The Vegetarian Society defines a vegetarian as follows:
"A vegetarian is someone who lives on a diet of grains, pulses, legumes, nuts, seeds, vegetables, fruits, fungi, algae, yeast and/or some other non-animal based foods (e.g. salt) with, or without dairy products, honey, and/or eggs. A vegetarian does not eat food that consists of, or have been produced with the aid of products consisting of or created from any part of the body of a living or dead animal. This includes meat, poultry, fish, shellfish, insects, by-product of slaughter or any food made with processing aids from these".
So it's obvious that it's more heavily plant based and animal based products are restricted.
The question to address then that often comes up is 'is choosing vegetarianism over a diet including animal based products a healthier way to live?', as is often presented.
Most, if not all, of the data is based on associations or correlations and these do not always equal causation.
What this means is that just because something appears to be the cause of something it doesn't mean it actually is. I've added a nice image I found on this and have added it below.
People use umbrellas when it rains. That doesn't mean it will rain if you take out an umbrella.
This is a really simplified version of the principle, but you can imagine that if you delve in to something more complex like nutritional science it gets a lot harder to decipher whether something being put to you is legitimate causation. It's quite common for individuals or groups with a bias to present something that looks like fact and it can take some pretty smart people to unravel the flaws in what has been put forward.
Also, a lot of the time when people talk about nutrition or other experiences it based on n=1. This means there is a sample size of one. That person alone has experienced something for themselves so it must be true because it happened to them.
This is not how science works. Things need to be repeated many times and rigorously to be accepted as consensus.
Now, n=1 can be fine for something like life experience such as 'I walked down that street, it's rough, don't go there', but when it comes to something like nutrition which can active health, well-being and longevity then you can see why you should want evidence based facts over opinion based facts.
In the time of social media everyone is opinionated, and you'll even see well educated but severely misguided figures put out information to suit an agenda. The agenda is usually 'make money'.
Vegetarianism versus a diet with meat and it's impact on health is an area that is tricky enough to unpack, but let's have a quick look to see why it isn't necessarily healthier.
Associational data consistently finds that vegetarians are healthier than meat eaters.
Very low meat intake is associated with greater longevity (Singh et al. 2003), lower cholesterol levels, lower mortality by 25% (Key et al. 1999) and on the surface it appears a switch to plants = better health and a longer life.
The flaw in the data however is that vegetarians are generally more health seeking individuals than meat eaters. Vegetarians are more likely to have all round better and more healthier lifestyles than meat eaters and it can distort the data.
Do vegetarians hang around in pubs all week smoking cigarettes and drinking heavily? A bit of an extreme example, but you get the picture.
A vegetarian will change many lifestyle behaviours as well as food and they are often matched in studies against meat eaters who's lifestyle choices are very poor around the fact they eat meat as well.
A study from Keys et al. 1996 showed that when vegetarians were matched with health seeking meat eaters there was no significance in health markers between the two groups.
If you look at the Blue Zones, the areas of the world where people have the longest life spans you see that there are a variety of diets and they are not all exclusively vegetarian.
It comes down to your overall lifestyle choices with diet being part of that cluster. There are many ways to eat a health promoting diet. Having plenty of plant based foods in there is ideal, but having an exclusively plant based diet does not hold some secret advantage.
Animal and dairy foods have nutrients we need for health and to prosper, and while those nutrients can be gotten through plants and supplementation it is easier to obtain them through those animal based sources.
As far as ethical reasons go, if someone is vegetarian because of that, that's as good a reason as any in my book.