Many people are vegetarians because they believe it is wrong to eat animals. The moral argument for being a vegetarian comes in a variety of flavours. One such flavour is as follows. Humans can live healthily without eating meat; it is wrong to cause unnecessary suffering; eating meat causes unnecessary suffering to animals; therefore, it is wrong to eat meat. Before I get to my main point, I want to take a slight digression vis-a-vis the first premise, namely that humans can live healthily without eating meat.
At present, there doesn't seem to be any evidence that, all else being equal, someone eating the healthiest possible vegetarian diet could become healthier by including some amount of meat in her diet. However, suppose there was such evidence. It is not inconceivable that scientists might one day find some. How big would the effect of meat supplementation have to be before the argument in the first paragraph no longer held? It is reasonable to hypothesise that, if there were an effect, it would be described by an inverse-U relationship (see below). For example, suppose that healthiness were measured in QALYs, and Δh* = 3. Would that be sufficiently great to justify eating meat?
Now to return to my main point. A non-trivial fraction of people who don't eat meat for moral reasons are not opposed to animal research. One prominent example is the moral philosopher Peter Singer. After all, there are good reasons for thinking that animal research has contributed significantly to advances in medicine over the last century. Incidentally, this is not to say that we couldn't have made advances in medicine without animal research, but just that they would have taken longer, or in a few cases, might not have occurred at all. The typical argument from someone who opposes eating meat but does not oppose animal research is as follows. Humans can live healthily without eating meat; humans cannot live healthily without animal research; therefore, although animal research causes suffering to animals, it is morally justified. Another way of stating this argument is as follows. The benefits of eating meat are small relative to the costs; the benefits of animal research are large relative to the costs; therefore, animal research is morally justified, whereas eating meat is not.
Notice that the argument is full-bloodedly utilitarian. The person who opposes eating meat but not animal research is not defending the absolute rights of animals, but is arguing that they should not be harmed unless the benefits to humans are sufficiently great. In other words, he is willing to use animals for his own purposes so long as the benefits to him and other members of his species are large enough. And once it is acknowledged that the argument in the third paragraph is a utilitarian one, establishing its veracity becomes a matter of empirical enquiry.
Short of empirical enquiry, a little reasoning suggests the argument may not be correct. The main reason is that healthiness (or QALYs lived) is not the sole criterion by which utilitarian propositions are evaluated. Even if supplementing the healthiest possible vegetarian diet with some amount of meat does not make someone on that diet any healthier, it does not follow that eating meat has zero benefits. Importantly, people seem to get a lot of pleasure from eating meat. It is almost certainly true that some fraction of people would be willing to trade-off some QALYs for the opportunity to eat a small amount of meat. And contrary to some versions of the argument in the third paragraph, there is no obvious reason why wanting to live longer is less selfish than wanting to enjoy the taste of meat.
Notice that I am not claiming that eating meat is morally justified. I am simply arguing that it is not obvious that animal research is morally justified, while eating meat is not.