The correlations between eating red meat and getting heart disease are suggestive. But the mechanism has always been a bit dim. Most of the research on saturated fat that was religiously followed in the 1990s has turned out to be less robust than we originally thought. That's led "paleo" diet followers and Atkins types to focus on eating almost nothing but meat. But the correlation still seems to be there.

I've long suspected that it was a case of selection bias. The people most likely to follow any diet fad are upper-middle-class people who are likely to live longer than everyone else anyway. So if you tell them to get on a low-fat diet, ten years later, your long-term study of dietary patterns and mortality will show that lo and behold, low-fat diets improve mortality. If they all switch to eating red meat, then ten years later, red meat will look like some life-lengthening elixir. But over the last 20 years, the elites have been switching away from red meat, so red meat is apt to look deadly.

Well, today I got a big challenge to that theory. Scientists have isolated what looks like a plausible mechanism by which red meat damages your heart: the gut bacteria of frequent meat eaters process carnitine, a chemical found in red meat, into something called TMAO. And TMAO is associated with a higher risk of heart attacks.

Interestingly, this only happened to frequent meat eaters; vegans who ate a steak did not show elevated blood-levels of TMAO. Over time, their gut bacteria had changed, so they no longer had lots of bacteria that like to eat carnitine.

I tend to discount dietary fads, but the association between red meat and heart disease is sufficiently long-standing that I'm trying to cut back to once a week. If we see more studies like this, I may cut back even further.

Of course, as many of you will no doubt point out in the comments, it's impossible to be sure. New studies could overthrow these results any time.

But that's always a problem in this uncertain world. In this case, my best guess is that red meat probably promotes heart disease (which anyway runs in my family). And the cost of eating more poultry and fish and less steak seems relatively small. Much smaller than dying of a heart attack in my sixties.