A vegetarian diet is defined as an eating pattern that excludes all meat products, including red meat, poultry, and seafood as well as any byproducts of slaughter, such as gelatin.^[1] This definition can be confusing because of a lack of standardization. Variations on the vegetarian diet include the lacto-ovo vegetarian diet, which includes dairy products and eggs, and lacto-vegetarian and ovo-vegetarian, which include only dairy products or only eggs, respectively. Some people who eat fish and other seafood products — usually called a pescatarian diet — also refer to their diet as vegetarian, although this is not within the usual definition. Other types of diets that are sometimes grouped in with the vegetarian diet include the flexitarian diet — a diet that focuses on including mostly plant-based foods but allows animal products in moderation — and the semivegetarian diet that also focuses on plant-based foods and excludes red meat products but still includes fish, poultry, or both.^[2][1][3][4] Within research, vegetarian and vegan diets are often studied together or referred to interchangeably, which can introduce confusion. Vegan diets are different from vegetarian diets in that they do not include any animal products (no meat, eggs, seafood, poultry, dairy products, or honey), and people who follow a vegan lifestyle often avoid animal products outside of food, such as leather goods, wool, or fur.

A vegetarian diet has several potential benefits. People who eat a vegetarian diet tend to have lower total cholesterol levels, LDL cholesterol levels, triglyceride levels, and apolipoprotein B (ApoB).^[5][6][7][8] There is also some evidence, although it is not consistent, that people who follow a vegetarian diet may have lower blood pressure.^{[9][5][10][11]} These factors contribute to a lower risk for ischemic heart disease (IHD) and a lower risk for cerebrovascular disease, including stroke.^[6]

There is some evidence showing that a vegetarian diet is associated with a lower risk of developing type 2 diabetes.^[12][13][14] It can also be used as an adjunct to standard treatment to further improve glycemic control in people with type 2 diabetes, as it can lower fasting blood glucose and HbA1c levels.^{[15][16][17][18]} However, the evidence for a vegetarian diet for the treatment of diabetes is inconclusive, and it should not be used in isolation to treat type 2 diabetes.^[19]

As with any diet that excludes certain food groups, there is a concern that nutritional deficiencies could develop. In the case of vegetarianism, the micronutrients of concern are the ones that occur primarily in animal products, such as iron, zinc, and vitamin B12.^[2]

Low iron levels are more common in vegetarians, especially in premenopausal women.^[20][21] Meat contains heme iron, but plant foods contain non-heme iron, which is more difficult for the body to absorb. Eating plant sources of iron together with sources of vitamin C may improve absorption. While iron stores might be lower, the prevalence of iron deficiency anemia is not higher in vegetarians, suggesting that the low stores are not necessarily a problem.^[22][21] It could be worth monitoring iron levels in some people who are vegetarian, such as children and during pregnancy.^[22]

Another concern is zinc. In omnivorous diets, the majority of the zinc intake comes from meat, especially from beef. When eating a vegetarian diet, it is possible to get a similar zinc intake through legumes, whole grains, nuts, and seeds, but this requires careful planning.^[22][23] However, so far there is no clear evidence that the lower zinc intake associated with a vegetarian diet causes any long term health concerns.^[24]

Vitamin B12 is less of a concern for vegetarians than for vegans, because dairy products and eggs contain a substantial amount of vitamin B12, but vitamin B12 deficiency is still more common among vegetarians than people who eat an omnivorous diet.^[25] Including foods fortified with vitamin B12, such as cereals and grains, can help increase the daily vitamin B12 intake. In some cases, a supplement might be necessary. Monitoring or supplementing vitamin B12 is especially important during pregnancy and for early childhood development.^[26]

People who follow a vegetarian diet tend to eat more fruits, vegetables, whole grains, and legumes or add them into their diets as an alternative to animal products. These foods all have a high amount of fiber, a range of vitamins and minerals, and are often low in saturated fats.^[2]

A lower intake of saturated fat combined with an increased intake of unsaturated fat and fiber can significantly lower the levels of LDL cholesterol in the blood. An increased intake of fiber can also help moderate blood glucose levels, improving blood sugar levels for people with type 2 diabetes. Fiber can also help with weight loss, because it slows the rate at which the stomach empties, and the high volume and low energy density of certain high-fiber foods can help with appetite control by increasing satiety.^[2][27]

The increased intake of fruit and vegetables leads to an increased intake of vitamin E and vitamin C, minerals like magnesium, and antioxidants including polyphenols and tannins. These factors contribute to the antioxidant and anti-inflammatory effects of the vegetarian diet that could improve cardiovascular health.^[27][28][2]