Researchers from three institutions have recently put together a comprehensive review and meta-analysis looking at the effects that consuming a plant-based diet has on the level of plasma lipids, or the lipids, cholesterol, and triglycerides, found in blood.
High levels of low-density lipoprotein (LDL) cholesterol, which is also known as “bad cholesterol,” have been associated with an increased risk of developing coronary heart disease (CHD).
Total cholesterol levels and the level of triglycerides have also been linked with an increased risk of CHD, although high-density lipoprotein (HDL) cholesterol, which is also known as “good cholesterol,” is thought to play a protective role in the system.
The team suggests that hyperlipidemia, or high cholesterol, can often go undiagnosed and untreated, which is one of the reasons why it can become a dangerous health factor. However, cholesterol levels can be kept under control through an appropriate diet and physical exercise.
The review was conducted by Dr. Yoko Yokoyama, from Keio University in Fujisawa, Japan, in collaboration with Susan Levin, who is director of nutrition education at the Physicians Committee for Responsible Medicine in Washington, D.C., and Dr. Neal Barnard, from the George Washington University School of Medicine and Health Sciences, also in Washington, D.C.
The scientists’ findings were published yesterday in the journal Nutrition Reviews.
Vegetarian diets reduce cholesterol
Dr. Yokoyama and team say that their initiative was triggered by the lack of reviews targeting “the association between vegetarian diets and long-term effects on plasma lipids.” An existing meta-analysis, they say, had suggested that a vegetarian diet could reduce plasma lipids, but it did not linger on the long-term potential of this effect.
The researchers examined 30 observational studies and 19 clinical trials focusing on the relationship between vegetarian-type diets and plasma lipids. Studies and trials included in the meta-analysis had to focus on vegetarian, semi-vegetarian, and vegan diets. In the context of the review, the term “vegetarian diets” refers to diets that only include meat products less than once per month.
The term “Semi-vegetarian diets” refers to diets that include meat products “more than once per month but less than once per week,” while “vegan diets” exclude the consumption of animal products. Some of the vegetarian-type diets might include eggs, dairy products, or fish.
The researchers found that vegetarian diets, in general, were associated with significantly lower levels of total cholesterol.
Observational studies suggested that vegetarian diets were associated with a mean concentration of total cholesterol that was lower by 29.2 milligrams per deciliter. Similarly, clinical trials showed that following a vegetarian diet resulted in a mean concentration of total cholesterol that was lower by 12.5 milligrams per deciliter.
Some alterations in the levels of triglycerides as a result of a plant-based diet were observed, but they were not considered significant.
Levin explained that “the triglyceride levels […] were 5.8 mg/dL [milligrams per deciliter] higher in the clinical research studies and 6.5 mg/dL lower in the observational studies. We often see this since new dietary changes, like a vegan diet that’s naturally higher in carbohydrates, increases lipid levels. It stabilizes over time.”
All the findings were recorded in comparison with an omnivorous, or meat-inclusive, diet.
These outcomes, the researchers explain, are consistent with those reported by previous reviews. The authors hypothesize that vegetarian diets impact body weight, as well as overall health, positively, which may help to regulate plasma lipids.
“Those [individuals] who have followed vegetarian dietary patterns for longer periods may have healthier body compositions as well as better adherence to a vegetarian diet, both of which may have an effect on blood lipids.”
‘Start making dietary changes early’
The authors admit that “observational studies present a higher risk of bias, compared with clinical trials,” yet they highlight the fact that observational studies are better at showing long-term effects. “A meta-analysis provides a quick view of the long-term benefits of taking a specific action, like a adopting a new diet,” Levin told MNT.
Although they were able to control for age and sex variables, Levin added, not all the studies offered information on the participants’ body mass index (BMI) or their physical activity patterns.
“The weakness,” she told us, “is [that] there may be some differences in people who are already lean or among people who maintain an active lifestyle.”
Nevertheless, the main strength of this review, the author explained to us, is providing important information regarding the impact of diet on health both to healthcare practitioners and to the people they care for.
“The immediate health benefits of a plant-based diet, like weight loss, lower blood pressure, and improved cholesterol, are well documented in controlled studies,” says Levin. She also refers to previous research conducted by her team, suggesting that vegetarian diets can regulate the metabolism and prevent the risk of cardiovascular diseases.
Levin and her colleagues encourage individuals to work closely with dieticians to try to steer toward plant-based diets. The researcher told us that she encourages individuals to start being more mindful of their nutrition as early in their life as possible.
“As a dietitian, my take-home message is to encourage anyone to start making dietary changes early on in his or her life. It’s easier to maintain optimal health than it is to change your diet at age 60 or 75, although, to be clear, it’s never to late to make healthful lifestyle changes.”
She also says that the next step from this meta-analysis might be to “continue to measure [the] net metabolic changes of using a whole-food, plant-based vegan diet” in people going through the first stages of heart disease.
She and her colleagues would like not just to further test the effectiveness of a plant-based diet in treating heart disease, but also “to measure the economic impact of having more physicians and primary care specialists talk to their patients about nutrition.”
“Seeing the health benefits and tangible cost-savings figures may incentivize providers and insurers to integrate nutrition therapy into primary care,” Levin concluded.