The transition to a low-fat diet originated in 1955 with the scientist Ancel Keys. Keys first introduced the “lipid hypothesis” which theorized that dietary fats raised cholesterol and lead to heart disease (Hyman, p. 38). Put simply, if high fat diets caused heart disease, then low fat diets must be good for you. He went on to demonstrate this idea by publishing the Seven Countries Study in 1970. This study showed that countries that ate low amounts of saturated fat had lower incidence of heart disease.  However, we now know that this information was unreliable. He used data from a small number of counties and excluded countries that didn’t conform to his hypothesis. Since then, many studies have shown no link between saturated fat and heart disease. The PURE study, for example, examined 135,000 individuals from 18 countries over 10 years. This study found no link between saturated fat and heart disease. The authors concluded that carbohydrates were found to increase the risk of heart disease and death, while fats (saturated, and poly and monounsaturated) were all found to decrease risk (p. 158).

We now know that fats are actually good for you.  They help us fee satiated, increase our metabolism, and don’t increase our blood sugar levels and subsequent insulin spikes.  Fats contribute to healthy cell membranes, help make hormones and immune cells, and regulate inflammation and metabolism. 60% of our brains area fat (p. 152). Without fat, our bodies wouldn’t be able to absorb fat soluble vitamins in plant foods.  Fats also contribute to healthy gallbladder function.  Fat is what stimulates the release of cholecystokinin, which in turn stimulates the release of bile from the gallbladder. A low fat, or bad fat diet, will not trigger this bile release. Without bile, our bodies can’t absorb the fats we need.

Fat also makes food taste good. When Americans were told to eat less fat, food makers had to come up with ways to make their products taste good. This led to the addition of increased amounts of sugar, chemicals, and trans fats to our food. Trans fats are vegetable oils that are hydrogenated to make them solids. Examples include margarine, vegetable shortening (ie; Crisco), and fake butter spreads. Trans fats are cheap to make and give foods a longer shelf life, but they have detrimental consequences to our health. They increase our LDL cholesterol, which causes blockages in our arteries, and contribute to chronic inflammation. They’ve also been linked to obesity, diabetes, and an increased risk of cancer (p. 160). It is estimated that the shift to trans fats caused up to 228,000 heart attacks every year (p. 150). Luckily, the FDA has finally ordered food manufacturers to phase them out of our food products, but searching labels for hydrogenated oils will still be imperative.

A study published in the Annals of Internal Medicine in 2014 found that the fats in the blood that cause heart attacks (palmitic and stearic acid) actually come from eating sugar and carbohydrates (p. 157). Additionally, the fats that reduce heart disease risk are the omega-3 and saturated fats. Diets lacking omega-3 fatty acids put us at greater risk for heart disease, chronic inflammation, Alzheimer’s disease, dementia, ADD, and depression (p.155). Inflammatory omega-6 fatty acids, which are found in refined vegetable oils like canola and soybean oil, were also found to increase heart disease risk.  And while our bodies need omega-6 fatty acids, they need to be consumed with the proper ratio to omega-3 for optimal health. Ideally, a ratio of 1:1, which is similar to the way our ancestors ate. With the use of refined vegetable oils in the processed foods we eat, the ratio of omega 6:3 we consume today is far out of balance.

Governmental dietary recommendations are slowly catching up to the research. In 2015, the US Dietary Guidelines finally removed the upper limits on total dietary fat and cholesterol.  They do, however, still recommend limiting saturated fat to no more than 10% of total daily calories.  They also still recommend to use vegetable oils over saturated fats (butter, coconut oil, lard) when cooking (health.gov).  For optimal health, however, it may be best to ignore the guidelines and follow the diets of our ancestors. Eat a diet rich in healthy, whole-food based fats from fish, grass-fed animal products, olive and nut oils, and avocados, and eliminate highly processed unsaturated vegetable fats.

Hyman, M. (2018). Food What the Heck Should I Eat. New York, NY: Little, Brown and Company.

U.S. Department of Health and Human Services and U.S. Department of Agriculture. 2015 – 2020 Dietary Guidelines for Americans. 8^th Edition. December 2015. Available at https://health.gov/dietaryguidelines/2015/guidelines/.