See also "Sugar as a Slow Poison"
I highly recommend Jason Fung's book "The Obesity Code." Jason Fung lays out what I consider the most credible theory for what causes obesity—and implicitly what has led to the continuing dramatic rise in obesity across the developed world over the last century. In order to understand the overall argument, which I will discuss in a future post, it is important to know certain facts that the nutritional establishment is loath to communicate because they run counter to the message they have been giving for so long. Among these, one of the key facts is that there is no evidence that dietary fat is bad for health. In saying this, I leave aside the trans-fats, which with good reason are close to being banned. The use of dangerous trans-fats was encouraged by the suspicion cast on more time-tested dietary fats.
The lack of evidence that dietary fat is bad for health is an important and credible null result because so much effort was expended looking for proof that dietary fat is bad, by researchers who believed that it is. Here are three passages that give the core of Jason Fung's account of that research, from Chapter 18, "Fat Phobia":
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, 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 immediately 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.
IN 1948, HARVARD University began a decades-long community-wide prospective study of the diets and habits of the town of Framingham, Massachusetts. Every two years, all residents would undergo screening with blood work and questionnaires. High cholesterol levels in the blood had been associated with heart disease. But what caused this increase? A leading hypothesis was that high dietary fat was a prime factor in raising cholesterol levels. By the early 1960s, the results of the Framingham Diet Study were available. Hoping to find a definitive link between saturated-fat intake, blood cholesterol and heart disease, the study instead found... nothing at all.
There was absolutely no correlation. Saturated fats did not increase blood cholesterol. The study concluded, “No association between percent of calories from fat and serum cholesterol level was shown; nor between ratio of plant fat to animal fat intake and serum cholesterol level.”
Did saturated fat intake increase risk of heart disease? In a word, no. Here are the final conclusions of this forgotten jewel: “There is, in short, no suggestion of any relation between diet and the subsequent development of CHD [coronary heart disease] in the study group.”
This negative result would be repeatedly confirmed over the next half century. No matter how hard we looked, there was no discernible relationship between dietary fat and blood cholesterol. Some trials, such as the Puerto Rico Heart Health Program, were huge, boasting more than 10,000 patients. Other trials lasted more than twenty years. The results were always the same. Saturated-fat intake could not be linked to heart disease.
But researchers had drunk the Kool-Aid. They believed their hypothesis so completely that they were willing to ignore the results of their own study. For example, in the widely cited Western Electric Study, the authors note that “the amount of saturated fatty acids in the diet was not significantly associated with the risk of death from CHD.” This lack of association, however, did not dissuade the authors from concluding “the results support the conclusion that lipid composition of the diet affects serum cholesterol concentration and risk of coronary death.”
All these findings should have buried the diet-heart hypothesis. But no amount of data could dissuade the diehards that dietary fat caused heart disease. Researchers saw what they wanted to see. Instead, researchers saved the hypothesis and buried the results. Despite the massive effort and expense, the Framingham Diet Study was never published in a peer-reviewed journal. Instead, results were tabulated and quietly put away in a dusty corner—which condemned us to fifty years of a low-fat future that included an epidemic of diabetes and obesity.
Once the skewing effect of trans fats was taken into account, the studies consistently showed that high dietary fat intake was not harmful. The enormous Nurses’ Health Study followed 80,082 nurses over fourteen years. After removing the effect of trans fats, this study concluded that “total fat intake was not significantly related to the risk of coronary disease.” Dietary cholesterol was also safe. The Swedish Malmo Diet and Cancer Study and a 2014 meta-analysis published in the Annals of Internal Medicine reached similar conclusions.
And the good news for saturated fats kept rolling in. Dr. R. Krause published a careful analysis of twenty-one studies covering 347,747 patients and found “no significant evidence for concluding that dietary saturated fat is associated with an increased risk of CHD.” [Siri-Tarino PW et al. Meta-analysis of prospective cohort studies evaluating the association of saturated fat with cardiovascular disease. Am J Clin Nutr. 2010 Mar; 91(3):535–46. In fact, there was even a small protective effect on stroke. The protective effects of saturated fats were also found in the fourteen-year, 58,543-person Japan Collaborative Cohort Study for Evaluation of Cancer and the ten-year Health Professionals Follow-up Study of 43,757 men.
To back up Jason Fung's interpretation of the the Framingham Diet Study, take a look at Michael Eades's blog post "Framingham follies," which Jason Fung cites. And certainly don't dismiss the idea that "dietary fat is innocent of the charges leveled against it without reading "Framingham follies" as an indication of the kind of scientific conduct one needs to be at least alert for in the area of nutritional research.
Also see the Wikipedia article "Saturated fat and cardiovascular disease controversy."
When checking out all the links, remember the problem of multiple hypothesis testing. There are many possible health outcomes. If a researcher tests 20 of them, then even if there is no real relationship between a variable and any of the outcomes, there should on average be an apparent association with one of them by chance that can be reported as "significant at the 5% level." And the number of tests actually done can easily exceed the number of tests reported.