The Guardian for instance writes: "High levels of dietary animal protein in people under 65 years of age was linked to a fourfold increase in their risk of death from cancer or diabetes", adding that "people should eat a low-protein diet until old age" (this is based on a study with a press release claiming that "eating animal protein may be as harmful as smoking").
Given the way the results of (generally honest and competent) scientific studies are reported, it may seem (if not explicitly claimed) that animal proteins cause diseases that plant proteins do not. However, it is important to understand that drawing useful conclusions from such studies can be hard (if not impossible).
Number of new cases of colon cancer (per 100 000 women per year) as a function of the daily meat consumption (in grams) for various countries.
These studies compare several populations: for instance people who (turn out to) eat more meat and who people eat less. If the former are more obese or die more of heart diseases, then it is clear that there is some sort of link between eating meat and such health issues. A plot such as the one on the right for instance can certainly convince us of a link between colon cancer and meat. However, these people differ in many other ways (meat consumption is certainly not the only difference between Africa, Japan and New Zealand), so how do we know that meat is actually the root of the problem? This only shows a correlation: it does not prove that eating meat causes the problem.
Why can we not conclude that meat is the cause of the problem? Because meat consumption itself correlates with other possible causes. Let us think concretely: what is different about big meat eaters?
All these explanations are compatible with a correlation between eating more meat and certain diseases without meat itself being the culprit. And even if meat
When properly looked at, studies of individual diseases have the nasty habit of showing that X correlates with (or causes, depending on the type of study) a certain disease but not total mortality. Or a certain diet is successful at reducing death from a certain disease but does not curb the overall death rate. This is a great diet to follow if you really want to avoid dying of cancer and prefer myocardial infarction instead (or the other way round); but if your want to live a long and healthy life, this is useless.
Proteins are chains of amino acids, like beads on a string. There are 20 different amino acids your body need, half of which it cannot make itself (these essential amino acids must come from food). Animal proteins are complete, which means that they contain all the amino acids you need. Plant proteins are incomplete (except soy), which means that you need different sources of proteins to get all the amino acids you need, e.g. grain plus legumes (lentils, peas).
Moreover, animal proteins are used more efficiently by your body. If you eat 10 g of animal proteins your body will be able to use about 9 g, against only 6 g with grain proteins. So it is necessary to consume more plant proteins for the same results; for instance if the recommended intake is 50 g a day, vegans need to consume about 75 g.
Neither superiority of animal protein makes it impossible to get proteins from plants (or even meet all your protein needs). But claims to the superiority of plant protein are more likely to refer to ideological superiority than to nutritional superiority.
The body breaks proteins down into amino acids. Once this happens, there is no telling which amino acid came from a plant and which from an animal (this is unlike fat: saturated fatty acids remain different from unsaturated ones). So how can animal proteins be bad and plant proteins good?
The fact that vegan propaganda against animal proteins has no reasonable ground does not mean that there are actually no plausible reasons for eating less meat.
First, we eat far more proteins than needed (this means more than the minimum, not necessarily too much). Our daily needs are something like 0.8 or 1 g of protein per kg of body weight (i.e. with a weight of 72 kg, I need something like 60 g of proteins a day). Protein-rich foods have about 20–25 g of proteins per 100 g, so I need about 250–300 g of meat, fish, milk, legumes, etc. a day (in total, not of each). (Given that foods such as whole grains contain proteins as well, the amount needed from the conspicuous protein providers is in fact lower.) People in rich countries would not have protein deficiencies if they ate less meat. So the key question is not why we should eat less meat but rather why we should eat as much as we currently do.
By eating less meat, you would also consume less saturated fat, and if you did not replace it with anything this would reduce your total calorie intake (whether the former is good and the latter meaningful is contentious). If you replace meat with fish, legumes or whole grains (brown rice, whole-wheat pasta), you will get proteins too, but also omega-3 fatty acids, vitamins, minerals, fibers, etc. (People who currently eat more meat than necessary to get enough amino acids would be at no risk of deficiency in vitamin B12, zinc or iron by lowering their intake.)
Meat is bad for the planet because it is produced inefficiently — not by farmers or industry, but by the animals themselves. It takes 2–5 kg of feed to produce 1 kg of chicken or pork, and something like 10 kg for beef (eggs and milk are more efficient). If all the plants (mostly corn and soybeans) grown for animal feed were used for humans, the agricultural pressure on the environment would be milder.
However, the inefficiency of meat is not the worst. With beef, 90% of the feed is wasted (10 kg of corn and soybeans are turned into 1 kg of meat); with pets the waste is 100%. I am always puzzled when people who keep cats or dogs say that meat is bad for the planet and causes hunger in poor countries.
Also, it is not meat itself that causes the problem but rather the production system. (There are intrinsic problems, such as methane emissions from cattle, but a better system would still be a huge improvement.) Cows raised in pastures in mountains or in lowlands where no crop would grow do not take land away from food production for humans. Turning 10 kg of grass into 1 kg of meat does not waste 9 kg of food, rather it creates 1 kg of food. (Also see the last chapter in Vaclav Smil's "Should we eat meat?".) I would like to see people who disagree with this argument eat the grass themselves.