Although motivations for following a vegetarian lifestyle are complex, research on Western populations identifies common reasons for being vegetarian, including: Ethical and environmental concerns, religious concerns, health reasons and gustatory reasons. Some individuals with eating disorders may adopt a vegetarian diet as a means of weight control. Research also indicates that dietary patterns vary depending on the motivation behind a vegetarian lifestyle.

• Grade I means there is Good/Strong evidence supporting the statement;
• Grade II is Fair;
• Grade III is Limited/Weak;
• Grade IV is Expert Opinion Only;
• Grade V is Not Assignable.
• High (A) means we are very confident that the true effect lies close to that of the estimate of the effect;
• Moderate (B) means we are moderately confident in the effect estimate;
• Low (C) means our confidence in the effect estimate is limited;
• Very Low (D) means we have very little confidence in the effect estimate.
• Ungraded means a grade is not assignable.

Vegetarianism is a fluid concept. Research shows that it is common for individuals who consider themselves vegetarian to change dietary patterns over time, sometimes becoming more restrictive, sometimes less so. Additionally, societal perspectives on vegetarianism change over time. It is more common now for individuals to be vegetarian for ethical or environmental reasons vs. 30 years ago. Likewise, it is more common for research to focus on the health benefits of vegetarianism and plant-based diets than in decades past when the focus tended to be on potential nutrient deficiencies associated with vegetarian diets.

• Grade I means there is Good/Strong evidence supporting the statement;
• Grade II is Fair;
• Grade III is Limited/Weak;
• Grade IV is Expert Opinion Only;
• Grade V is Not Assignable.
• High (A) means we are very confident that the true effect lies close to that of the estimate of the effect;
• Moderate (B) means we are moderately confident in the effect estimate;
• Low (C) means our confidence in the effect estimate is limited;
• Very Low (D) means we have very little confidence in the effect estimate.
• Ungraded means a grade is not assignable.

The two most common ways of defining vegetarian diets in the research are:

Vegan diets: Diets devoid of all flesh foods,

Vegetarian Diets:  Diets devoid of all flesh foods, but also include egg (ovo) and/or dairy (lacto) products.

However, these very broad categories mask important variations within vegetarian diets and dietary practices. These variations within vegetarian diets make absolute categorization of vegetarian dietary practices difficult and may be one of the sources of unclear relationships between vegetarian diets and other factors.

• Grade I means there is Good/Strong evidence supporting the statement;
• Grade II is Fair;
• Grade III is Limited/Weak;
• Grade IV is Expert Opinion Only;
• Grade V is Not Assignable.
• High (A) means we are very confident that the true effect lies close to that of the estimate of the effect;
• Moderate (B) means we are moderately confident in the effect estimate;
• Low (C) means our confidence in the effect estimate is limited;
• Very Low (D) means we have very little confidence in the effect estimate.
• Ungraded means a grade is not assignable.