Photograph: Moyan Brenn

When it came to good nutrition, I thought I was doing pretty well. I survived the 2005 transition from a readable food pyramid to an incomprehensible one, and the 2011 change from the incomprehensible one to choosemyplate.gov. Ever since I read The Omnivore’s Dilemma (2006), I have ferreted out and deleted high-fructose corn syrup from my diet. And it goes without saying that I try to limit my intake of salt, saturated fats, and cholesterol.

But new government guidelines released in February have taken the brakes off dietary intake of cholesterol. It seems I am free to eat eggs, lobster, and oysters without fear for my life.

How, in a mere five years, could our ideas about nutrition do such an about-face? There are several possible explanations. First, it may be really hard to do a good study linking cholesterol intake to ill health. Over time scientists may have designed better and better study methods, until, finally, a more justified truth has emerged. Second, vested interests—giant agribusinesses (purveyors of lobster, eggs, and well-larded beef) and pharmaceutical companies (purveyors of cholesterol-lowering drugs such as statins)— may have influenced the guidelines. Third, foods have changed. Perhaps newer studies differ from older ones because an egg circa 1960 is not an egg circa 2000. Today’s chickens are more inbred. Their own food intake has changed, possibly altering the cholesterol in their eggs. Fourth, we have started to focus on human metabolism at the level of multi-organ interactions. Instead of treating diabetes as a disease of the pancreas and obesity as a problem of fat storage, we now talk about a metabolic syndrome, which links high blood sugar, high blood pressure, excess midriff fat, and abnormal cholesterol levels to increased risk of heart disease and diabetes. As conceptions of disease change, so do ideas about the sources of disease. And fifth, the tried-and-true “all of the above.” Let us briefly consider each possibility.

First, the Dietary Guidelines for Americans Committee (DGAC), a joint committee of the U.S. Departments of Agriculture and Health and Human Services, is required by law to issue new dietary guidelines every five years. In pure governmentese, the 2015 report states, “Available evidence shows no appreciable relationship between consumption of dietary cholesterol and serum cholesterol.” Therefore “cholesterol is not a nutrient of concern for overconsumption,” a conclusion the DGAC says is consistent with recent findings from the American Heart Association and American College of Cardiology.

But there is some slippage here. One of the DGAC’s cited references found no relationship between egg consumption and cardiovascular disease in the general population, but it did not mention cholesterol specifically. However, the same study found egg consumption was associated with a higher incidence of type-2 diabetes and cardiovascular disease in a diabetic population. A second 2013 study concluded that evidence linking restricted cholesterol intake to improved cardiovascular health was weak, meaning not that cholesterol is irrelevant to health but that better studies are in order.

Why do our ideas about nutrition change so quickly?

Why would the new DGAC report be so misleading, drawn from reports that barely mention cholesterol except to conclude there is not enough evidence to make a recommendation? This points to my second explanation: vested interests. In 2000 seven of eleven DGAC members had relationships with two meat associations, one sugar association, one grain association, five food companies (big ones, such as McDonald’s), six industry-sponsored associations, and twenty-eight pharmaceutical companies. In 2010 nine of thirteen members had such relationships. In addition, there are almost a thousand public comments on the draft 2015 guidelines.

I did not attempt to rigorously dissect the comments, but, at a glance, they appear to originate with three broad groups: representatives of the food and agriculture industries, individual citizens and public interest organizations urging strong reforms in our patterns of food consumption and production, and individual citizens furious with the government for intervening in their right to eat whatever they choose, no matter how bad it might be for them. So while I can’t say for sure how “not enough evidence” in the supporting literature turned into “not a nutrient of concern” in the report, clearly the DGAC is buffeted by diverse interests and beliefs. Giving them the benefit of the doubt, members do their best to produce a consensus from scientists who are in turn influenced in a variety of ways by agribusiness, food retailers, pharmaceutical companies, public interest and patient advocacy groups, and the citizens who care enough to publicly comment.

The jury is still out on option number three—changes in food quality over time. The locavore, free-range, organic, and grass-fed farming movements have made it possible to eat meat from animals that are not fattened on grain and fed antibiotics and hormones. This meat does differ nutritionally from conventionally produced alternatives. But it is too soon to say if these trends have changed the effects of animal products on human cardiovascular systems. Similarly, the composition of fatty acids in eggs changes—whether for better or for worse is unclear—according to what chickens eat, for instance corn as opposed to a natural diet of grass seed and insects. But, again, it is too soon to conclude what this means for human well-being. In the future, however, food-quality changes might lead to new understandings of what is healthy to eat.

Another factor in the volatility of nutritional advice is the shifting concept of diet itself. Physiology and medicine have become more holistic scientific endeavors. High blood sugar is understood to result from a multi-organ syndrome of malfunction, not just pancreas-based diabetes. Cardiovascular diseases, which result from and cause problems in many organs, have displaced single-organ heart disease as an object of study. Food guidelines have followed suit, becoming more holistic and including exercise as part of a dietary system aimed at the whole organism. Thus the trend is away from recommending X milligrams of one substance and Y of another and toward a food and exercise system. For instance, the 2010 Government Dietary Guidelines introduced the DASH and Mediterranean eating patterns, which focus on styles of consumption that essentially incorporate the food pyramid and choosemyplate.gov guidelines in a form replete with recipes and advice about healthier eating. Numerous scientific studies show that these diets reduce high blood pressure and risk of cardiovascular disease and can be incorporated into a healthy weight loss program. The idea, then, is to think globally about intake rather than try to obey a growing number of specific injunctions aimed at individual nutrients.

So, why does official dietary advice change? My answer, of course, is number five: all of the above, and then some. Our scientific accounts of the body and our methods of studying diet and exercise improve, one hopes, but the science and science policy produced by government agencies are influenced by the agricultural, food, and pharmaceutical industries as well as by public interest groups—both ones I agree with and ones I don’t. The DGAC is probably right that it is myopic to focus on 300 milligrams daily consumption of cholesterol. But, as a recent New York Times piece says, “Bacon and egg yolks are [still] not health foods.” I, for one, am sticking to my two eggs per week.