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The artificial sweetener aspartame is found in most diet sodas and more than 6,000 other food products consumed by hundreds of millions of people around the world. So
on July 14 when a group working under the auspices of the World Health Organization warned that the sweetener “possibly” causes cancer and that “high consumers” of aspartame-sweetened products were at risk, the news went viral. So did pronouncements a few days later that aspartame was, in fact, pretty safe. “The WHO announcement doesn’t mean aspartame is linked to cancer,” an official from the U.S. Food and Drug Administration told NPR, and a second committee of the WHO agreed.
Public health messaging is often a difficult tightrope walk (see: COVID-19) and that is especially true for anything related to food—a matter of profound importance and intense public interest, about which science can’t seem to make up its mind. Nutrition scientists and other food-and-health experts have for years been feeding the public conflicting and muddled advice about food, diet and health. Is it a good idea to count calories, avoid carbs, load up on meat? Is it best to eat only natural foods, avoid gluten, go vegan? Science can’t provide definitive answers.
It’s no wonder, then, that the recent wave of seemingly magical cures and miracle regimens. Diets that emphasize intermittent fasting, nurturing good gut bacteria and cutting out carbs have surged in popularity, but their basis in science is sketchy. The arrival of a new generation of astonishingly effective weight-loss drugs offers the tantalizing prospect of blunting America’s obesity problem. But it’s not clear if a lifetime drug regime will be safe or affordable for the 42 percent of Americans who are obese.
“It’s very hard for consumers to find consistent dietary advice,” says Regan Bailey, a professor of nutrition at Texas A&M University, where she is associate director of precision nutrition for the Institute for Advancing Health Through Agriculture. “For every study that finds a particular approach works, another one comes out that’s against it.”
Getting to the bottom of diet science won’t be easy. Part of the problem is that research on food is terrifically challenging. To do it right requires tracking what people eat over long periods of time, which is expensive, and keeping them on diet plans that they may not like, which is as easy as herding cats. Because health problems tend to emerge over many years from complex causes, it’s difficult to trace them directly to diet alone, never mind particular foods or individual ingredients.
That’s why so many theories abound that may not have a sound scientific basis. What’s needed more than new theories are practical solutions grounded in the relatively small universe of unimpeachable facts about how to eat healthfully. That’s true in the realm of public policy, where a movement is afoot to protect children from the ravages of the food industry. In July, for instance, the WHO called for countries around the world to implement regulations with a particular eye to protecting children from junk-food marketing.
It’s also true in the myriad decisions individuals have to make when shopping, cooking and deciding what to have for lunch or feed the kids. Although much of the dietary advice currently circulating in the zeitgeist is questionable, Newsweek has sifted through the myriad competing and confounding claims and separated what is factual about healthful eating from what is merely speculative. What’s left are a few bedrock principles to keep in mind.
Do Calories Really Matter?
Making the diets of Americans healthier has long been primarily about reducing the rate of obesity. And for good reason, notes Howard Sachs, a physician and associate professor at the University of Massachusetts Chan Medical School. “When I see a patient who’s overweight, I have to consider whether this person is at risk of cardiac disease, high blood pressure, diabetes, arthritis and sleep apnea,” he says.
For more than half a century doctors and public-health experts have been pushing out the message that recommending changes in diet is the best way of tackling the problem of excess weight. (Research strongly links exercise to better health but not in any clear way to weight loss.) The conventional recommendation for healthful weight loss has been some form of calorie tracking, with a goal of reducing daily total calorie consumption. This approach tends to place less emphasis on which foods to eat and more on an overall diet that is as satisfying as possible while still trimming calories.
Research has consistently found that diets involving some form of calorie-tracking—such as WeightWatchers, which uses a “point” system loosely based on calories—are on average more effective than other approaches to losing weight and keeping it off. But there’s a big catch: Most people fail on these diets anyway. Studies have shown, for example, that the great majority of people with obesity who are encouraged by a physician to lose weight through dietary changes don’t lose weight at all. And more than two-thirds of those who do lose weight on a doctor’s advice gained more than two-thirds of the weight back, on average, within three years, according to a 2018 review study by the National Institutes of Health (NIH) and Johns Hopkins University.
The poor track record of the calorie-based approach to healthy eating is one reason it has been under fire in recent years. Some experts have called into question the notion that people should be encouraged to make dietary changes to overcome obesity at all on the grounds that it’s too difficult and can be pointlessly shaming.
Recognizing that most people struggle to cut calories, various dietary scientists and clinicians have pushed alternatives to the calorie-tracking paradigm. One that gets a lot of attention is the very-low-carbohydrate diet, often referred to as a “keto” diet—the most popular diet among those that restrict particular food types, according to a 2022 study published in the journal Current Developments in Nutrition. More than 23 million people say they follow keto diets, according to the International Food Information Council, fueling a $12-billion-a-year industry supplying foods tailored to keto, according to the market-research firm Mordor Intelligence.
The theory behind the diet, most prominently championed by Harvard University researcher and physician David Ludwig, is that people gain excess weight not when they consume too many calories, but when their bodies lose the ability to deliver the energy from food to muscles.
That happens, the theory goes, because refined sugar and carbohydrates from grains and fruits (which the body breaks down into another type of sugar) tend to flood too quickly into the bloodstream, leading the body to produce large amounts of insulin to try to get the sugar to the muscles. Over time, muscles can become resistant to those big hits of insulin, and the body ends up converting the sugars to body fat instead. The keto solution to this “insulin resistance” is to stop eating just about all carbs. People on full keto diets typically try to get about three-quarters of their calories from fat, and almost all the rest from protein.
A big attraction of keto is that adherents can eat large portions of whatever non-carb food they want, feasting freely, for example, on steak, cheese and eggs. As unhealthy as that may sound, research tends to back the notion that sticking with a keto diet can lead to losing excess weight, working on average about as well as a calorie-tracking approach. Keto also reduces the risk of diabetes.
Most experts, though, are skeptical of the keto diet. The large amounts of saturated fats typically consumed by keto adherents can lead to high cholesterol levels closely linked to heart disease, as confirmed by a 46-year-long study involving more than 18,000 subjects published in 2021 in JAMA Cardiology among other studies. Fruits and whole grains, which get cut out in a keto diet, are also generally considered important components of a healthy diet. In addition, when people deprive themselves of ubiquitous, convenient, much-loved foods such as pasta, bread and most desserts, they’re likely to develop cravings that make it nearly impossible for them to stick with the diet over the long term, researchers say.
“It’s not helpful to focus on just one component of diet,” says Michelle Cardel, a registered dietitian and assistant professor at the University of Florida and senior director of global clinical research and nutrition at WeightWatchers.
The Mediterranean diet is a more highly-regarded healthy-eating scheme. While there isn’t a specific Mediterranean diet industry, WeightWatchers and most other popular services offer ways to adhere to a diet more or less in keeping with its principles. There’s no official definition of the diet, but a key component is eating more unsaturated fats such those found in olive oil, nuts and especially oily fish. Advocates also advise eating vegetables, whole grains and lean sources of protein such as poultry and beans, while cutting down on sugar, red meat and saturated fat.
Studies show that people who follow a Mediterranean diet tend to show a reduced risk of heart disease. They’re also less likely to be obese, even though calorie tracking isn’t part of the diet. However, these studies haven’t established beyond a doubt that people who eat this way are healthier from the diet alone. It’s possible that people who follow a Mediterranean diet are healthy due to other factors, such as exercise and moderate calorie intake, and are simply more likely than others to eat this way. A 2022 Stanford University study, however, did convincingly show that the Mediterranean diet worked as well as the much more restrictive keto diet in fending off insulin resistance.
Other diet approaches include plant-based diets, gluten-free diets, “microbiome” diets that aim to regulate gut bacteria and fasting during certain times each day. Studies show some health benefits to each approach, but there is no convincing evidence that any one is better than the others at making its adherents healthier. For most of the public, they hold little appeal as a way of eating.
Still, these diets have their vocal supporters. Intermittent fasting, in particular, has quickly become the most popular diet in the U.S., according to the International Food Information Council. About one out of 10 people who say they’re following a diet claim intermittent fasting is their preferred approach. The theory behind it is that frequent fasting can help people get out of the habit of constantly eating and gives the body time to focus on burning the food that has already been eaten, rather than on digesting new food. Fasting can also trigger a stress response in the body that promotes cell repair. Usually, intermittent fasters say they fast either for entire days two or more days a week, or else for a large chunk of the day. There is no agreed-upon measure for what counts as intermittent fasting other than following some sort of timing scheme in eating. The big appeal to this approach is being able to eat without restrictions (except, of course, when fasting).
Intermittent fasting has its downsides. The biggest one: having to fast off and on for the rest of your life. Fasting also doesn’t do a better job of taking off excess weight than conventional calorie-tracking, according to a joint Northwestern University and University of Illinois study published in July in the Annals of Internal Medicine. And weight loss via the calorie approach delivers the same cell-repair benefits, without having to fast, according to a 2017 study published in Ageing Research Reviews.
Microbiome diets are fast gaining in popularity. The idea is to eat food that is friendly to good bacteria that lives in the intestines, which has been shown to have an influence on mental and physical well-being, and unfriendly to bad bacteria. The approach encourages high-fiber foods like fruits and vegetables. Certain vegetables—such as leeks, asparagus and seaweed—are held to be especially helpful to maintaining a healthy microbiome. Research has shown that the type and quantity of bacteria in the gut can indeed be linked to both weight and health, but there has yet to be any solid proof that switching to particular microbiome-promoting foods can reliably produce improvements.
One of the most promising approaches, still in its infancy, is that of “precision nutrition,” which aims to tailor a diet to suit each individual’s unique biology, health, lifestyle and goals. An $8 billion industry has already sprung up to cater to would-be precision-nutrition eaters, offering a range of lab tests and questionnaires—assisted, in some cases, by artificial intelligence. Research is in the early stages, most notably an NIH study announced in May that will enroll 10,000 people to examine how AI can determine which foods raise or lower health risks for individuals. Most experts say that effective precision-nutrition schemes are still years off.
Any of these diets can work—as long as they reduce calorie intake, says Jamy Ard, a physician, epidemiologist and vice dean for clinical research at Wake Forest University, and president-elect of the Obesity Society, an influential academic research association. “Yes, calories matter. Downplaying that does people a disservice by getting them to focus on the wrong things,” he says. “I see people eating only organic, natural, healthy foods, and they can’t figure out why they’re gaining weight.”
Many of the new diets are too expensive for most people. They often involve having to buy more fresh, whole foods—which can be out of reach of people with low incomes, who also tend to be at highest risk of diet-related poor health. And some of the diets may simply be unappealing for many. “The right diet has to come down to what’s livable and sustainable for you,” says University of Florida’s Cardel.
A New Villain
As if dietary science hasn’t already given us a large enough crop of competing diet theories, researchers have been busy promoting a new one: Blame it all on “ultra-processed” foods.
The food industry is all too eager to ply the public with the fatty, sugary, calorie-dense foods it craves—Americans get about half their calories from junk food, according to a 2022 study from the University of North Carolina and other researchers, with a predictable impact on health. But a spate of studies over the past five years have made the increasingly strident claim that the food industry is churning out food products that are attacking health in ways that go beyond merely cramming them with fat and sugar. Ultra-processed foods, insist some researchers, are the sole cause of climbing obesity rates and all our food-related health problems. The evidence, however, does not definitively support these claims.
The term “ultra-processed” was introduced in 2009 by Carlos Monteiro, a nutrition researcher at the University of Sao Paulo in Brazil, who defined it as food prepared with ingredients and equipment not found in most kitchens. More than two-thirds of all food sold by the packaged food industry falls into that definition—including most plain yogurt sold in supermarkets—leading some scientists to question the term’s value. “There’s no agreed-upon scientific meaning for it,” says Texas A&M’s Bailey. The label, she notes, can imply baby carrots from a supermarket are bad for you, while equally improbably deeming candy made in a home kitchen perfectly healthy. To avoid confronting that apparent contradiction, most researchers seem to treat the term as a catchier, more scientific-sounding way of describing junk food.
Studies that link the consumption of junk food to obesity and chronic disease have failed to establish cause and effect. In other words, it is not clear whether junk food alone is making healthy people sick, or people who eat junk food also tend to be more vulnerable to health problems for other reasons, such as lack of exercise and lack of access to good healthcare, education and other resources. A 2019 study from Harvard found that junk-food eaters are not only more vulnerable to heart disease but they also have higher rates of death from injury, which supports the notion that junk food may not be the only factor.
The claims about ultra-processed foods grew louder after a 2019 National Institutes of Health study in which 20 people were put on an ultra-processed diet (examples: mac and cheese, chicken nuggets) for two weeks, and on a “minimally processed” diet (oatmeal, salad, grilled chicken) for two weeks. The balance of fat, protein and carbs were the same for the two diets, and the subjects were allowed to eat as much as they wanted at each meal. The results: They ate about 500 calories per day more on the ultra-processed diet, gaining weight from it.
The study didn’t make clear how the ultra-processed foods led to more calorie intake. But those foods did pack in far more calories per mouthful than the non-ultra-processed versions, and many researchers—including those who ran the study—point out such “calorie density” promotes excess eating. That’s because cramming more fat and sugar into a single bite, as most junk foods do, makes foods taste and feel extremely stimulating and satisfying. They’re “hyperpalatable.”
Many researchers and healthy-eating advocates interpret the results of that NIH study to claim that it represents a newly discovered, special harm from the food sold by the prepared-food industry beyond its well-known appeal and calorie-density. It’s possible that something as of yet unidentified about junk food is causing special harm. But scientists haven’t clearly established that claim. However, that hasn’t stopped some researchers from insisting that it’s real and doing damage.
Monteiro, the researcher who coined the term, is one of those researchers claiming special harm. “Sugar in ultra-processed food creates a lot of problems, but not the sugar in homemade desserts,” he says. “The fact that we still don’t know what links ultra-processed to different diseases can’t be used as a reason to ignore the evidence.”
The claim of special harm beyond hyperpalatability might be settled by studies comparing, say, food-company mac-and-cheese to home-cooked mac-and-cheese, rather than comparing it to a salad. But researchers don’t seem eager to clarify that point. “The key questions about ultra-processed foods aren’t being addressed,” says Cardel. Another point that needs to be taken into account is the difficulty many people would have in giving up highly processed foods, she says. “I don’t find the fear mongering over ultra-processed food helpful. It’s elitist and unrealistic to assume everyone can just switch to an unprocessed diet.”
Drugs for Weight Loss
Amid the confusion, meanwhile, there’s some good news for the multitudes who have struggled unsuccessfully against the pull of junk food: A new generation of diabetes and weight-loss drugs is flipping the long, discouraging odds for successfully losing weight for many people with obesity. These drugs, including Ozempic, Wegovy and Mounjaro, imitate certain
hormones that can reduce appetite and cravings.
“They are a game-changer,” says the University of Massachusetts’ Sachs. “I have patients who have never been able to lose an ounce losing 20 pounds in three months. It’s phenomenal.” Currently, these drugs must be injected every two or four weeks, but pill versions are on the drawing board, which will likely add to the interest in taking them.
These drugs come with drawbacks. For one thing, health insurance covers most of the costs only for those people diagnosed with diabetes or, in some cases, for patients with another significant obesity-related health problem, such as heart disease.
Others who hope to enlist the drugs for weight loss and can get a prescription would have to shell out nearly $1,000 a month—as would anyone who doesn’t have good healthcare insurance, no matter how sick they are. Obesity and its related ills disproportionately fall on the poor and those in groups traditionally underserved by healthcare. “The people who are most at risk for obesity usually don’t have insurance,” says Bailey. Costs may eventually come down, but right now there’s little sign of that happening any time soon.
Other drawbacks to the new drugs include gastrointestinal distress and other side effects. And stopping the drugs tends to result in regaining all the weight, which means most people would need to stay on them for the rest of their lives to keep the weight off. The drugs are so new that the potential long-term side-effects have not yet been studied.
For these reasons, dietary changes are likely to remain the main approach to losing excess weight for some time. “I don’t know if we’ll ever see a day when people can just take a drug and not have to worry about what they eat,” says Wake Forest’s Ard. He notes that even people who are on the drugs have to pay close attention to what they eat, because a poor diet could lead to muscle loss or other health problems, or might even interfere with weight loss.
The Limits of Knowledge
The problems of dietary science stem from the enormous complexity of the relationship between food and health, and the length of time—the better part of a lifetime—it takes for many health problems to appear.
Most diet-and-health research is based on “cohort” studies, in which the health of people who eat a certain way is compared to the health of those who eat differently. But while cohort studies can show that a certain diet seems associated with certain health problems, they can’t show whether or not the diet causes the problems. That’s because people who choose to eat a certain way may be different in other ways, such as income, ethnicity, education, community resources, exercise habits, and much more—and there’s usually no good way to say for sure which factors cause a given health problem.
One way to avoid the limitations of cohort studies is to fashion a randomized controlled trial, or RCT, in which people are randomly assigned to different diets. Any emerging differences in the health of the different diet groups could then be attributed with some confidence to the diet, because everything else about the groups should, on average, be the same.
There are reasons why dietary researchers don’t do many RCTs. Getting people to agree to adopt a diet they may not like is difficult. And those who do agree may have trouble sticking to the diet—which is why a good dietary RCT requires carefully monitoring what the subjects eat 24/7, at enormous inconvenience and cost. Requiring people to eat junk food for long periods of time would also be unethical, given the health risks. That’s why the NIH study had only 20 patients eating ultra-processed food for a mere two weeks—numbers considered too small for reliable results.
No wonder scientists and clinicians can’t agree on the best approach to a healthy diet. And even if they did, would it have much of an effect on Americans’ health? Haunting the entire endeavor of improving the public’s diet is the fact that most people simply can’t, or won’t, make the long-term eating changes that experts recommend. Everyone hears that junk food isn’t good for them, but it still dominates the diet of Americans, with no sign of change. Junk food is not only hyperpalatable, it’s also cheap and convenient.
“Not everyone has access to a farmer’s market or the time to cook food,” says Bailey. “We haven’t begun to understand all the socioeconomic factors involved in helping people make changes.” No number of new study conclusions is going to change those facts.
Having largely failed at convincing most people to switch to some type of healthier diet—and facing growing criticism for even trying, out of concern that it’s a form of shaming—experts have taken to calling for regulation that would force the food industry to stop selling junk food. Although there isn’t a lot of hard evidence that restricting the marketing or even sale of junk food would improve the public’s dietary health, the assumption seems reasonable. That’s why in July the WHO called for countries around the world to implement such regulation to protect children from junk-food marketing.
A regulation like that may be a pipe dream in the U.S., where many people, and most Republicans in Congress, oppose restrictions or taxes on food marketing or sales. “These intrinsically healthy foods have to be regulated,” says Monteiro. “But the culture in the U.S. is very much against it.” (Republicans in Congress are willing to make one exception to that resistance to food regulation: They are currently pushing a ban on junk food purchases made with federal food-aid benefits. Democrats insist such a law would unfairly target the poor.)
Marion Nestle, a New York University professor emeritus of nutrition and a leading advocate for reforming America’s diet, proposed a series of modest junk-food-related regulations in a 2022 paper in the American Journal of Public Health. But she conceded that the proposed regulations “may seem unrealistic,” and “would confront formidable attitudinal, legal, and legislative hurdles.” She called them “aspirational.”
Neilson Barnard/The International Culinary Center/Getty
Other than the proposed legislation impacting food-aid recipients, no junk-food restrictions are currently under active consideration by Congress. Two California cities, Berkeley and Perris, have banned grocery stores from displaying junk food at check-out counters. Even the U.K., which has proven far friendlier to anti-junk-food regulation, was forced in June to back off a planned ban of two-for-one discounts for junk food items in grocery stores until at least 2025, due to objections over consumers having to pay more for food—even if it’s junk food.
Lacking any realistic hope of curbing the massive output of junk food, it’s unclear what dietary changes might realistically be sold to the American public. “Diet isn’t simple,” says Bailey, “but people need to take home a simple message.”
Still, there may be some wiggle room for improvement. Although most doctors can’t convince their patients to make dietary changes, studies suggest most doctors don’t follow the recommended approaches for counseling patients who are overweight, which include tailoring advice to each patient’s specific eating habits and scheduling a follow-up conversation. Several studies indicate that doctors who follow these recommendations usually succeed in guiding patients to at least modest weight loss.
This sort of practical advice, rather than the hawking of impressive-sounding theories, is where the most progress can be made, says Ard. “We need to spend more time on the science of implementing our ideas and moving them into practice,” he says. “Yes, the environment promotes unhealthy eating, but that’s not the same as saying we can’t encourage people to take some level of responsibility for managing what they can in their diet. We can help them make small changes that can lead to significant impact.” Getting people to cut down on sugary sodas, he says, would be a good place to start.
As long as critics lambast such counseling as useless and shaming, and as long as nutrition scientists clash over differing views of the ideal diet while howling for regulation that has no chance of happening, little is likely to change. The consequences are measured in the millions of lives cut short.