Technically, there are six types of vegetarian diet. Most people who claim to be vegetarian will fit into one of these categories:

• Semi-vegetarian – dairy foods, eggs, chicken, and fish, but no other animal flesh.
• Pesco-vegetarian – dairy foods, eggs, and fish, but no other animal flesh.
• Lacto-ovo-vegetarian – dairy foods and eggs, but no animal flesh.
• Lacto-vegetarian – dairy foods, but no animal flesh or eggs.
• Ovo-vegetarian – eggs, but no dairy foods or animal flesh.
• Vegan – no animal foods of any type.

In general, vegetarian diets are lower in saturated fats, cholesterol and animal protein, and higher in fiber and folate than non-vegetarian diets. Consequently, vegetarians have substantially reduced risks for obesity, heart disease, high blood pressure, diabetes mellitus, osteoporosis, and some forms of cancer - particularly lung cancer and colon cancer. In western countries, vegetarians often live longer, on average, than non-vegetarians.

However, differences in lifestyle – not just eating habits – also play a huge role in vegetarian health. People who choose to follow a vegetarian diet are often non-smokers, have a lower body weight, and exercise regularly.

Of course, vegetarians can also be notoriously unhealthy. Some people simply cut meat out of their diet, only to replace it with more junk food. They don’t take care to satiate their appetite with plenty of protein and fiber. This often leads to cravings and overindulgence in high-sugar, high-fat, empty calories.

Replacing meat with poor substitutes can also lead to nutrient deficiencies in protein, iron, calcium, zinc and vitamin B12. Athletes, children and pregnant females are particularly at risk as their nutrient needs are especially high. Vegans (who eat no animal products of any type) in particular need to supplement their vitamin B12 intake. Poor meal planning, illness, stress and the excessive use of supplements can also cause problems.