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It seems generally agreed that (even fresh, unprocessed) red meat is at least somewhat bad for human health. But why exactly is this the case? It's difficult to find any attempt at an explanation, especially for anyone not too familiar with diving deep into the medical literature.

Is it because the myoglobin in red meat is carcinogenic or otherwise harmful? That would seem to be the most direct explanation, and there are a few studies on this, but they don't seem to be widely cited in discussions on health effects of red meat. If that is true, would it mean that one should avoid dark, myoglobin-rich fish and poultry such as tuna and ostrich just as much as mammal meat?

Is it only because red meat tends to be more fatty? In that case, would leaner cuts of red meat have the same health effects as white meat?

Or is it possible that there is some deep genetic reason why the flesh of mammals is more harmful than the flesh of birds and fish for human consumption? (I had wrongly assumed this was a strange idea, but acvill's answer gives a plausible explanation.)

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The gut microbiome is partly to blame, at least for the associations of red meat consumption with heart disease. Red meat has a high concentration of L-carnitine, which is metabolized by bacteria in the gut to produce trimethylamine, which is further metabolized by FMO3 in the liver to produce trimethylamine-N-oxide (TMAO). High blood levels of TMAO are associated with higher risk for both cardiovascular disease and early death from any cause.

Sources on the relationship between the gut microbiome, L-carnitine, and heart health:

New study links L-carnitine in red meat to heart disease

Red meat, TMAO, and your heart

Or is it possible that there is some deep genetic reason why the flesh of mammals is more harmful than the flesh of birds and fish for human consumption?

The NIH gives the following values for the carnitine content of different foods:

Food Milligrams (mg)
Beef steak, cooked, 4 ounces 56–162
Ground beef, cooked, 4 ounces 87–99
Milk, whole, 1 cup 8
Codfish, cooked, 4 ounces 4–7
Chicken breast, cooked, 4 ounces 3–5
Ice cream, ½ cup 3
Cheese, cheddar, 2 ounces 2
Whole–wheat bread, 2 slices 0.2
Asparagus, cooked, ½ cup 0.1
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