It’s that time again. Time to revise the Dietary Guidelines for Americans, something that happens every five years. This time, those revisions will happen in the face of charges that, over the past 40 years or so, the guidelines not only haven’t helped Americans eat better, they’ve contributed to our obesity problem.

Have they?

Here’s the theory. By suggesting that Americans limit fat intake, the guidelines led to decreased fat and increased carbs. That change was at least partly responsible for the rise in obesity.

Let’s take a tour of what the government told people to eat, and what they actually ate. The first iteration of the guidelines, in 1980, recommended that fat be limited to 30 percent of calories. That recommendation stood until 2005, when the limit changed to 20 to 35 percent.

Was anyone listening? I asked Mal Nesheim, emeritus nutrition professor at Cornell and chairman of the 1990 Dietary Guidelines Advisory Committee. “It’s hard to say,” he told me. “The average fat consumption didn’t come down much.” He’s right. From 1980 to 1997, fat went from 41.3 percent to 37.3 percent of calories (although actual fat consumption increased, carbohydrate consumption increased more) and then started to tick back up.

That’s a small drop, but it could easily mean that some people decreased their fat consumption dramatically. It wouldn’t be surprising, since the prevailing dietary wisdom at the time was to eat less fat. Nesheim points out that two major reports that came out just before the 1990 guidelines — one from the National Academy of Sciences and one from the U.S. surgeon general — both recommended lowering fat intake.

I’m old enough to remember all this, and I, too, was persuaded that fat was a big part of the problem. There was evidence aplenty to support that idea. I even wrote a book about low-fat food, back in the Pleistocene Era. I asked Nesheim if there was pushback at the time. “I don’t remember much pushback,” he said, with the caveat that it was a long time ago.

The problem that critics focus on, though, is not just that the percentage of calories from fat dropped. It’s that we turned to sugar and refined grains. It’s what New York University nutrition professor Marion Nestle calls the “Snackwell’s Phenomenon,” which she says took the nutrition community by surprise.

Nestle edited the 1988 Surgeon General’s Report on Nutrition and Health, which recommended reducing fat, and she told me the assumption was that if people reduced meat and dairy to avoid saturated fat, they would automatically turn to more healthful substitutes such as fruit and vegetables and would eat fewer calories overall. Instead, the food industry substituted sugars for fats in processed foods, and consumers bit. No-fat, high-sugar Snackwell’s proved more popular than, say, lentils.

Sure enough, sugar and refined carbohydrate consumption did increase (sugar by 27 percent from 1980 to its peak in 1999; flours and cereals by 36 percent in about the same period). But in that same time period, we ate more of everything, and total calories increased 24 percent. Small wonder we gained weight. “The point,” Nestle says, “is calories.”

The guidelines have always advised limiting sugar. They’ve also encouraged robust vegetable consumption and calorie levels consistent with a healthy weight. The lesson is not that the guidelines caused obesity, but rather that we all take the little bits of dietary advice that allow us to eat what we want and ignore the rest. Anyone who actually followed the guidelines did fine.

The low-fat recommendation was dropped in the 2015 guidelines, reflecting a widespread change of heart in the nutrition community, but that change of heart hit the zeitgeist long before it hit the guidelines; sugar and refined grain consumption have been dropping for almost two decades as fat consumption has been on the rise. If you subscribe to the “low-fat made us fat” theory, you would expect to see our waistlines reverse course. But in that time, obesity has risen by nearly a third, from 31 percent to 40 percent. It is pretty clear that we Americans can overeat any combination of macronutrients.

I asked Nina Teicholz, adjunct professor at NYU and the author of “The Big Fat Surprise: Why Butter, Meat & Cheese Belong in a Healthy Diet,” about the conundrum. “There’s a sizable body of evidence — far greater than for other diets — that shows that if you reduce carbs you can reduce obesity,” she told me. A prominent critic of the dietary guidelines, she was glad to see the 2015 iteration drop advice to limit fat, but asks, “why not include a low-carb dietary pattern” alongside the other eating patterns that get the governmental imprimatur?

I’ve got a slew of studies that review diet research and say that low-fat and low-carb diets are about equally effective (which is, of course, not very effective at all), and I asked Teicholz about them. She clarified that most of them are not low-carb enough: “Those kinds of analyses fail to capture the far greater efficacy when you go down to 20 percent [of calories from carbohydrates] or less.”

In super low-carb diets, the lowest-carb are ketogenic, meaning the low level of carbohydrates reduces blood sugar so much that your body converts stored fat into ketone bodies, which are then used as fuel. If you search for weight-loss trials that compare ketogenic to other diets, you find only a handful. Some have showed significantly better results for keto, but a review done in 2013 found that subjects on a ketogenic diet lost only two pounds more than those on a low-fat diet in studies that lasted one to two years. More recent reviews find either that there is no difference for weight loss, or that there is a small difference in the short term, but it does not last.

To make sure I wasn’t missing anything, I checked in with a couple of people who are knee-deep in the evidence. Kevin Hall is a researcher and section chief at the National Institute of Diabetes and Digestive and Kidney Diseases, and Stephan Guyenet is an independent obesity researcher and author of “The Hungry Brain: Outsmarting the Instincts that Make Us Overeat.”

“When you advise people to cut carbohydrates, or count calories, or advise a low-fat diet, in the first several weeks to couple of months, people tend to lose more weight on lower-carb diets,” Hall says. “All of these diets are equally poor for long-term weight loss.”

Both Hall and Guyenet think the most likely reason for this is simply that people eat fewer calories; there’s no metabolic magic that ups the calorie burn. “You can make extremely wide changes in fat and carbs and keep protein the same and there’s very little change, calorie-wise,” says Hall.

Which doesn’t mean switching your body’s fuel source from glucose to ketone bodies doesn’t have an effect. “Other things happen,” says Hall. A ketogenic diet can affect glucose, insulin and blood lipids, and there’s evidence that it can help manage diabetes.

Both Hall and Guyenet point out that advocates for super low-fat diets (with fat calories in the range of 10 percent of your total) also claim remarkable results. Way back in the 1950s, the Rice Diet, which was nearly all carbohydrates, reversed kidney disease that was routinely killing people, and it was later deployed for weight loss. “It’s at the extremes that the magic happens,” says Guyenet.

Few of us, though, are capable of sticking to those extremes for the long haul, and Hall points out that we do not know whether those diets are safe in the long-term. Meantime, we are looking at garden-variety carb restriction and asking, as Teicholz has, whether that kind of eating pattern shouldn’t be included in the dietary guidelines.

“For some people with certain conditions, it may be the diet of choice,” Hall says. But the evidence doesn’t yet “rise to the standard of recommending a population to do this.” For Teicholz, that first part is important. Shouldn’t the guidelines take into account the fact that so many Americans are dealing with diabetes or some kind of metabolic dysfunction?

That is an excellent question. The evidence for the diets that are included in the guidelines is not particularly strong, either, and the more choices we have, the better. For a dietary pattern to make the guidelines, it has to be defined specifically and must meet all nutrient requirements. So there are some technical hurdles, but I hope the advisory committee for the next version will tackle it.

As to the original question, I’ve got to say no, the guidelines did not make us fat. I’d go so far as to say that low-carb advocates might have an easier time persuading skeptics if they didn’t make that kind of unreasonable claim. But reducing carbohydrate intake seems like a perfectly healthful way to eat, and it has helped a lot of people lose weight and manage disease. It deserves a good, hard look.

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