A month ago, I wrote about a reader "Steve" who reversed his fatty liver using a change in diet. Non-alcoholic fatty liver disease (NAFLD) is a truly disturbing modern epidemic, rare a few decades ago and now affecting roughly a quarter of the adult population of modern industrialized nations. Researchers cause NAFLD readily in rodents by feeding them industrial vegetable oils or large amounts of sugar.
We've been having an interesting discussion in the comments about a recently published paper by Dr. Stephen C. Benoit and colleagues (free full text). They showed that a butter-rich diet causes weight gain and insulin resistance in rats, compared to a low-fat diet or a diet based on olive oil. They published a thorough description of the diets' compositions, which is very much appreciated!
They went on to show that infusing palmitic acid (a 16-carbon saturated fat) directly into the brain of rats also caused insulin resistance relative to oleic acid (an 18-carbon monounsaturated fat, like in olive oil). Here's a representation of palmitic acid. The COOH end is the acid end, and the squiggly line is the fatty end. Thus it's called a "fatty acid", various forms of which are the fat currency of the body.
One of the most interesting things about this study is the butter group that the investigators fed the same number of calories as the low-fat group (this is called pair-feeding). This group did not become overweight, and did not experience elevated fasting insulin and blood glucose relative to the low-fat group*. This shows clearly that the adverse effects of the butter diet were primarily due to the fact that rodents overeat when fed a high-fat diet.
Unfortunately, the paper doesn't provide longitudinal food intake data so we have no idea how many calories the rats in each group ate, beyond knowing that the low-fat group and the pair-fed butter group ate the same amount. We have no assurance that rats in the butter group and olive oil group ate the same number of calories over time. Rats eat less of foods they find bitter. This probably accounts, at least in part, for the beneficial effects of things like blueberry extracts on rodent models of disease. Olive oil may taste bitter to a rat, particularly when it's 20% of the diet by weight. Butter is tasty to calves, humans and rats alike.
This diet trial was conducted between 1959 and 1971 in two psychiatric hospitals near Helsinki, Finland. One hospital served typical fare, including full-fat milk and butter, while the other served "filled milk", margarine and polyunsaturated vegetable oils. Filled milk has had its fat removed and replaced by an emulsion of vegetable oil. As a result, the diet of the patients in the latter hospital was low in saturated fat and cholesterol, and high in polyunsaturated fat compared to the former hospital. At the end of six years, the hospitals switched diets. This is known as a "crossover" design.
Just to show it wasn't a fluke, reader "Steve" replicates the experiment: