A vegetarian diet confers a wide range of health benefits. Research has proven that vegetarians suffer less from many of the diseases linked to a modern Western diet: obesity, coronary heart disease, hypertension, type II diabetes, diet-related cancers, diverticular disease, constipation and gall stones.
Compared with non-vegetarians, Western vegetarians have:

-   A lower average Body Mass Index (BMI) (by about 1 kg/m2). 

-   A lower mean plasma total cholesterol concentration (by about 0.5 mmol/l).

-   A lower mortality from IHD (by about 25 percent).

-   They may also have a lower risk for some other diseases such as constipation, diverticular disease, gallstones and appendicitis.

This is the only major ongoing study on the general health and mortality of vegetarians in the U.S. Data was collected since 1976. Of the 34,192 participants, all members of the Seventh-day Adventist church, 29 percent were vegetarian, while 7-10 percent of the vegetarians were vegan. Compared to non-vegetarians the above vegetarians had about: 

-   1/2 the high blood pressure and diabetes

-   1/2 the colon cancer 

-   2/3 the rheumatoid arthritis and prostate cancer

-   Breast, lung, & uterine cancers tended to be lower in vegetarians but could have been due to random chance.

Life expectancies in the Adventist Health Study have recently been published. They show that this group of Seventh-day Adventists appears to be the longest-lived, formally studied population in the world (with an average life span of 78.5 years for men, 82.3 for women.

Diets high in vitamin C and beta-carotene provide significant protection against Parkinson's disease. American researchers have concluded that a high intake of animal fats is associated with a five-fold increase risk.

Many of the diet and lifestyle changes recommended against allergies are a natural consequence of adhering to a more animal-free lifestyle: avoiding saturated fats (meats and dairy products), eggs, shellfish; consuming fresh fruits and vegetables (be careful of citrus in particular though), whole grains, nuts, seeds, and fresh juices; using synthetic materials such as acrylic instead of animal products such as wool.

Multiple Sclerosis
A vegan/vegetarian diet satisfies several of the requirements set out in the Swank Diet for Multiple Sclerosis. When the disease is treated early in its course, dietary measures can provide dramatic improvement. The dietary regimens which contain the most pro-inflammatory food types (e.g. gluten, dairy, saturated fat) and the least anti-inflammatory nutrients ( vitamin D, omega 3 fats) occur in areas in which MS and other autoimmune diseases are most common.

Stroke
A low incidence of cerebrovascular disease was associated with geographical regions where fresh fruit and vegetable consumption (increased potassium) was high

Constipation
A vegetarian diet is generally higher in fiber than non-vegetarian diets. Constipation amongst those on vegan and raw-food diets is extremely rare.

Cholesterol
Cholesterol levels are much lower in vegetarians. Vegetarian diets reduce serum cholesterol levels to a much greater degree than is achieved with the National Cholesterol Education Program Step Two diet. In one study published in The Lancet  total cholesterol in those following a vegetarian diet for 12 months decreased by 24.3%.

Obesity
Obesity is a major contributor to many serious illnesses, and is much less common among vegetarians, compared to the general population. Vegetarians are, on average, about 10% leaner then omnivores.

Bone Strength
Calcium loss is reduced with a vegetarian type diet.

A study published in the January, 2001 edition of the American Journal of Clinical Nutrition examined the diets of 1,035 women, particularly focusing on the protein intake from animal and vegetable products. Deborah Sellmeyer, M.D., found that animal protein increases bone loss. In her study, women with a high animal-to-vegetable protein ratio experienced an increased rate of femoral neck bone loss. A high animal-to-vegetable protein ratio was also associated with an increased risk of hip fracture. Dr. Sellmeyer states: "Sulfur-containing amino acids in protein-containing foods are metabolized to sulfuric acid. Animal foods provide predominantly acid precursors. Acidosis stimulates osteoclastic activity and inhibits osteoblast activity."
Higher consumption of fruits, vegetables and cereals was associated with greater femoral bone mineral density (in men) in a study of 907 older adults.

Gall and Kidneystones
In a study published in the British Medical Journal, it was shown that vegetarian women had a much lower incidence of gallstones than non-vegetarian women. Of the 632 vegetarians, overall occurrence of gallstones was 25%. Vegetarians had only half as many gallstone problems, with 12% being found to have gallstones
Sixty men with recurrent kidney stones who were on a low animal protein and low salt diet were compared with 60 men who were on a low calcium diet. It was found during a 5 year followup that only 12 out of 60 men on the low animal protein, low salt diet had a recurrence of kidney stones while 23 of 60 men on the low calcium diet had a recurrence.

Hypertension
The prevalence of hypertension among vegetarians is about one-third to one-half that of non-vegetarians. A study of Caucasian Seventh-day Adventists found hypertension in 22% of omnivores, but only 7% of vegetarians. Among African Americans, the prevalence was 44% of omnivores and 18% of vegetarians. Adopting a vegetarian diet significantly lowers blood pressure in both normal and hypertensive individuals.

Cancer
Cancer rates for vegetarians are 25 to 50% below population averages, even after controlling for smoking, body mass index, and socioeconomic status.Vegans show even better results. One study found that people who include generous amounts of fruits and vegetables in their daily diets have lower rates of cancers of the lung, breast, colon, bladder, stomach, mouth, larynx, esophagus, pancreas, and cervix compared to people who avoid such foods.
Fruits and vegetables contain antioxidant substances, such as vitamin C, vitamin E, and carotenoids, which protect cells against oxidative damage, which is related to cancer risk and other health problems. The multitude of phytochemicals found in various fruits, vegetables, grains, legumes, and nuts are thought to protect against heart disease and cancer.

Headaches, Migraine
A long-term trial of a vegan diet (elimination of all animal products) provided significant improvement in 92% of the 25 patients who completed the study. Drinking water was limited to spring water (chlorinated tap water was prohibited), and coffee, tea, chocolate, sugar and salt were excluded.
When migraine and tension headache patients are placed on low-protein, natural plant-based diets, with no refined sweets of any type, they often improve  within a month. 

Gout
Preferably no meat should be eaten as it is rich in uric acid forming components. Raw fruit, vegetables, grains, seeds and nuts are highly recommended.

Arthritis
Rheumatoid arthritis patients consumed either the typical Western diet or an antiinflammatory diet, which was a modified lactovegetarian diet for an 8-month period. Patients in both groups received either placebo or an oil which provided 30mg of total fish oil omega-3 fatty acids (EPA, DHA) per kg of body weight. Those on the antiinflammatory diet compared with the Western diet experienced a reduction in tender and swollen joints by 14% during the placebo treatment while during the fish oil supplementation, the anti-inflammatory diet compared with the Western diet had a reduction in the number of tender joints at 28% versus 11%, respectively, and swollen joints at 34% versus 22%, respectively.

Conclusion
It is obvious that a diet mainly based on plant derived foods has clear health benefits. This does not mean that switching to a vegetarian or vegan diet will change your health overnight or have you thrown out your medication. A change in diet can have an effect on your health on the longterm but just as medicines work for some people better then others it does not necessarily be a cure for you. It reflects strongly that lifestyle (diet and exercise) is the cornerstone for general good health.