Meat, how often, how much, what type?

Meat is nutritious, providing macro and micronutrients – protein and fat, iron, zinc and B12 – although all its nutrients are available in sufficient quantities from non-meat foods given a balanced and varied diet. Humans have eaten meat for thousands of years; it’s just that it hasn’t been as intensively farmed as it often is now, and hasn’t been as highly processed as much of our meat is, or been eaten in such abundance. But is it healthy for us and the environment?