A carbs-rich diet has been blamed for the alarming explosion of obesity and chronic disease. What does the science show?
A Kellogg's magazine advert, 1952. Image courtesy of The Advertising Archives
Melinda Wenner Moyer is a science and health journalist. She teaches at the City of New York’s graduate school of journalism, and is the parenting columnist for Slate. She lives in Cold Spring, NY.
A decade ago, I said goodbye to wheat. I had been carrying around 15 extra pounds since high school and I was sick of it. A friend claimed that going wheat-free helped her lose weight and feel more energetic. A diet that didn’t require counting calories? Sounded good to me, so I gave it a shot. Six months later, those 15 pounds, close to seven kilos, were history.
Slowly, wheat found its way back into my world – oh how I missed bread! – and in 2009 I decided I was done. I haven’t regained much of the weight, nor do I feel different now. I can’t help but wonder, then, why my diet worked. Was it that my body truly functioned better without wheat? Was the claim true that carbohydrates, especially grains, promoted weight gain more than other types of foods?
Anti-carb rhetoric has been around for a long time, but it’s gotten fierce these past few years, and the public is lapping it up. Numerous best-sellers blame wheat, gluten and sugar for obesity, neurological disorders, and other chronic diseases. Carb critics fall into two basic camps: some make biochemical arguments, suggesting that the way our body processes carbs produces unique and harmful health effects, while others say carbs symbolise humanity’s fall from grace – that every time we stuff our faces with cookies, pasta and bagels, we are blatantly disrespecting the dietary blueprint our ancestors left in our genes. Our meat-eating hunter-gatherer forebears never ate these carbs, and we plainly lack the metabolic oomph to deal with them now. This argument implies a fixed human nature – and a prehistoric past that dictates how we should eat today. But to what extent are modern humans really Pleistocene hominins under the skin? And what can these evolutionary arguments tell us, if anything, about the Western world’s ongoing health woes?
The alarming increase in metabolic disease might be newly epic, but low-carb diets have been around for hundreds of years. The first fad took off in 1863 with William Banting’s Letter on Corpulence, Addressed to the Public. Banting was a formerly portly London undertaker who felt compelled to write about his weight-loss success after he began avoiding foods containing ‘starch and saccharine matter’. By replacing his buttered toast with bacon and his pastries with poultry, Banting dropped 46 pounds (21kg) in a year, despite eating what some have since estimated to be 2,800 calories a day.
But even back then, the idea that carbs might be uniquely fattening wasn’t news to the medical establishment. In Good Calories, Bad Calories (2007), the US science journalist Gary Taubes explains that, at the time, the editors of the esteemed medical journal the Lancet scoffed at Banting’s diet because, they said, the medical literature ‘supplies abundant evidence that all which Mr Banting advises has been written over and over again’.
Everything changed in the early 1960s. That’s when the American Heart Association officially backed the low-fat diet, thanks largely to the tireless work of the US physiologist Ancel Keys, who studied populations around the world, reporting correlations between fat consumption and heart disease. If Keys and his low-fat diet were in, then so were carbs – they were pretty much all that remained if you cut out dietary fat. For the next 40 years, low-fat diets were touted as the preferred – if not the only – recommended approach for preventing and combating obesity, diabetes and heart disease.
But rebels have continued to speak out. The late New York cardiologist Robert Atkins developed his low-carb diet after reading a paper on the approach in the Journal of the American Medical Association in 1963 and trying one himself. His plan recommended replacing carbohydrates, including grains, fruits, and legumes, with proteins and vegetables; it was based on the idea that without carbohydrates in the diet, the body would start burning its own, stored fat for fuel. In 1965, Atkins presented his program to the viewers of The Tonight Show, and millions signed on.
A decade later, another low-carb movement, one motivated by an entirely different concept in science, was born. In 1975, the US gastroenterologist Walter Voegtlin self-published The Stone Age Diet, in which he argued that our genes haven’t had the time to adapt to the vast changes we made to our diet after the agricultural revolution 10,000 years ago, when we began growing our own crops and dramatically increased our consumption of carbohydrates. The evolutionary mismatch between all the carbs our ancestors were suddenly eating, and the meat- and plant-rich diets their bodies had grown accustomed to, was causing ‘many of Man’s physical discomforts and health problems’ because carbs were unsuitable for our meat-conditioned digestive tracts.
Over the years, the concept was refined. By the mid-1980s, the physician Boyd Eaton and the anthropologist Melvin Konner, both of Emory University in Atlanta, were saying that our human ancestors ate mostly meat and plants and were free of modern chronic diseases such as heart disease, hypertension, diabetes and certain types of cancer.
All this caught the eye of the Colorado State University exercise physiologist Loren Cordain, who, with Eaton and others, analysed the dietary patterns of 229 modern hunter-gather societies from around the world. They found that some 73 per cent of the societies derived more than half their subsistence from animal products, and surmised that Paleolithic hunter-gatherers did much the same.
Cordain gave us the most recent iteration of the concept with his book The Paleo Diet (2002). In it, he approves of lean meats and fish but rejects grains, legumes and processed foods. He allows fruits, but tells dieters to avoid fatty meats and dairy products, because they weren’t eaten by our ancestors back in the day.
Yet as an argument against carbs, the premise of the Paleo diet turns out to be difficult to prove because we can’t be sure what our distant ancestors ate: some anthropologists argue that we were mostly meat-eaters; others argue that we evolved to eat starches; most say we just don’t know.
In fact, it makes sense that ancestral populations ate all sorts of foods, depending where and when they lived. Just because early humans had the ability to eat animals doesn’t mean that they ate them all the time, says the University of Colorado anthropologist Matt Sponheimer, who has researched the issue for years. Likewise, we can’t say that grains and sugars were never consumed by early humans. The remains of many early human ancestors had teeth loaded with cavities, ‘suggesting that sugar was a big part of their diet’, notes Peter Ungar, chair of anthropology at the University of Arkansas. Sponheimer recently published evidence collected from the dental enamel of early hominins suggesting they might have eaten certain types of cereal grains, though perhaps they simply ate animals that had eaten the grains; it is impossible to know.
Of course, all this is relevant only if our ancient ancestors were much healthier than we are. Why else would we want to eat like them? While we can't be certain, the predominant evidence suggests they were. Certainly, early humans perished young in violent encounters and childbirth. But Cordain cites studies suggesting that Paleolithic humans tended to have lower blood pressure, better insulin sensitivity, and a lower body-mass index than modern humans – which translates, he says, into a lower risk of chronic disease. Kim Hill, an Arizona State University anthropologist who has spent his career studying a handful of modern hunter-gatherer societies, concurs. Many of them ‘look like competitive athletes’, he says.
Yet even if our ancestors were healthy without grains, that sheds little light on the situation today. Diet is hardly the only culprit pundits bring up to explain the health crisis in our midst. What about all the chemicals and toxins in modern society; or our lit-up nights; our sedentary ways?
Nor do Paleo diet proponents consider how well-adapted we are to grains right now. Cordain argues that only 333 generations have passed since our diets drastically changed due to the advent of agriculture – not enough time for significant genetic adaptations. But the University of Minnesota biologist Marlene Zuk, author of the book Paleofantasy, cites several very recent genetic changes that aid the digestion and metabolism of grains and dairy, suggesting that modern humans have, in fact, evolved to meet our new dietary habits, and that we are continuing to do so now.
The mere existence of genetic changes doesn’t mean we’re physiologically compatible with grains in the long-term. ‘Europeans may have adaptations for drinking milk, but that doesn’t mean it’s good for us,’ says Sponheimer. ‘It just means we’ve had to evolve a capability for it.’ We might have acquired genetic changes that influence our ability to digest grains, but that tells us nothing about whether we’ve acquired the specific set of changes needed to thrive on a grain-based diet or avoid its health consequences over the long-term.
Resolving these issues remains critical because carbs have been invoked not just as a cause of obesity but also wide-ranging chronic disease. In Wheat Belly (2011), the US cardiologist William Davis points to gluten, the protein that gives wheat its stretchiness (and is the reason bread is so deliciously spongy), as a potential cause of immune problems, including coeliac disease, which has been increasingly diagnosed in the US over the past 50 years. Davis wonders whether the genetic changes that have arisen in wheat crops during that time as a result of selective breeding could be at the root: he cites a 2010 study reporting that some modern European wheat varieties contain a higher concentration of the types of glutens known to cause coeliac disease.
Since people who suffer from coeliac disease are also at increased risk for autoimmune diseases such as rheumatoid arthritis, lupus and Crohn’s, Davis suggests that wheat might also increase the risk for these diverse conditions. ‘In short, the reach of gluten consumption consequences is mind-bogglingly wide,’ he writes. While it’s a thought-provoking hypothesis, it’s also possible that all of these conditions share another root cause.
Then there are the claims that grains cause neurological diseases such as anxiety, depression, epilepsy, schizophrenia, attention-deficit hyperactivity disorder, and dementia. Grains are dangerous, says the US neurologist David Perlmutter in his highly controversial book Grain Brain (2013), in part because gluten incites inflammation throughout the body, including the brain. He argues that people with high blood sugar are more likely to experience brain shrinkage; that those with diabetes are more likely to develop dementia; and that someone with coeliac disease is at an increased risk for cognitive impairment. Yet when I asked Perlmutter why he opted to use such strong causal language when so many of his claims are based on associations, he said that, ‘In writing a book, I’m able to take a little more liberty and say, based on the best available knowledge today, here's what I as a physician would recommend’. In other words, none of this is established fact.
To prove that carbs are really behind these ills, we need careful scientific studies and plausible biological explanations for what we observe. Work along these lines over the past decade might not be conclusive, but a glimmering of insight has emerged.
In 2004, researchers at the US Veterans Affairs Medical Center and Duke University published the results of a trial in which they had assigned 120 overweight volunteers to follow either a low-fat or a low-carb diet for 24 weeks. By the end, the low-carb dieters had lost nearly twice as much weight – 12.9 per cent of their initial body weight, compared with 6.7 per cent in the low-fat dieters.
A 2010 trial compared low-carb versus low-fat diets in severely obese adolescents for 13 weeks. Teens on the low-carb regimen reduced their body-mass index 50 per cent more than the low-fat dieters; when the teens were followed up 11 weeks after the trial had ended, those on the low-carb diet had maintained their greater weight loss.
‘the Atkins Diet is the easiest to follow – you simply drive by a fast food window, order a burger, throw away the bun, and scrape off the pickles and ketchup’
A 2013 meta-analysis of 13 clinical trials concluded that, overall, people on very low-carb diets ‘achieve a greater weight loss than those assigned to a low-fat diet in the long-term,’ meaning a year or longer. And although there is evidence to suggest that low-carb diets reduce appetite, multiple studies have shown that low-carb dieters lose weight even when they consume upwards of 2,500 and even 3,000 calories a day.
A plausible, though unproven, explanation for these finds comes from David Ludwig of Boston University and Mark Friedman of the Nutrition Science Initiative. In a paper published in the Journal of the American Medical Association and an accompanying editorial in the New York Times, they suggest that excessive consumption of carbohydrates leads the body to overproduce insulin, a hormone that converts those carbs into fat and stores them in body tissues. When food is preferentially stored instead of made available as fuel in the bloodstream, we feel hungrier. In a nutshell, we don’t get fat because we eat too much, we eat too much because we get fat – and carbs catalyse that unfortunate sequence of physiological events.
Other solid research suggests that low-carb diets might be better for the heart, too, by improving blood cholesterol levels and insulin sensitivity – an idea that directly contradicts the American Heart Association’s repeated warnings that low-carb diets, which are typically high in fat, could ‘raise the risk of coronary heart disease, diabetes, stroke and several types of cancer’. A 2012 meta-analysis of 23 clinical trials published between 1966 and 2011 reported that, overall, low-carb dieters experienced bigger drops in total cholesterol, ‘bad’ LDL cholesterol, and blood triglycerides compared with low-fat dieters; they also had greater increases in ‘good’ HDL cholesterol.
There is, of course, some evidence that certain types of low-fat diets might be beneficial, too – but the data is difficult to interpret. For instance, the Cornell-Oxford-China Study, conducted in the 1980s by Cornell University, the University of Oxford, and Chinese research institutions, compared lifestyle and disease characteristics among 6,500 people from 65 counties in 24 provinces of rural China. It found that the more animal protein people consumed, the higher their blood cholesterol levels, and the greater the risk for ‘Western disease’ – cancer, atherosclerosis, diabetes, and stroke. The US physician Caldwell Esselstyn is famous for a small 1995 study in which he put 22 of his heart disease patients on a strict plant-based, low-fat diet. The 11 participants who stuck with it experienced a complete remission of their heart disease. And Dean Ornish, a physician at the University of California, San Francisco, has also published studies suggesting that very low-fat diets can reverse heart disease.
Cookie by cookie, we might be forging humanity into new evolutionary territory, re‑shaping our genes to handle our new dietary indulgences
Yet these dieting patients didn’t just diet – Ornish’s also exercised more, meditated, stopped smoking and attended psychosocial support groups, while Esselstyn placed his patients on cholesterol-lowering statins during their diets. And the Cornell-Oxford-China Study shows only associations: even though people in regions consuming more meat also had more cancer, we can’t necessarily blame the meat for the cancer, because these regions are different in many ways. In short, it is impossible to make conclusions about the specific impact that these low-fat diets had.
In addition, the low-fat diets recommended by these experts aren’t your typical American Heart Association-backed low-fat diets that advise limiting fat intake to 30 per cent of total calories. They are far more rigorous, allowing patients to consume only 10 per cent of their calories from fat – that means going vegan, never cooking with oil, butter or margarine, and eating ‘light’ tofu when possible. That these diets are so strict makes them very difficult to follow for long periods of time. In the only clinical trial to compare very low-fat diets (less than 10 per cent fat) with low-carb diets, Ornish dieters had an exceptional amount of trouble and managed to reduce their fat intake only to some 30 per cent on average – three times more fat than they should have been eating. John McDougall, a California-based internist and the author of The Starch Solution (2012), says ‘the Atkins Diet is the easiest to follow – you simply drive by a fast food window, order a burger, throw away the bun, and scrape off the pickles and ketchup, and you’re on the diet’. Of very low-fat diets, on the other hand, he says: ‘with such a steep learning curve, few people succeed’. Even if very low-fat diets can reverse heart disease at the extreme end of compliance, they are so difficult that only the most committed might be able to make them work.
In the midst of all the claims and counterclaims, there is a single clear piece of common ground. Experts of every stripe ask dieters to avoid refined sugars and grains. ‘Losing body weight on a plant-based diet is much less likely to occur if the diet includes too many refined carbohydrates,’ writes Cornell’s T. Colin Campbell in his book, The China Study , based in part on his Cornell-Oxford-China study research. Esselstyn instructs his dieters to consume only whole-grain products and avoid fruit juice. And McDougall urges his readers to eat complex carbohydrates instead of refined sugars and flours.
In essence, these scientists and doctors are recommending an Atkins diet that replaces the meat and fat with plants and certain complex (but never refined) carbohydrates. They attribute the success of their low-fat regimens to elimination of all animal products and fats, but can we be sure that the most important part isn’t the elimination of refined sugars and carbs instead? Many societies, including the Chinese and Japanese, have gotten sicker and fatter in recent decades as they have started eating more meat and dairy products, the low-fat proponents argue. But increases in meat and dairy consumption typically go hand in hand with increases in refined carbohydrate and sugar intake. In fact, back in 1957, British nutrition researcher John Yudkin had found that increases in sugar intake in various populations predicted their increasing heart disease death rates more strongly than did increases in fat or animal consumption.
So where does all this leave us, other than confused and wondering if we should stop eating cupcakes? On the health side, the science does collectively suggest, but not prove, that a calorie is not always just a calorie, and that carbohydrates – particularly refined ones – might have unique metabolic effects that increase risk for chronic disease. Indeed, the notion that sugar and refined carbs are dangerous seems to be the one point on which nutrition scientists at either end of the carb-fat spectrum agree. I suspect that my weight-loss success a decade ago had something to do with the fact that, by cutting out wheat, I was replacing some refined carbohydrates with other macronutrients.
It’s also safe to say that carbohydrates as we eat them today are indeed ‘unnatural’ for us. Even though our Paleolithic ancestors almost certainly enjoyed occasional treats of honey, they weren’t having Entenmann’s crumb coffee cake for breakfast; the technology to refine grains just wasn't available then. It’s likely that our bodies are not well-suited for such a regimen, either.
So Paleo dieters might be right – we could be more evolutionarily in tune with a diet like that of our ancestors, which almost certainly includes fewer refined carbs. We can’t say, based on today’s evidence, that carbs are the root cause of all our chronic ailments, but scientific evidence suggests that we might stay healthier if we take flour and added sugar off our plates. Still, human nature is a moving target. Cookie by cookie, we might be forging humanity into new evolutionary territory, re‑shaping our genes to handle our new dietary indulgences. Along the way, we will undoubtedly ease our problems with new medicines, technologies and lifestyle adaptations – the supine lifestyle depicted in the 2008 film WALL-E comes to mind. But we will undoubtedly have a smoother road ahead if we change our dietary ways, instead of letting our dietary ways change us.
17 June 2014