data from different studies have shown that the calorie intake of vegetarians is typically lower than that of non-vegetarians, with a mean difference of ~400 calories/day, as observed, for example, in the National Health and Nutrition Examination Survey (NHANES) 1999–2004 study. This energy deficit can contribute to a reduction in BP as a result of weight loss; however, a lower BP effect through a vegetarian diet has also been observed in individuals with normal body weight. The effects of the diet on BP cannot be explained by weight loss alone, and the different dietary components of vegetarian diets each individually contribute to lowering BP.
The dietary components capable of lowering BP include the following.
Salt intake. A reduced salt intake is one of the main dietary approaches to prevent and treat hypertension. But surprisingly, it was shown almost 30 years ago that while Seventh-day Adventist vegetarians had lower BP than non-vegetarian control subjects, their urinary sodium levels were similar.
Potassium intake. As shown in numerous observational and epidemiological studies, a high potassium intake, related to high consumption of fruits and vegetables, is associated with lower BP in both non-hypertensive and hypertensive individuals. In the Dietary Approaches to Stop Hypertension (DASH) Study, 8 weeks on a diet rich in fruits, vegetables and low-fat dairy products resulted in a mean reduction in systolic and diastolic BP of 5.6 and 3.0 mmHg, respectively.[20,21] These findings should be related to the diuretic and natriuretic effects of potassium.
Whether a cause or consequence, hypertension is commonly associated with CKD, and because of a potential increased risk of hyperkalaemia in CKD patients, their diet is restricted in terms of fruit and vegetable intake. At first glance, the risk of hyperkalaemia seems all the more real since it is potentiated by the frequent use of drugs, such as angiotensin-converting enzyme (ACE) inhibitors, angiotensin receptor blockers and non-steroidal anti-inflammatory agents, that can substantially impair urinary potassium excretion. However, in non-dialysed CKD patients, serum potassium concentration appears to be weakly associated with dietary potassium intake, and the alkaline load, linked to the increased intake of fruits and vegetables, favours the shift of potassium from extracellular to intracellular fluids. Lastly, fibres associated with high-potassium plant foods correct the frequent constipation observed in these patients. As a result, the shorter the intestinal transit time, the less dietary potassium will be absorbed.
Therefore it seems reasonable and safe, in CKD Stage 4 and 5 patients with retained urine flow, to set the potassium intake at 4.7 g/day (120 mmol/day), which corresponds to the potassium content of the DASH diet.
Dietary fibre intake. Results of the International Study on Macro/micronutrients and Blood Pressure study have shown that a higher intake of fibre, especially insoluble ones, may contribute to lower BP. Results from this study have confirmed previous meta-analyses of randomized controlled trials indicating that an increased intake of dietary fibre might contribute to the prevention of hypertension or to the reduction of BP in hypertensive patients. Enhanced insulin sensitivity, improved vascular endothelial function and improved magnesium intestinal absorption have been proposed to explain the BP-lowering effect of dietary fibres.
It was noticed that when the intake of fibre is increased, in isolation from other dietary changes, no effects on BP were observed.
Fat intake. Vegetarians eat less total fat and also more polyunsaturated fatty acids, resulting in an elevation in the dietary polyunsaturated:saturated (P:S) ratio (1.0 versus 0.3), but most studies do not support the view that these changes have a direct BP-lowering effect. However, in a cross-sectional study comparing age- and sex-matched vegetarians and non-vegetarians, a higher resting energy expenditure (REE) was found in vegetarians than in non-vegetarians, contributing to their lower BMI values, independent of exercise. This increase in REE was positively correlated with a specific component of the vegetarians' diet—vegetable fats.
Carbohydrate intake. The OmniHeart Study has shown that, under isocaloric conditions, partial substitution of carbohydrates with either protein, particularly from plant sources, or monounsaturated fat can reduce BP, improve lipid levels and reduce cardiovascular risks. It remains uncertain whether these effects result from a reduction in carbohydrate or from a compensatory increase in other macronutrients.
Protein intake. Data from several clinical trials suggest a small, but significant, inverse relationship between total dietary protein intake and BP, which was observed with protein intake from plant sources rather than with animal protein intake. This effect appears to be independent of changes in body weight and may be more apparent in populations with elevated BP and older populations.
The exact mechanisms linking plant proteins to BP have not yet been clarified and different explanations have been put forward. Dietary protein has been related to the synthesis of cellular ion channels inducing natriuresis, thus leading to lower BP. Dietary protein may result in a higher concentration of several amino acids that have BP-lowering effects. Arginine, which serves as a substrate for nitric oxide (NO), is a potent vasodilator; its production improves endothelial function and contributes to BP lowering. Tryptophan may also reduce BP by augmenting NO production and by reducing adrenaline and noradrenaline. Higher protein intake may result in a higher concentration of histidine, which triggers a vasodilatory response.
To close this section concerning the relationship between vegetarianism and BP, we must mention that several non-dietary aspects of a vegetarian lifestyle (low to moderate alcohol intake, minimal cigarette smoking and regular physical activity) might also contribute to lower BP and more generally have a beneficial effect on the outcomes of patients.
Vegetarianism and Type 2 Diabetes
Comparison of ovo-lacto-vegetarians with omnivores showed that the former group were more insulin sensitive than their omnivore counterparts and that the degree of insulin sensitivity was correlated with years on a vegetarian diet.
Whole-grain products and vegetables generally have low glycaemic index values, and individuals following vegetarian diets are less than half as likely to develop diabetes compared with non-vegetarians; these figures were confirmed by the Adventists Health Study-2 that included >60 000 individuals who participated in the study conducted in 2002–06. After adjustment for age and different components of lifestyle, the prevalence of type 2 diabetes increased from 2.9% in vegans to 7.6% in non-vegetarians, while the prevalence was intermediate in participants consuming lacto-ovo-, pesco- and semi-vegetarian diets.
A systematic review and meta-analysis including 255 type 2 diabetic patients (17 lacto-ovo-vegetarians and 238 vegans) showed that consumption of a vegetarian diet, combined with exercise, was associated with a dramatic reduction in the use of glucose-lowering medications and in haemoglobin A1c, as well as a non-significant reduction in fasting blood glucose concentration.
Lastly, vegetable proteins have a lower impact on renal haemodynamics than animal proteins. Replacing animal proteins with vegetable proteins may decrease renal hyperfiltration, proteinuria and, theoretically, in the long-term, the risk of developing renal failure.
Vegetarianism and MetS
MetS is defined as the presence of at least three of the following criteria: abdominal obesity, low high-density lipoprotein, hypertriglyceridaemia, elevated fasting glucose and hypertension. The prevalence of MetS, which occurs in ~20% of the population of the USA, is increasing in developing and developed countries. Various dietary patterns have been proposed for preventing MetS, among which is a vegetarian diet. Several cross-sectional and case–control studies have confirmed an association between consumption of a vegetarian diet and an estimated 2-fold reduction in the prevalence or risk of developing MetS (except for low-density lipoprotein).[4,36]
The proposed recommendation of a vegetarian diet was well illustrated in the prospective Atherosclerosis Risk in Communities (ARIC) Study, which included >10 000 middle-aged adults, free of diabetes and cardiovascular disease, with an estimated glomerular filtration rate (eGFR) >60 mL/min/1.73 m2. Twenty-one per cent of the participants met the criteria for MetS at their initial visit, with prevalences of zero, one, two, three, four or five traits of MetS of 26, 30, 23, 15, 5 and 1%, respectively. After a 9-year follow-up, 7% of the participants developed CKD, and the risk of incident CKD increased significantly with the baseline number of traits of MetS. These data confirm that, in addition to hypertension and diabetes, MetS is also independently associated with an increased risk for incident CKD.
Nephrol Dial Transplant. 2019;34(2):199-207. © 2019 Oxford University Press