Why there will never be a single key to weight loss

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There are a zillion variables in the interaction between bodies and food, yet, over and over, we’re told that one thing is the key.

Once upon a time, it was fat. Then it was gluten. Some people still think it’s sugar. Now, it’s insulin.

Insulin is the hormone that shuttles glucose from your bloodstream into fat storage. If you limit insulin, the thinking goes, you limit your body’s capacity to store calories as fat. Voila! We’ve found the key to weight management.

The insulin issue is hardly new. It’s at the heart of low-carb diets, which have been cycling through popularity for at least 150 years. But right now, insulin is in the ascendant. Although it’s tough to put hard numbers to the zeitgeist, Google searches on the hormone have increased steadily over the past decade, with a big bump this year.

This interest raises two questions. First, does insulin really matter that much? But the second is arguably more important: Why on Earth do we continue our population-wide case of one-thing-itis?

But let’s tackle that first question. How much does insulin matter for weight loss? (There are also questions about insulin’s effect on other health measures, but my editors limit me to 1,200 words, so I’m focused on the issue that seems to matter most to people.)

Insulin is at the heart of the debate about whether low-carb diets are more effective than other diets. Because insulin turns glucose into fat, the theory is that it changes the way your body uses energy – “fuel partitioning,” the scientists call it. So, when you eat foods that trigger insulin spikes (refined grains and sugar are very good at that, but other carbohydrates do it as well), your body sends more of those calories into fat stores and makes fewer available for you to function. Other kinds of foods, by contrast, don’t lead to insulin spikes, so those calories don’t get shuttled to fat but are instead available to be burned.

That’s the essence of the Carbohydrate-Insulin Model (CIM), the underpinning of low-carb diets.

If all that were true, we would expect to see a couple measurable results. First, you should burn more calories on a low-carb diet, because your body makes them available for use rather than sequestering them away in fat. Second, the depletion of glucose in the bloodstream should drive hunger, so food intake should correlate with insulin spikes – the higher the spike after a meal, the more the person should eat in the hours afterward.

Luckily, many studies over the years have addressed both those issues, and I’ve tackled them in previous columns. My read of the evidence is that low-carb diets do increase energy expenditure, but only by a little. The evidence is decidedly mixed on whether consumption increases after higher measured insulin levels (sometimes it does and sometimes it doesn’t, and monitoring insulin doesn’t seem to help with weight loss). I’ve sorted through the research, and I think it’s clear that any effect on weight loss is small.

See the theme there? It’s smallness, and it’s the key to understanding why we have so many fruitless debates about diet, and why the answer is never one thing.

Bodies are complex, diets are varied, humans are hard to study, and what we know about nutrition is dwarfed by what we don’t know. Amid all that uncertainty, there’s temptation to latch onto something we do know – insulin shuttles energy to fat stores – and turn it into the key to human health.

Invariably (at least so far), it turns out not to be. Even if you start with something that’s true, once you mix it up with everything else your body does, it translates to something much less consequential than the key to human health. Effects are often real! But they are almost always small. Studies find some differences, but not the kind that change lives, or even waistlines.

Kevin Hall, who was with the National Institutes of Health until last year, has done some of those studies, and his recent book, “Food Intelligence,” puts insulin’s role in context. “We can get by on carb-heavy potatoes or fat-rich meat, or myriad mixtures in between. The body’s fuel tank seems to be agnostic.” Hall and co-author Julia Belluz call it a “flex-fuel” system. (Full disclosure: Belluz, a journalist, is a friend of mine.)

I asked Harvard’s David Ludwig, a long-time proponent of low-carb diets, about that theory, and he doesn’t disagree. “Clearly, humans can derive energy from a variety of fuels,” he told me. “It’s an exquisite system.” He was very clear that he believes the exquisite system still burns more calories on low-carb diets, but fully acknowledges that carbs aren’t the only fuel.

The Hall-Ludwig debate has been framed as two conflicting theories of weight gain: Hall says the reason for weight gain is primarily a food environment humans can’t navigate, while Ludwig says carbohydrates, primarily refined, drive hunger, which drives overconsumption, which drives weight gain. While those of you who come here often know I’m essentially in the Hall camp, I think this argument isn’t one of competing principles – it’s one of degree.

Hall’s research finds a small increase in energy expenditure on a very low-carb ketogenic diet. Ludwig believes in Hall’s flex-fuel model. What they disagree on is the magnitude of the metabolic impact of consuming carbohydrates, an issue that’s tough to adjudicate because humans are hard to study. Research that could tease out these differences would require fairly long-term studies of people on very tightly controlled diets. To do that, scientists would need both robust funding and subjects willing to live on a metabolic ward for months.

In light of the complexity and uncertainty, it’s easy to see the appeal of the “insulin spikes are bad” message.

Psychologist William Hallman, a distinguished professor at Rutgers University, studies why we eat what we eat. “Humans are drawn to the idea of a single ‘magic’ solution because it simplifies complexity,” he wrote me in an email. Focusing on one thing “creates an important illusion of control over complex systems involving interacting variables and provides a sense of emotional comfort because ‘I’m doing something powerful to improve my health.'”

Every nutrition scientist I’ve talked with, including Hall and Ludwig, acknowledges both the complexity and the uncertainty. Harvard’s Walter Willett told me that “obesity is very multifactorial with important interactions.” He believes that it’s “likely that insulin is a mediating factor, but probably not the only factor.”

Yet when the science gets filtered through the media – both social and traditional – the message invariably gets boiled down to the magic solution we all want. In fact, the science is an integral part of our attachment to one-thing solutions: A key part of magic-solution thinking is “a plausible, convincing explanation for why it supposedly works,” Hallman told me.

When people thought the magic solution was fat, they were wrong. Ditto gluten and sugar. Sometimes, you’re supposed to eat more of the one thing, but protein and fiber haven’t panned out either. And insulin isn’t the first one-thing answer that looks at hormones. I’m old enough to remember leptin. But every time there’s a new one thing, we fall for it yet again.

Really, what are the odds?

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