When the federal government’s 1980 “Dietary Guidelines for Americans” warned about the baleful effects of saturated fats, public interest activists joined the fight and managed to persuade major food companies to switch to the shiny new alternative: trans fats. Thirty-five years later, the Food and Drug Administration finally determined that trans fats are not just useless but unsafe, and ordered them removed from all foods. Oops.

So much for settled science. To tell the truth, I never paid much attention to the fat fights in the first place. From my days as a medical student (and prodigious consumer of junk food), I’ve seen so many solemnly proclaimed “findings” come and go that I decided long ago to ignore — and outlive — them all.

So far, I’m ahead. Never had an egg substitute in my life. I figured trans fats were just another fad waiting to be revoked and renounced. Moreover, if I was wrong, the green eggs and ham would take so long to kill me anyway that I was more likely to be hit by a bus first. Either way, win-win.

Don’t get me wrong. I don’t advocate this kind of jaunty fatalism for everyone. This is a private affair. I do, however, preach skepticism. Remember that most venerable piece of received medical wisdom — 98.6 degrees as the average adult human temperature? In 1992, three researchers bothered to measure — and found that the conventional wisdom (based on an 1878 German study) was wrong. Normal is 98.2.

After that — 114 years of error — one is inclined to embrace Woody Allen’s “Sleeper” theory that in 200 years we’ll discover that smoking is good for you, fruits are not. I still love peaches, but I eat them for the taste — and the memories — not because they might add a month to my life (in the ICU when I’m 90).

I don’t mean to be cynical, just realistic. Take fish oil. For at least 10 years the National Institutes of Health has strongly recommended omega-3 fatty acids and fish oil for the prevention of cardiovascular disease.

I held out, trusting both my gastronomic prejudices (more turf than surf) and my faith that time ultimately undoes all of life’s verities. I waited. My orneriness has not been fully vindicated — NIH still recommends dietary fish oil — but it does find omega-3 supplements to be useless.

Exhibit A for medical skepticism, however, remains vitamin C. When Linus Pauling, Nobel laureate in chemistry (not nutrition), began the vitamin-C megadose fad to fend off all manner of disease, the whole thing struck me as bizarre. Yes, you need some C to prevent scurvy if you’re seven months at sea with Capt. Cook and citrus is nowhere to be found. Otherwise, the megadose is a crock. Evolution is pretty clever. For 2 million years it made sure Homo erectus, neanderthalensis, sapiens, what have you, got his daily dose without having to visit a GNC store.

Sure enough, that fashion came and went. But there are always new windmills to be tilted at. The latest is gluten.

Now, if you suffer from celiac disease, you need a gluten-free diet. How many of us is that? Less than 1 percent. And yet supermarket shelves are groaning with products proclaiming their gluten-freedom. Sales are going through the roof.

Another crock. Turns out, according to a massive Australian study of 3,200 products, gluten-free is useless. “The foods can be significantly more expensive and are very trendy to eat,” says Jason Wu, the principal investigator. “But we discovered a negligible difference when looking at their overall nutrition.”

Told you so.

Why then am I not agitating to have this junk taken off the shelves? Because of my other obsession: placebos. For which I have an undying respect, acquired during my early years as a general-hospital psychiatrist. If you believe in the curative powers of something — often encouraged by the authority of your physician — a sugar pill or a glass of plain water can produce remarkable symptom relief. I’ve seen it. I’ve done it.

So I’d never mess with it. If a placebo can alleviate your pain, that’s better than opioids. If going gluten-free gives a spring to your step, why not? But please, let the civility go both ways. Let the virtuous Fitbit foodie, all omega-3’d and gluten-free, drop the self-congratulatory smugness. And I promise not to say it’s all in his head.

Live and let eat. Merry Christmas.

© 2015, The Washington Post Writers Group