Scientific Update
A Review of Recent Scientific Papers Related to Vegetarianism
By Reed Mangels, PhD, RD
Vegan Diet Has Lowest Impact on the Environment
Food-related procedures, including production, cultivation, processing, packaging, transportation, and distribution, are some ways how the foods we eat affect the environment, often in a big way. For example, the global food system accounts for 20-30% of total greenhouse gas emissions.1 A recent study estimated the impact of five different dietary patterns on land use, water use, and greenhouse gas emissions.2 The dietary patterns were the current U.S. diet, a "healthy" U.S. diet, a Mediterranean diet, a lacto-ovo vegetarian diet, and a vegan diet. The estimated environmental impact of the dietary patterns was determined by using information from databases about the environmental effects of specific foods and by making assumptions about the foods that would be eaten by those following the dietary patterns.
In terms of land use, the vegan diet was found to use the least land; the current U.S. diet and the "healthy" U.S. diet used the most. The amount of land used solely for red meat production for each of the nonvegetarian diets was greater than the total land used to produce a vegan diet. Red meat, dairy milk, cheese, and grains were the food groups that made the largest contribution to land usage.
The vegan diet required the least amount of water; the "healthy" U.S. diet and the Mediterranean diet required the most. Red meat and dairy milk were the largest sources of water usage for nonvegan diets; nuts and seeds were responsible for 40% of water use for the vegan diet.
The three nonvegetarian diet patterns contributed the most to greenhouse gas emissions. The vegan diet was associated with the lowest greenhouse gas emissions, with the lacto-ovo vegetarian pattern intermediate. There was a 4.75-fold difference in greenhouse gas emissions related to the Mediterranean diet compared to the vegan diet. Red meat alone in the current U.S. diet was responsible for 2.5 times more greenhouse gas emissions than the total vegan diet.
The study's authors concluded, "The most impactful diet-related change that Americans can make towards a more sustainable diet is a shift towards a vegetarian or vegan diet and reducing the consumption of red meat and potentially dairy products." They also pointed out that this shift could have a highly beneficial effect on individual health.
1 Watts N, Amann M, Arnell N, et al. The 2020 report of The Lancet Countdown on health and climate change: responding to converging crises. Lancet. 2021;397(10269):129-170.
2 Jennings R, Henderson AD, Phelps A, Janda KM, van den Berg AE. Five U.S. dietary patterns and their relationship to land use, water use, and greenhouse gas emissions: Implications for future food security. Nutrients. 2023;15:215.
Diet and Depression
Many factors, including genetic, environmental, and psychological factors, play a role in depression. A focus of some recent research is the possible connection between dietary choices and depression. Two studies from Australia examined the association between the quality of vegetarian, vegan, and nonvegetarian diets and depressive symptoms. Both of these studies used online surveys to ask subjects about their food choices and about how often they experienced symptoms associated with depression, such as poor appetite, feelings of loneliness, and restless sleep. Both studies assigned dietary quality scores to participants' reported dietary intakes. Higher scores meant that participants ate more fruits, vegetables, legumes, whole grains, nuts, seeds, and soy products and fewer processed foods. Lower scores meant that participants ate more fried food, salty and sugary snacks, and sugary drinks.
One study of self-identified vegans and vegetarians found that higher dietary quality scores were associated with a lower risk of having depressive symptoms.1 A second study compared self-identified vegans, vegetarians, and nonvegetarians.2 The dietary quality score was highest in vegans, followed by vegetarians, and then nonvegetarians. Depressive symptoms were highest in the nonvegetarian group, followed by the vegetarian and vegan groups. In all groups, higher dietary quality scores were associated with a lower risk of having depressive symptoms.
Having depressive symptoms, such as poor appetite and sadness, could affect food choices. Thus, we can't say with any certainty that choosing a higher quality diet will reduce the risk of having symptoms associated with depression. More research is needed in this area.
1 Lee MF, Eather R, Best T. Plant-based dietary quality and depressive symptoms in Australian vegans and vegetarians: a cross-sectional study. BMJ Nutr Prev Health. 2021;4:479-486.
2 Walsh H, Lee M, Best T. The association between vegan, vegetarian, and omnivore diet quality and depressive symptoms in adults: A cross-sectional study. Int J Environ Res Public Health. 2023;20:3258.
Healthier Vegetarian Diets Are Associated with a Reduced Risk of Major Cardiovascular Events
A recent study from the United Kingdom included more than 390,000 adults; almost 2% were lacto-ovo vegetarians and 98% ate red meat. Vegans and those who avoided red meat but ate fish and chicken were not included in the study. Diets of meat-eaters and vegetarians were classified as "healthier" or "less healthy" based on current U.K. guidelines. Study participants were followed for a median of slightly more than 10 years. During that time, participants had more than 40,000 major cardiovascular events, such as heart attacks and the development of heart disease.
Meat-eaters, whether their diet was classified as healthier or less healthy, ate fewer fruits and vegetables and had less dietary fiber than the vegetarians. Vegetarians with healthier diets ate less saturated fat and ate more fiber, fruits, and vegetables than did vegetarians whose diets were classified as less healthy. Vegetarians with healthier diets had a 16% lower risk of major cardiovascular events than did meat-eaters with less healthy diets; meat-eaters with healthier diets had a 5% lower risk of these events than meat-eaters with less healthy diets.
The difference in risk between vegetarians with less healthy diets and meat-eaters with less healthy diets was not statistically significant. These results suggest that there are benefits to choosing a healthier vegetarian diet compared to a less healthy vegetarian diet.
Petermann-Rocha F, Celis-Morales C, Pell JP, Ho FK. Do all vegetarians have a lower cardiovascular risk? A prospective study. Clin Nutr. 2023;42:269-276.
Microalgae Supplementation in Vegan Diets
by Melinda Benz, RDN/VRG volunteer
Two omega-3 fatty acids, DHA and EPA, primarily found in seafood due to fish consuming microalgae, are associated with promoting brain, eye, and heart health. Plant-based omega-3 supplements derived from microalgae are available to provide DHA for vegans and others who do not eat seafood. Previous studies evaluating the effectiveness of algae-based DHA supplements in increasing DHA levels in humans have used high amounts of DHA. A recent study from Spain used relatively lower amounts of DHA. This randomized, double-blind trial analyzed 98 healthy adults. Subjects, all of whom consumed little or no DHA, were vegans, lacto-ovo vegetarians, and omnivores. The study aimed to determine if taking a 250 milligram per day algae-based DHA supplement (from the microalgae Schizochytrium sp) could effectively increase serum fatty acid DHA levels. Nearly half of the participants received one daily capsule of the DHA supplement for five weeks followed by an olive oil placebo for five weeks. The other half received placebo for five weeks followed by DHA for five weeks.
The study results showed that consuming the 250 milligram microalgae DHA supplement effectively increased DHA levels in all three diet groups. Vegans had the greatest increase in DHA levels, followed by lacto-ovo vegetarians, and then omnivores.
García-Maldonado E, Alcorta A, Zapatera B, Vaquero MP. Changes in fatty acid levels after consumption of a novel docosahexaenoic supplement from algae: a crossover randomized controlled trial in omnivorous, lacto-ovo vegetarians and vegans [published online ahead of print]. Eur J Nutr. 2022;1-15.
