Fox News claims Americans are obese primarily because they eat too many carbs. The science behind this idea is still not compelling.
“Overeating isn’t fueling obesity, it’s too many carbohydrates in our diet, researchers say,” Fox News reported on June 7. Despite its recent vintage, the story outlined the arguments advanced by Harvard endocrinologist Dr. David Ludwig and several co-authors in a December 2021 opinion piece for the American Journal of Clinical Nutrition.
Their thesis is well known by this point, but for anyone who hasn't followed the diet wars, the argument goes like this: people necessarily overeat when they gain weight, but overeating is not the ultimate cause of weight gain. Instead, obesity is primarily a hormonal disorder driven by excessive intake of rapidly digestible carbohydrates. This is known as the “carbohydrate-insulin model” (CIM) of obesity. It's often contrasted with the energy-balance model (EBM) that stresses calories consumed vs calories burned as the cause of obesity. Ludwig and his co-authors summarized it this way:
“...[A]ccording to the CIM, hormonal and metabolic responses to the source of dietary calories, not merely calorie content, lie upstream in the mechanistic pathway. In other words, the CIM proposes a reversal of causal direction: over the long term, a positive energy balance does not cause increasing adiposity; rather, a shift in substrate partitioning favoring fat storage drives a positive energy balance ...”
Many ideas in science suffer an untimely death because there's no evidence behind them. The GMOs-are-dangerous thesis is my favorite example. This isn't the case with the CIM; a wealth of peer-reviewed literature supports carbohydrate restriction as an effective weight-loss intervention. But what ultimately sinks this hypothesis as an explanation for obesity is all the evidence that doesn't fit its predictions.
These data points are often dismissed as exceptions to the rule. The Fox News report is a reminder that this is a convenient explanation, not a sound scientific judgment.
Little Debbie's snack cakes: the ultimate weight-loss treat
I took the CIM as Gospel truth for many years after losing weight on a low-carb diet. But I remember when my faith first began to waiver in 2010—"Twinkie diet helps nutrition professor lose 27 pounds," CNN memorably reported that November:
For 10 weeks, Mark Haub, a professor of human nutrition at Kansas State University, ate one of these sugary cakelets every three hours, instead of meals. To add variety in his steady stream of Hostess and Little Debbie snacks, Haub munched on Doritos chips, sugary cereals and Oreos, too.
Like an anti-vaccine mom confronting Dr. Paul Offit's work for the first time, I was troubled by Haub's experiment. How could this guy eat so much carb-laden snack food and lose almost 30 pounds?
Several prominent critics alleged that Coca-Cola funded some of Haub's research. That may have incentivized him to reach a certain conclusion, but it didn't refute his results. Adding insult to injury, Haub later ran the reverse experiment by eating a balanced diet built largely on salads—and gained 17 pounds. The key variable in both cases was energy intake. Eating fewer calories led to weight loss, eating more to weight gain.
Just an anomaly?
One man's experience doesn't refute the CIM. If anything, Haub's experience could be the exception that proves the low-carb rule. Ludwig and his co-authors took this view, asserting that “conceptual adoption of the EBM has failed to stem the obesity pandemic.” Telling Americans to eat less and move more hasn't prevented them from gaining weight in recent decades. Elsewhere they argued this may be because
“Restricting energy intake when consuming a high-GL [glycemic load] diet would neither lessen the predisposition to fat storage nor diminish hunger during dieting. Rather, by further reducing metabolic fuel availability, hunger would increase and energy expenditure may decline.”
This isn't the case, though. As I argued in a recent article challenging veganism as the weight-loss solution, there is clear evidence that people slim down on all sorts of diets. Studies have documented massive weight loss (north of 100 lbs) in many people who consume calorie-restricted diets consisting of white rice, fruit, fruit juice, and refined table sugar. This makes little sense if the carbohydrate-insulin model is valid.
Critics of the CIM have likewise pointed to rural populations in Asia who subsisted on high-carb grains yet didn't experience widespread obesity. Ludwig and his co-authors don't have a particularly good response to these anomalies. In a section of their article titled “criticisms,” they claimed that “people in these rural societies had high levels of occupational physical activity and restricted food availability, both of which might offset dietary influences on weight gain.” Which is another way of saying that calorie restriction prevents obesity. 
Ludwig and co. also pointed to clinical research in which participants lost more weight on carb-restricted diets than those who ate a calorie-restricted, low-fat diet. This provides evidence that some people can slim down by cutting carbs, but it isn't a consistent result across multiple studies. As they acknowledged in reference to the DIETFITS trial, “weight loss was greater when consuming a low-carbohydrate than when consuming a low-fat diet at 3 and 6 mo, a difference that did not persist at 12 mo [my emphasis].”
DIETFITS is an especially interesting example because it was financed and conducted by the Nutrition Science Initiative (NUSI), a non-profit co-founded by Gary Taubes, a prominent advocate of the CIM and one of Ludwig's co-authors. NUSI summarized the trial's results in no uncertain terms:
“The two groups prescribed the 'healthy low-fat' or 'healthy low-carbohydrate' diets showed significant differences in their carbohydrate and fat intakes. However, despite these differences, the low-carbohydrate and low-fat diet groups lost similar amounts of body weight during the trial.”
RealClearScience editor Ross Pomeroy was even more blunt in a September 2021 article.
“So it seems that Taube's initiative actually refuted his hypothesis, yet here he is, nine years later, still trumpeting the same ideology! True to form, he actually published a new book pushing low-carb eating last winter.”
I don't mean to insult Ludwig, Taubes, and others who share their views about obesity. They've done an excellent job of debunking the hysteria surrounding saturated fat and refuting the idea that palatable “ultra-processed” foods drive obesity—a popular meme among food scolds who want to regulate everyone's diet.
That said, the CIM doesn't account for all our data. Calorie reduction appears to be the only factor that consistently predicts weight loss, whatever diet someone follows. I think Ludwig's article got very close to the frustrating truth about obesity with this observation:
“In any event, many dietary and nondietary factors undoubtedly contribute to these trends.”
 This isn't to minimize the suffering of people who don't have enough to eat. Hunger is an enormous problem globally. My point here is only to illustrate a weakness in the CIM.