Complicating matters is that there is also a lot of sugar in the good stuff we eat. Your liver doesn’t know whether the sugar you consume comes from an apple or a candy bar. A sixteen-ounce bottle of Pepsi has about thirteen teaspoons of added sugar and no nutritive value at all. Three apples would give you just as much sugar but compensate by also giving you vitamins, minerals, and fiber, not to mention a greater feeling of satiation. That said, even the apples are a lot sweeter than they really need to be. As Lieberman has noted, modern fruits have been selectively bred to be vastly more sugary than they once were. The fruits that Shakespeare ate were, for the most part, probably no sweeter than the modern carrot.
Many of our fruits and vegetables are nutritionally less good for us than they were even in the fairly recent past. Donald Davis, a biochemist at the University of Texas, in 2011 compared the nutritive values of various foods in 1950 with those of our own era and found substantial drops in almost every type. Modern fruits, for instance, are almost 50 percent poorer in iron than they were in the early 1950s, and about 12 percent down in calcium and 15 percent in vitamin A. Modern agricultural practices, it turns out, focus on high yields and rapid growth at the expense of quality.
In the United States, we are left in the bizarre and paradoxical situation that we are essentially the world’s most overfed nation but also one of its most nutritionally deficient ones. Comparisons with the past are a bit difficult to make because in 1970 Congress canceled the only comprehensive federal nutrition survey ever attempted after the preliminary results proved embarrassing. “A significant proportion of the population surveyed is malnourished or at a high risk of developing nutritional problems,” the survey reported, just before it was axed.
It is hard to know what to make of any of this. According to the Statistical Abstract of the United States, the amount of vegetables eaten by the average American between 2000 and 2010 dropped by thirty pounds. That seems an alarming decline until you realize that the most popular vegetable in America by a very wide margin is the French fry. (It accounts for a quarter of our entire vegetable intake.) These days, eating thirty pounds less “vegetables” may well be a sign of an improved diet.
A striking marker of just how confused nutrition advice can be was a finding by an advisory committee for the American Heart Association that 37 percent of American nutritionists rate coconut oil—which is essentially nothing but saturated fat in liquid form—as a “healthy food.” Coconut oil may be tasty, but it is no better for you than a big scoop of deep-fried butter.
“It is,” says Lieberman, “a reflection of how deficient dietary education can be. People just aren’t always getting the facts. It’s possible for doctors to go through medical school without being taught nutrition. That’s crazy.”
Perhaps nothing is more emblematic of the unsettled state of knowledge on the modern diet than the long and unresolved controversy over salt. Salt is vital to us. There is no question of that. We would die without it. That’s why we have taste buds devoted exclusively to it. Lack of salt is nearly as dangerous to us as lack of water. Because our bodies cannot produce salt, we must consume it in our diets. The problem is in determining how much is the right amount. Take too little and you grow lethargic and weak, and eventually you die. Take too much and your blood pressure soars and you run the risk of heart failure and stroke.
The average American consumes about 3,400 milligrams of sodium a day. It is very difficult not to. A lightish lunch of soup and a sandwich, none of it conspicuously salty, can easily push you over the limit. Many authorities believe that 3,400 milligrams is way too much and that it vastly increases the risk of heart attack and stroke. The World Health Organization suggests that we consume no more than 2,000 milligrams of sodium a day. But other authorities say that reducing sodium intake to that level has no proven health benefit and may actually be harmful.
One study in Britain estimated that as many as 30,000 people a year died in the U.K. from consuming too much salt over too long a period, but another study concluded that salt did no harm to anyone except for those with elevated blood pressure, and yet another concluded that people who ate a lot of salt actually lived longer. A meta-analysis at McMaster University in Canada of 133,000 people in four dozen countries found a link between high salt intake and heart problems only for those with existing hypertension, while low salt intake (less than three thousand milligrams a day) had an increased risk of heart problems for people from both groups. In other words, according to the McMaster study, too little salt is at least as risky as too much.
A central reason for the lack of consensus, it turns out, is that both sides indulge in what statisticians call confirmation bias. Simply put, they don’t listen to each other. A 2016 study in the International Journal of Epidemiology found that researchers on both sides of the argument overwhelmingly cite papers that support their own views and ignore or dismiss those that do not. “We found that the published literature bears little imprint of an ongoing controversy, but rather contains two almost distinct and disparate lines of scholarship,” the study’s authors wrote.
To try to find an answer, I met Christopher Gardner, director of nutrition studies and professor of medicine at Stanford University in Palo Alto, California. He is a friendly man, with a ready laugh and relaxed manner. Though nearing sixty, he looks at least fifteen years younger. (This seems to be true of most people in Palo Alto.) We met at a restaurant in a neighborhood shopping center. He arrived, almost inevitably, on a bicycle.
Gardner is a vegetarian. I asked him if that was for health or ethical reasons. “Well, actually originally it was to impress a girl,” he said, grinning. “That was in the 1980s. But then I decided I liked it.” In fact, he liked it so well he decided to start a vegetarian restaurant but felt he needed to understand the science better, so he did a PhD in nutrition science and got sidetracked into academia. He is refreshingly reasonable about what we should and shouldn’t eat.
“In principle, it’s really pretty simple,” he says. “We should eat less added sugar, less refined grain, and more vegetables. It’s essentially a question of trying to eat mostly good things and avoiding mostly bad things. You don’t need a PhD for that.”
In practice, however, things are not so straightforward. We are all habituated, at an almost subliminal level, to go for the bad stuff. Gardner’s students demonstrated that with a beautifully simple experiment in one of the university’s cafeterias. Each day they gave the cooked carrots a different label. The carrots were always the same and the labels always truthful, but they just emphasized a different quality each day. So one day the carrots were labeled as plain carrots, then the next day as low-sodium carrots, then as high-fiber carrots, and finally as twisted glaze carrots. “The students took 25 percent more of the sugary-sounding glazed carrots,” Gardner says with another broad smile. “These are smart kids. They are aware of all the issues about weight and health and all that, and yet they took the bad option anyway. It’s a reflex. We had the same results with asparagus and broccoli. It’s not easy to overcome the dictates of your subconscious.”