Washington - Last year, eggs were declared safe.
After demonising the cholesterol in them for a generation, nutritionists finally acknowledged that there was overwhelming scientific evidence that eggs were not artery-clogging killers after all.
But wait. What's this? The government's latest nutrition guidelinescame out this month and they're not egg-friendly. They say people should consume as little cholesterol as possible.
That's even stricter than the 2010 standard allowing 300ml a day, about the amount in one egg.
Scientists are supposed to change their minds when confronted with new evidence - whether it's reclassifying Pluto as not quite a planet or admitting that Neanderthals contributed to the modern human gene pool.
When it comes to diet, though, even scientists sometimes get stuck in a rut. Then they drive the rest of us into a baffling morass of nutrition advice, in which the cholesterol paradox is a world-class stumper.
Why would the same nutrition scientists who said last year that “cholesterol is not considered a nutrient of concern for overconsumption” keep warning people not to eat it?
The answer lies in some of the less than scientific beliefs held by nutritionists. Underlying their endeavour is the faith that there are good foods and bad foods - and that by strictly avoiding the bad foods we can conquer heart disease, cancer, and perhaps put off death itself.
That faith has led them to warn people away from anything that presents even the remotest possibility of causing harm. It's a misuse of the precautionary principle: the idea that substances should be treated as dangerous until scientifically proven to be safe.
Reasonable precaution makes sense - most people expect extensive safety testing on new artificial sweetener or drugs given to pregnant women for morning sickness, for example. Food choices can certainly influence health. There's a strong consensus that too much sugar is a risk factor for obesity and diabetes, for example. But too much caution can do more harm than good.
The problem with applying the precautionary principle to food is that it fails to take account of alternatives. When told not to eat one thing, we reach for something else.
Provisional evidence that butter and cream caused heart attacks led to increased consumption of margarine and non-dairy creamer instead. Many heart attacks and bypass operations later, research determined that the trans fats in these substances were much worse.
Trans fats - aka hydrogenated vegetable oils - are manufactured through a process that renders them chemically distinct from the fats coming from plants and animals. For much of the 20th century, they were a major component of margarine as well as commercial pastries, processed foods and snacks. The stuff not only raises bad cholesterol, it lowers good cholesterol and boosts triglycerides.
The health strictures against eggs went along with a general demonisation of fats. So for years people ate more carbohydrates - a prescription that many experts now admit played a role in the current epidemic of obesity and type 2 diabetes.
Scientists painted such a fearsome picture of fat and cholesterol, said one heart specialist, that gummy bears and other candies were being promoted because they were fat free.
Meanwhile there were never good evidence that eggs had more than a minor effect on blood cholesterol or that eating them in moderation was harmful. Top heart specialists such as Dan Rader at the University of Pennsylvania say humans break down most of the cholesterol in food. Most of the cholesterol in the bloodstream is made in the liver. The body uses it to make everything from cell membranes to sex hormones.
Some people develop abnormally high blood cholesterol because the mechanism for cleaning up the excess gets broken. The biggest risk factors for inadequate clean-up are genes, trans fats, and, to a lesser extent, saturated fats. Not eggs.
Why can't the guidelines reflect this? The USDA's explanation is that foods high in cholesterol also have lots of saturated fat, but that's misleading. Eggs have very little saturated fat. The same goes for shrimp and shellfish - which, contrary to conventional wisdom, may not even be high in cholesterol.
Oh, and about those saturated fats - found in meat, poultry, cheese and butter - the kind the French eat while remaining quite healthy. Their deadly reputation may be exaggerated or undeserved.
The scientific literature is full of contradictory claims. A 2013 meta analysis concluded that cutting back on saturated fat didn't help prevent heart disease. Some nutritionists say that study was misleading because people were substituting carbohydrates for saturated fat. (Who could possibly have led them to do that?) The new guidelines tell people to replace the saturated fats with unsaturated fats - the stuff found in vegetable oils.
Much of the science of saturated-fat risk does not come from experiments. Instead, it's based on observational studies that rely on self-reporting, which is notoriously unreliable. Steve Nissen, head of cardiology at the Cleveland Clinic, said he doesn't believe science knows yet whether saturated fats belong on the bad list and unsaturated fats on the good. Other experts agree.
The reaction of many nutritionists was to say that the USDA didn't make its recommendations scary enough. They blamed the food industry (The egg lobby must have been out on a company picnic). But if the nutritionists had their precautionary way, we'd all be subsisting on kale salad. With no cheese - and no assurance of living better or longer.
* Faye Flam writes about science, mathematics and medicine. She has been a staff writer for Science magazine and a columnist for the Philadelphia Inquirer. She is author of “The Score: How the Quest for Sex has Shaped the Modern Man.”