It is now increasingly recognized that the low-fat campaign has been based on little scientific evidence and may have caused unintended health consequences.
HARVARD RESEARCHERS DRS. FRANK HU & WALTER WILLETT, 2001
ONE OF THE giants of modern nutritional science, Dr. Ancel Keys (1904– 2004) received his first PhD in oceanography and biology and a second PhD in physiology from the University of Cambridge. He spent most of the rest of his career at the University of Minnesota, where he would play a dominant role in defining the current nutritional landscape.
During World War II, Dr. Keys led the development of the K-rations, which would form the basis of military nutrition in the United States. He studied the effects of severe caloric restriction in the famous Minnesota Starvation Experiment (discussed in chapter 3). However, his crowning achievement is considered to be the Seven Countries Study, a long-term observational study of diet and heart disease.
In the post–World War II years, starvation and malnutrition were the major nutritional challenges. But Dr. Keys was struck by an odd inconsistency.
Americans, despite far better nourishment, were suffering from rising rates of heart attack and stroke. In war-ravaged Europe, those rates remained low.1 In 1951, Dr. Keys noticed low rates of heart disease in Italian laborers. The Mediterranean diet, as he observed in Naples, was substantially lower in fat (20 percent of calories) than the American diet of the period (approximately 45 percent of calories).2 Most striking, though, was the lower rate of consumption of animal foods and saturated fat. He hypothesized that high blood cholesterol levels caused heart disease and that the low dietary intake of fat was protective. In 1959, he published his dietary advice for the prevention of cardiovascular disease.3 Prominent among his recommendations were the following guidelines:
These recommendations survived relatively intact and defined nutritional orthodoxy for the next half century. In 1977, they became enshrined in the Dietary Guidelines for Americans.4 The main message, then as now, is that all fat is bad, but saturated fats particularly so. Dietary fat was thought to “clog up the arteries” and cause heart attacks.
The ambitious Seven Countries Study compared rates of coronary disease with various diet and lifestyle factors across nations. By 1970, with five years’ worth of data, the study had several main conclusions regarding fats:5
Significantly, total dietary fat was not correlated to heart disease. Rather, saturated fat was dangerous, but mono-unsaturated fats were protective. Dietary cholesterol was also not identified as a risk factor for heart disease.
Heart disease is caused by atherosclerosis—the process by which arteries in the heart become narrowed and hardened by the buildup of plaque. But atherosclerosis is not simply the result of high cholesterol levels clogging arteries. Current opinion holds that plaque develops as a response to injury: the wall of the artery becomes damaged, resulting in inflammation, which in turn allows infiltration of cholesterol and inflammatory cells into artery walls, in addition to the proliferation of smooth muscle. The narrowing of the artery may cause chest pain (also called angina). When plaques rupture, a blood clot forms, which abruptly blocks the artery. The resulting lack of oxygen causes a heart attack. Heart attacks and strokes are predominantly inflammatory diseases, rather than simply diseases of high cholesterol levels.
This understanding, however, came much later. In the 1950s, it was imagined that cholesterol circulated and deposited on the arteries much like sludge in a pipe (hence the popular image of dietary fat clogging up the arteries). It was believed that eating saturated fats caused high cholesterol levels, and high cholesterol levels caused heart attacks. This series of conjectures became known as the diet-heart hypothesis. Diets high in saturated fats caused high blood cholesterol levels, which caused heart disease.
The liver manufactures the overwhelming majority—80 percent—of the blood cholesterol, with only 20 percent coming from diet. Cholesterol is often portrayed as some harmful poisonous substance that must be eliminated, but nothing could be farther from the truth. Cholesterol is a key building block in the membranes that surround all the cells in our body. In fact, it’s so vital that every cell in the body except the brain has the ability to make it. If you reduce cholesterol in your diet, your body will simply make more.
The Seven Countries Study had two major problems, although neither was very obvious at the time. First, it was a correlation study. As such, its findings could not prove causation. Correlation studies are dangerous because it is very easy to mistakenly draw causal conclusions. However, they are often the only source of long-term data available. It is always important to remember that they can only generate hypotheses to be tested in more rigorous trials. The heart benefit of the low-fat diet was not proven false until 2006 with the publication of the Women’s Health Initiative Dietary Modification Trial and the Low-Fat Dietary Pattern and Risk of Cardiovascular Disease study,6 some thirty years after the low-fat approach became enshrined in nutritional lore. By that time, like a supertanker, the low-fat movement had gained so much momentum that it was impossible to turn it aside.
The association of heart disease and saturated fat intake is not proof that saturated fat causes heart disease. Some recognized this fatal flaw immediately7 and argued against making dramatic dietary recommendations based on such flimsy evidence. The seemingly strong link between heart disease and saturated fat consumption was forged with quotation and repetition, not with scientifically sound evidence. There were many possible interpretations of the Seven Countries Study. Animal protein, saturated fats and sugar were all correlated to heart disease. Higher sucrose intake could just as easily have explained the correlation to heart disease, as Dr. Keys himself had acknowledged.
It is also possible that higher intakes of animal protein, saturated fats and sugar are all merely markers of industrialization. Counties with higher levels of industrialization tended to eat more animal products (meat and dairy) and also tended to have higher rates of heart disease. Perhaps it was the processed foods. All of these hypotheses could have been generated from the same data. But what we got was the diet-heart hypothesis and the resulting low-fat crusade.
The second major problem was the inadvertent triumph of nutritionism, a term popularized by the journalist and author Michael Pollan.8 Rather than discussing individual foods (spinach, beef, ice cream), nutritionism reduced foods to only three macronutrients: carbohydrates, proteins and fats. They were then subdivided further as saturated and unsaturated fats, trans fats, simple and complex carbohydrates, etc. This sort of simplistic analysis does not capture the hundreds of nutrients and phytochemicals in foods, all of which affect our metabolism. Nutritionism ignores the complexity of food science and human biology.
An avocado, for instance, is not simply 88 percent fat, 16 percent carbohydrate and 5 percent protein with 4.9 grams of fiber. This sort of nutritional reductionism is how avocados became classified for decades as a “bad” food due to their high fat content, only to the reclassified today as a “super food.” Nutritionally, a piece of butterscotch candy cannot be reasonably compared to kale simply because both contain equal amounts of carbohydrate. Nutritionally, a teaspoon of trans-fat–laden margarine cannot be reasonably compared to an avocado simply because both contain equal amounts of fats.
Dr. Keys made the unnoticed and unintentional claim that all saturated fats, all unsaturated fats, all dietary cholesterol, etc., are the same. This fundamental error led to decades of flawed research and understanding. Nutritionism fails to consider foods as individuals, each with its own particular good and bad traits. Kale is not the same nutritionally as white bread, even though both contain carbohydrates.
These two fundamental but subtle errors of judgment led to the widespread acceptance of the diet-heart hypothesis, even though the evidence supporting it was shaky at best. Most natural animal fats are chiefly composed of saturated fats. In contrast, vegetable oils such as corn are chiefly omega 6 polyunsaturated fatty acids.
After remaining relatively stable from 1900 to 1950, animal-fat consumption began a relentless decline. The dialogue began to change in the late 1990s due to popularity of higher-fat diets. The unintended consequence of the saturated fat reduction was that intake of omega 6 fatty acids increased significantly. Carbohydrates, as a percentage of calories, also started to climb. (To be more precise, these were intended consequences. They were unintentionally detrimental to human health.)
Omega 6s are a family of polyunsaturated fatty acids that are converted to highly inflammatory mediators called eicosanoids. The massive increase in the use of vegetable oils can be traced to technological advances in the 1900s that allowed modern production methods. Since corn is not naturally high in oil, normal human consumption of omega 6 oils had been quite low. But now we could process literally tons of corn in order to derive useful amounts.
Omega 3 fatty acids are another family of polyunsaturated fats that are mainly anti-inflammatory. Flax seeds, walnuts and oily fish such as sardines and salmon are all good sources. Omega 3 fatty acids decrease thrombosis (blood clots) and are believed to protect against heart disease. Low rates of heart disease were originally described in the Inuit population and subsequently in all major fish-eating populations.
High dietary ratios of omega 6:3 ratios increase inflammation, potentially worsening cardiovascular disease. It is estimated that humans evolved eating a diet that is close to