Impact of Vegetarian Diet on Heart Disease, Diabetes
Here is an alarming statistic: The Centers for Disease Control and Prevention (CDC) reports that more than one in three adults nationwide has one or more types of cardiovascular disease.1 In fact, cardiovascular disease (CVD) is the leading cause of death among men and women in the United States.1
Several studies suggest that a vegetarian diet is effective at reducing almost all controllable risk factors for heart disease, which include total cholesterol, low-density lipoprotein (LDL) cholesterol, systolic blood pressure, diabetes, and obesity.
Other established predictors of CVD include age, smoking status, high-density lipoprotein (HDL) cholesterol, and body mass index (BMI).2 Although there are a number of variables to consider, vegetarian diets as a way to treat and prevent CVD should be emphasized in patient treatment plans.
Types of Vegetarian Diets
“Vegetarianism” is an umbrella term for many dietary variations, some of which permit meat consumption and others of which entirely exclude or significantly limit the consumption of meat (See Table 1 below).
TABLE 1: Various vegetarian diets
|Vegan||Exclusively plant-based foods|
|Lacto-ovo vegetarian||Eggs, milk, and milk products, but no meat|
|Pesco-vegetarian||Same as lacto-ovo, plus fish or seafood|
|Semi-vegetarian||Primarily plant-based diet with occasional meat consumption|
The higher a person's cholesterol, the greater his or her risk for CVD. A study of 800 persons that examined the effect of diet on lipid metabolism revealed that with the exception of HDL, vegetarians had significantly lower plasma lipid levels than did non-vegetarians.3
Considering that long-term vegetarians tend to have low plasma lipid levels, an important question remains: can non-vegetarian individuals who adopt a vegetarian diet expect a reduction in plasma lipid levels? And if so, how long does it take for those changes to become apparent?
The results of one randomized controlled trial suggest that the maximum benefit from a cholesterol-lowering diet involves more than simply reducing dietary intake of saturated fat and cholesterol: The amount of vegetables, legumes, and whole grains consumed seems to play a marked role in decreasing cholesterol levels.4
Researchers compared two diets of participants with moderately elevated cholesterol levels over the course of 4 weeks: Diet A, a low-fat diet with a few vegetables, legumes, and whole grains; and Diet B, a low-fat, plant-based diet consisting largely of vegetables, legumes, and whole grains. In terms of cholesterol, saturated fat, and total fat intake, Diets A and B were identical.
Both diets were associated with significant reductions in LDL cholesterol and total cholesterol, but participants following Diet B, the plant-based diet, showed greater overall reductions. Diet B yielded decreases in total cholesterol of 17.6mg/dL and in LDL cholesterol of 9mg/dL, whereas Diet A reduced total cholesterol by 13.8mg/dL and LDL cholesterol by 7mg/dL.
Reductions in serum cholesterol levels can occur within a relatively short time period after initiating a vegetarian diet, as evidenced by one study in which participants were randomized into three groups that consumed vegetarian diets for four weeks.5
A control group consumed a low-fat dairy and whole-wheat-cereal diet; one intervention group consumed the same diet plus 20mg of lovastatin; and the second intervention group consumed a diet high in plant sterols, soy protein, viscous fiber, and almonds. Significant decreases in cholesterol were seen as early as 2 weeks after diet initiation, with reductions at four weeks in LDL levels of 8.0% in the control group, 30.9% in the statin group, and 28.6% in the final group.
These results suggest that the efficacy of dietary modification in the treatment of hyperlipidemia is not only similar to that of a statin, but also that the lipid-lowering benefit of dietary modification can be evident as early as 2 weeks after implementing dietary change.
Another randomized controlled trial found that people with untreated high cholesterol attained a reduction in serum LDL levels and risk for coronary heart disease (CHD) that was nearly half of what might be expected from statin therapy when they were placed on a vegetarian diet emphasizing plant sterols, soy protein, viscous fiber, and nuts.6
Specifically, researchers found that plant sterol intake lowered serum LDL by 5% (0.94g plant sterols per 1,000kcal diet), viscous fiber intake by 4% (9.8g viscous fiber per 1,000kcal diet), soy protein intake by 2% (22.5g soy protein per 1,000kcal diet), and nuts by 2% (22.5g nuts per 1,000kcal diet).
The study measured serum LDL levels (LDL-C) and total cholesterol/HDL ratio (TC/HDL) over the course of six months in 351 participants with untreated high cholesterol who were assigned either to a control group, in which members followed a low-saturated-fat diet and received 2 consultations with dietitians; or to one of two intervention groups (low-intensity and high-intensity, receiving two and seven consultations with dietitians, respectively), in which participants followed a vegetarian diet emphasizing plant sterols, soy protein, viscous fiber, and nuts. Significant reductions were seen in both the low-intensity group and the high-intensity group in terms of LDL-C (13.1% and 13.8%, respectively) and TC/HDL (8.2% and 6.6%, respectively).
The risk for CHD was significantly lowered, by 10.8% in the low-intensity intervention group and by 11.3% in the high-intensity group, whereas the control group showed insignificant reductions in CHD risk.
Many popular diets that encourage low carbohydrate and high protein intake may be effective in terms of weight reduction, but they have not been shown to improve an individual's cholesterol profile. In one study, a low-carbohydrate, plant-based diet significantly reduced LDL and total cholesterol over four weeks while producing weight loss similar to that seen with low-carbohydrate, high-protein diets.7
For patients who are hesitant to take medication for high cholesterol, determining ways of modifying their diet would be worth exploring.
As practitioners, we should encourage patients to consider cholesterol-lowering foods, such as plant sterols and nuts. In addition, we should help them explore ways of cutting back in other areas of their diet, such as saturated fats.
Research indicates that a vegetarian diet protects against the development of hypertension. One study of 500 persons found significantly lower systolic and diastolic blood pressures among vegetarians compared with omnivorous subjects.8 Not surprisingly, the vegetarians also were less likely to use any blood pressure medications.
Vegans in particular were less likely to have hypertension compared with omnivores (odds ratio [OR], 0.37), lacto-ovo vegetarians (OR, 0.57) and partial vegetarians (OR, 0.92). Meat-eaters self-reported high blood pressure most often (men, 15%; women, 12.1%) and vegans least often (men, 5.8%; women, 7.7%), even after adjusting for age.9 Approximately 9% of fish-eaters and vegetarians reported hypertension, regardless of gender.
As with changes in cholesterol, changes in blood pressure can happen fairly quickly with dietary modification, and certain combinations of foods yield greater benefits than others. A diet rich in fruits and vegetables decreased blood pressure in one randomized, controlled trial, but the reduction was greater when the diet also included low-fat dairy products and less saturated and total fat10.
The 459 adults in the study spent the first three weeks on a control diet that was low in fruits, vegetables, and dairy products and had a fat content that was consistent with the average U.S. diet. For weeks after that, half of the participants followed a diet rich in fruits and vegetables while the other half was assigned a “combination” diet that was rich in fruits and vegetables but also included low-fat dairy products and reduced saturated and total fat.
Although both groups had lower systolic and diastolic blood pressure by the end of the intervention, these measurements were significantly lower in the combination-diet group than in the control group (systolic, 11.4mmHg less; diastolic, 5.5mmHg less). It is important to note that body weight was maintained during the study, which excluded the possibility that the effects on blood pressure were due to an associated weight loss.