We foolishly thought that our experiment to improve on Mother Nature’s options for fatty food and replace saturated fat with “healthier” trans fats was a win-win. But trans fats have turned out to carry some of the same health risks as saturated fats, and it has taken an unfortunate amount of time for that to become clear.

Trans fats don’t have to be artificial—some are also found in nature. Bacteria in cows and sheep are known to create trans fats that appear in the milk we drink and the mutton we eat. But for 94 percent of human existence, we consumed small amounts of trans fats. Even after the domestication of cattle, our consumption of natural trans fatty acids did not typically exceed half of 1 percent of daily calorie intake until modern times.

Creating the trans fats we typically think of involves chemically processing an oil with acceptable health characteristics (such as cottonseed oil) to make it mimic saturated fat. In 1901, a German chemist discovered that exposing polyunsaturated oil to a metal catalyst made it denser. This property caught the eye of Crisco, and the company introduced a special kind of shortening in 1911—the first American food product to include industrial trans fats. Crisco’s shortening is semi-solid at room temperature, and the oil remains stable when used for deep frying, which made it appealing to cooks and food manufacturers. It gave food a longer shelf life and cost less than butter, lard, or tallow. Not only that, a pie made with Crisco lacked the animal flavors you might find in one made with lard or tallow.

Trans fats began to appear in baked goods in the 1920s but really took off during World War II, when lard was scarce and expensive. Margarine replaced butter—and manufacturers embraced trans fat as a cheaper and more desirable alternative to saturated fat.

Trans fats then got a boost from the medical community. In the 1980s, health authorities responded to the results of studies such as the Seven Countries study of heart disease by urging Americans to reduce their intake of saturated fat to minimize the risk of heart disease. In one of history’s greater ironies, the Center for Science in the Public Interest lauded fast food companies in the early 1980s for replacing heart disease-risky saturated fat with trans fats made from what they described as “heart-healthy” polyunsaturated fats.

By 1989 more than 10,000 food products sold in the U.S. contained trans fats. In the early 1990s, 95 percent of prepared cookies and 100 percent of crackers manufactured in the U.S. contained trans fats, according to FDA estimates.

Then scientists discovered alarming health effects of trans fats. In one clinical trial, the people who consumed trans fats had significantly higher levels of LDL-cholesterol (the “bad cholesterol) in their blood and lower levels of HDL-cholesterol (the “good” cholesterol), both reliable predictors of cardiovascular disease. Trans fats consumers showed even worse cholesterol levels than those who consumed similar amounts of saturated fat. Other studies revealed additional negative health effects. When trans fats were substituted for saturated fats, inflammation increased lesions in artery walls; trans fats also increased stiffening of the arteries.

In 1994, epidemiologists estimated that 30,000 cardiovascular deaths per year in the U.S. could be attributed to the consumption of trans fats. That same year, the Center for Science in the Public Interest did a complete about-face. They petitioned the FDA to require that food products containing trans fats include information about the amount per serving on the label and urged the public to limit its intake.

In 1999, the FDA announced that food product manufacturers had seven years to reformulate their products and list the trans fat content on labels—or at least the labels on any product with 0.5 grams or more. By 2006—when those labels first began appearing on products—the damning evidence against trans fats now included 12 clinical trials, one of which suggested that eliminating industrial trans fats from the food supply could prevent at least 72,000 heart attacks and strokes every year.

U.S. food manufacturers initially opposed FDA regulation of trans fats on the grounds that trans fats had been used for decades and were generally regarded as safe. But they began reformulating products anyway because of the bad press. Since then, manufacturers have reduced or replaced the trans fats in thousands of products—mostly energy-dense and nutrient-poor foods such as cakes, cookies, pies, pastries, and savory snacks. The McDonald’s baked apple pie, for instance, no longer uses trans fats and instead uses palm oil, a tropical plant oil with levels of saturated fat that are unusually high for a plant-based oil.

But we are mistaken if we think eliminating trans fats from American food products will result in a significant reduction in obesity-related diseases like heart disease and diabetes. We won’t see the results we’re hoping for until we change how and what we eat.

As any parent of a vegetable-rejecting child knows, it can take time to learn to like fruits and vegetables. My diet in college was heavy on cheeseburgers, French fries, ice cream, and pizza, with maybe a piece of fruit and a few vegetable servings. Over time, I’ve trained my body to prefer a healthy lifestyle. I eat six servings of fruit and six or more servings of vegetables every day. Now, even a single potato chip makes me feel queasy. Eating this way takes time—about 30 minutes to prepare dinner each night, plus three trips a week to the grocery store for fresh produce. Based on a sophisticated measure of my heart functioning, my current biological age is 27. (My real age is 62.) At this point, I’m addicted to the DASH diet, which calls for eight or more servings of fruits and vegetables combined a day. It may be the one addiction that is unequivocally good for you. And it contains negligible trans fats.

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Last week in the same newspaper, food columnist and author Mark Bittman called the toddler obesity plunge a “tribute to the improved Special Supplemental Nutrition Program for Women, Infants and Children (WIC), which encourages the consumption of fruits and vegetables; to improved nutrition guidelines; to a slight reduction in the marketing of junk to children; and probably to the encouragement of breast-feeding.”

But in fact, this spectacular drop is likely an artifact of the data sample, for which 2- to 5-year-olds are a small portion. Indeed, as Paul Campos has pointed out in The New Republic, rates of obesity among this group have gone up and down several times within the past decade. The real story is less spectacular but clearer: Rates of obesity for Americans of all ages—which increased during the 1980s and 1990s—have plateaued since 2000.

The question is why, and the answer is that we have no idea. We know no more about why rates of obesity stabilized in the 2000s than we do about why they increased in the 1980s and 1990s.

There is no shortage of hypotheses, as I discovered when doing research for my book on scientific and political debates over body weight. Some point to fluctuating levels of caloric intake, but this has been difficult to demonstrate. Indeed, one study found that that children and adolescents in 1988 to 1994 were consuming the same amount of calories as their counterparts had in the 1970s, despite increases in the numbers of overweight children and adolescents during this time. Others point to increased consumption of sugared drinks, meat, or carbohydrates, but hard evidence of any of this is hard to come by.

According to another line of research, reviewed in Julie Guthman’s 2011 book Weighing In, the weight gain of the 1980s and 1990s may be largely an adaptive response to toxins in our water, air, clothing, and elsewhere. According to this argument, the body creates fat deposits as part of an effort to store these toxins away from vital organs. In this scenario, weight gain may not be—in itself—a bad thing, although it may be indicative of other problems. Some research suggests that a small but significant part of the weight gain of the 1980s and 1990s can be attributed to smoking cessation, clearly a positive development that we do not wish to reverse.

The more interesting theories underscore the importance of broad societal trends. Some researchers argue that increases in obesity rates in the 1980s and 1990s were due to growing levels of inequality. They point to evidence that obesity rates rose highest in those nations with the greatest levels of inequality and with the stingiest welfare states, as well as among the poorest members of all societies. They reason that economic inequality and insecurity produce high levels of stress, raising levels of cortisol, which, in turn, leads to weight gain. Generous welfare states buffer people somewhat from the effects of economic insecurity and the related impact on their waistlines.

It’s clear that the poor and socially disadvantaged are more likely to be categorized as overweight or obese, though why that is isn’t so clear. The answer may lie in unequal access to information, fruits and vegetables, safe opportunities to exercise, greater exposure to toxins, or elevated stress and cortisol levels. There is also evidence that weight-based discrimination leads to downward mobility among those biologically predisposed to corpulence. According to this view, being fat makes you poor, rather than the other way around.

What’s frustrating about not being able to isolate precisely why our population’s weight skyrocketed at the end of the 20th century and has since stabilized is that it then becomes difficult to rely on the right public policy measures to further the trend.  Many of the policy interventions that are being credited for creating our current obesity plateau were developed a decade or more after the start of this trend. More to the point, even public health interventions designed to help people—children in particular—lose weight have been shown to produce no weight loss, even when they were successful in changing diet and exercise behaviors.

This is not to say that policy efforts—such as improving access to fruits and vegetables in low-income neighborhoods, improving the nutritional value of school lunches, or building more bike paths and parks—are not laudable. Having access to delicious and healthy foods and safe opportunities for movement are likely to improve our overall quality of life and perhaps even health outcomes. Working to alleviate poverty and economic insecurity is also likely to have all sorts of social benefits, even if these efforts do not lead us to lose weight.

And maybe that is OK. There is growing evidence that people benefit from physical activity and improved diet even in the absence of weight loss. For instance, several studies have shown that physically fit “obese” individuals have a lower incidence of heart disease and mortality from all causes than do sedentary people of “normal” weight. Similarly, a clinical trial published in the New England Journal of Medicine showed that adopting a Mediterranean diet reduced cardiovascular risk without inducing weight loss. And economic security has consistently been shown to positively affect health and longevity, which is—after all—what all this is about.

Or isn’t it?

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