This article is from the Food Science FAQ, firstname.lastname@example.org (Paul E. J. King) with numerous contributions by others.
Not necessarily. Current nutritional advice, to eat less fat,
more fibre, more fresh fruit and vegetables and more starchy foods,
may indeed be easier to achieve with a vegetarian diet. However,
animal foods provide a concentrated source of protein, vitamins and
minerals. These nutrients can be obtained from a vegetarian diet, but,
unless it is expertly-designed, there could be difficulties with
protein quality and with some micro-nutrients, especially with
calcium and vitamin B2 (riboflavin) if milk products are rejected.
[Note: Whereas vegans are very well informed about problems of
obtaining sufficient vitamin B12 in a vegan diet, and there are
numerous yeast-based spreads and supplements for their use, no
warning is given anywhere in vegetarian/vegan literature about
vitamin B2. In a typical western diet, some 40 per cent of the
vitamin B2 intake derives from milk products. Someone switching to a
vegan or strict vegetarian diet that excludes milk products will not
only lose a major source of calcium, but will (in most cases,
unknowingly) lose that 40% of vitamin B2. That is why we rectify that
information deficiency in this FAQ, so that the deficits can be made
up from other sources].
There is an increase in the number of people who are vegetarian;
either because they are concerned about animal welfare, especially of
farm animals, and do not wish to eat meat or animal products, or
because they believe that there are health benefits in following a
vegetarian diet. The Vegetarian Society provides a wealth of
vegetarian nutrition information to help ensure the nutritional
adequacy of such diets.