The potential advantages or disadvantages of either a vegetarian or omnivorous diet will vary to a great degree depending on the selection of specific food items that comprise the diet of the individual. However, general qualities of these different dietary approaches may be summarized in relation to their capacity to influence metabolic processes and the probable outcomes of such dietary patterns.
Vegetarians usually have a greater in take in foods that are high in dietary fiber such as whole grains, legumes, fruit, vegetables, nuts and seeds. A low intake of dietary fiber has been associated with a number of conditions including cardiovascular disease, diabetes, gall bladder disease, diverticulitis and bowel cancer. Omnivores generally have a lower intake of fiber and may have a potentially increased risk of developing these conditions. A vegetarian diet may confer protection against such conditions especially if the diet limits the consumption of refined carbohydrates, as this will significantly reduce fiber intake and in addition most of the advantages of a high fiber diet are significantly reduced if refined carbohydrates contribute to >18% of energy intake.
A high dietary fiber intake is also associated with increased satiety and as such a vegetarian diet may assist in prevention of development of obesity. Fiber is also effective in reducing plasma cholesterol levels and specifically a high consumption of fruit, vegetables, soy and oats has been shown to exert a cholesterol reducing effect. Although it is possible for omnivores to achieve a high fiber intake it is much easier to reach these objectives with a vegetarian approach.
On the other hand, not all of the effects of a high fiber intake are beneficial. Fiber may potentially interfere with nutrient absorption, which could possibly reduce the availability of amino acids, essential fatty acids, vitamins and minerals. However in regard to minerals reduced availability has been demonstrated only when fiber is added to the diet as a supplement and not when it is a naturally occurring component of the diet.
An omnivorous diet is associated with a higher intake of saturated fats which is linked with the development of cardiovascular disease and some cancers and current dietary recommendations are that saturated fats contribute to 10% or less of total energy. The inclusion of animal products such as beef, eggs and dairy will substantially increase the intake of saturated fats.
It is entirely possible for a lacto-vegetarian or lacto-ovo vegetarian to have a saturated fat intake that exceeds that of an omnivore, especially if they rely heavily on eggs and/or dairy as a major component of the diet. However, vegans or vegetarians that consume only small quantities of eggs or dairy products will in most cases have a very low saturated fat intake.
Total fat intake will similarly be reduced in a vegetarian diet however this will vary greatly among individuals.
Essential Fatty Acids
Vegetarian diets are also generally higher in essential fatty acids however in vegetarians with a deficiency of nutrient cofactors required for the conversion of essential fatty acids into prostaglandin precursors, there may be an inability to efficiently utilize these fatty acids for eicosanoid production.
In these situations an omnivorous diet can be advantageous because the inclusion of fish provides a source of the omega 3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), which reduces the need for metabolic conversion of alpha-linoleic acid to EPA prior to production of the series 3 prostaglandins.
Achievement of Ideal Body Weight
Vegans are generally reported to exhibit lower body weights, which may indicate that a vegan diet can prevent development of obesity. The precise mechanisms involved have not been fully substantiated but may related to a reduced fat intake, a reduced efficiency of conversion of carbohydrates into fat, the satiety increasing effects of high fiber foods or several other factors.
Improving Diabetic Health with a Vegetarian Diet
A low fat, high fiber diet has also been shown to improve metabolic parameters in diabetics and can assist diabetics with achievement of ideal weight. A vegetarian diet that limits or eliminates consumption of eggs and full-fat dairy products will in many cases be an effective dietary approach for diabetics who are overweight or obese.
A low fat, high fiber diet is more difficult to achieve with an omnivorous diet however it is possible if very low fat meats such as chicken breast are consumed along with the restriction of other sources of fats. However a proportion of diabetics do not respond well to such a regime as is the case when triglycerides or insulin remain elevated and these individuals may fare better if a portion of the carbohydrate intake is substituted with monounsaturated fats as is the case with elevated triglycerides or insulin. This approach may be achieved with either an omnivorous or vegetarian diet.
Protein and a Vegetarian Diet
The major disadvantage of a vegetarian diet is the difficulty in obtaining an adequate intake of high quality protein.
This is less likely to be a concern with lact-ovo vegetarians as both egg and dairy products are considered to have an indispensable amino acid that closely matches human requirement and are also of high digestibility. However obtaining adequate protein intake and absorption may be challenging with a vegan diet as plant proteins generally have reduced digestibility and are deficient in one or more of the indispensable amino acids.
A larger quantity of plant proteins will need to be consumed in order to obtain the same amount of usable protein in comparison to animal sources and the diet may require more planning so that complementary protein sources are included in the daily diet.
In addition the vegan sources of protein are accompanied by significant quantities of carbohydrates, as with legumes and grains, or fats, as with nuts and seeds. Thus when attempting to increase the protein intake in a vegan diet as may be required in illness or convalescence, this may be difficult to achieve without also increasing the carbohydrate and/ or energy intake to a level above that which is desirable.
In these situations it may be necessary to rely on protein powders to supplement the dietary intake. The question of adequate protein intake especially applies to vegetarians following restrictive diets for in order to reduce weight as total energy consumption and subsequently protein intake will be reduced.
These obstacles are less likely to be encountered in an omnivorous diet as animal proteins have a high biological value.