Making Sense of the Latest Evidence on Dietary Fats

Fats may be on a public opinion roller coaster, but between complicated terminology and oversimplified diet trends there is new evidence that challenges age-old wisdom.

It wasn't long ago that the general recommendation for most dietary fats was to avoid them — and depending on whom you ask, it still is. But fixating on the calories in fat and its relationship to weight may preclude a fuller understanding of dietary fat quality, sources of different fats and fats’ impact on health. While all fats have about the same calories, unsaturated fatty acids continue to be the healthful hero and trans fatty acids the villain. When it comes to saturated fatty acids, however, the paradigm is shifting.

Decades of research shows that heart health can be improved by replacing most unhealthy dietary fats with unsaturated fats. Because of their impact on total and LDL cholesterol and the risk of cardiovascular disease, saturated fatty acids have been in the medical community’s crosshairs.

The 2010 Dietary Guidelines for Americans recommend limiting saturated fats to up to 10 percent of calories and replacing them with unsaturated fats. But in 2013, the American Heart Association/American College of Cardiology Lifestyle Management guidelines further slashed saturated fat intake to 5 to 6 percent of calories.

However, saturated fats are not the sole culprit for increased risk of cardiovascular disease, and it’s not enough to use a single biomarker such as LDL cholesterol to assess risk. Additional factors include lifestyle, being overweight or obese, increased waist circumference and other lipids, says Penn State University heart health researcher Penny Kris Etherton, PhD, RD.

Current dietary guidance to limit saturated fat also does not account for the food source — whether dairy fat (not typically associated with an increased risk of weight gain, cardiovascular disease or type 2 diabetes), coconut oil (92-percent saturated and with potential, albeit not well-supported, health benefits) or other plant or animal fats.

Additionally, while American consumers have been successful in following heart-health advice to eat less fat, many have replaced it with refined carbohydrates and sugars. This may escalate rates of obesity, diabetes, metabolic syndrome and heart disease — and ultimately contribute to heart disease at a higher rate than saturated fats. Instead, replacing saturated fat with PUFAs (which offer additional protective properties), followed by MUFAs (prevalent in the Mediterranean diet) and then whole-grain carbohydrates may more favorably impact lipid profiles.

“Grouping all saturated fatty acids together is not a useful context because they have different structures and biologic effects. What is consumed in place of saturated fatty acids matters, and so do food choices and overall diet,” says Harvard cardiovascular researcher Dariush Mozaffarian, MD, DrPH. A diet restricting saturated fats to 6 percent of caloric intake is not definitively healthy; it depends on nutrient density and other characteristics of the total diet.

Meanwhile, omega-3 fatty acids have enjoyed a virtual health halo. Strong evidence suggests omega-3 fatty acid consumption from marine and plant sources has a significant cardioprotective effect and decreases cardiovascular mortality. For the primary prevention of cardiovascular disease, the American Heart Association and Academy of Nutrition and Dietetics recommend at least two servings of preferably fatty fish per week for an average daily intake of 450 to 500 milligrams.

Beyond cardiovascular disease, omega-3 fatty acids in sufficiently high amounts are anti-inflammatory agents that can reduce symptoms of rheumatoid arthritis. Brain health is another area of emerging research, and a link between low levels of omega-3 fatty acids and depression has led the American Psychiatric Association to recommend consuming fatty fish at least twice a week. Other studies suggest a relationship between lower levels of docosahexaenoic acid and cognitive decline and Alzheimer’s disease — although the results are mixed and noted as “fair” in the Academy’s Evidence Analysis Library.

There seems to be more consensus when it comes to trans fats: Guidance from the American Heart Association, National Cholesterol Education Program, Institute of Medicine and Dietary Guidelines recommends eliminating or limiting trans fats from artificial sources to 1 percent of total calories, or as low as possible, because they lower HDL and increase LDL. A 2-percent increase from trans fatty acids is associated with a 23-percent increase in cardiovascular risk.

In November 2013, the Food and Drug Administration proposed strict measures to eliminate artificial trans fatty acids, no longer declaring them “generally recognized as safe.” Cited in the document is the Institute of Medicine’s conclusion that there is no safe level of trans fat consumption.

Many food manufacturers are taking note; according to the Centers for Disease Control and Prevention, efforts to reformulate foods have led to a significant reduction in trans fat measured in blood levels and a reduction in intake to about 1 gram per day in 2012. Innovative steps have led to fat blends and use of low-linoleic, mid-oleic or higholeic oils. Yet saturated fats such as palm and palm kernel oil are still used for their physical properties. “MUFAs blended with small amounts of saturated fatty acids are common replacements, although ideally more PUFAs would be used, but anything is better than artificial trans fatty acids,” says Mozaffarian, who studied the impact of food reformulation.

So is the low-fat diet passé? For decades, followers of the Mediterranean diet have demonstrated that a diet rich in unsaturated fats can be healthy. The PREDIMED study — which studied nearly 7,500 participants over five years on Mediterranean diets enriched with tree nuts or olive oil — showed less weight gain, risk of type 2 diabetes, cardiovascular disease and stroke on a 41-percent healthy-fat diet compared to a low-fat diet.

Concentrating on the total diet and individualizing dietary fats based on risk factors may be a more effective approach to nutrition and health. The 2013 ACC/AHA guidelines emphasize vegetables, fruits, whole grains, low-fat dairy, poultry, fish, legumes, nontropical vegetable oils and nuts while limiting intake of sweets, sugar-sweetened beverages and red meats. The Dietary Guidelines advise replacing solid fats with oils when possible, increasing the amount and variety of seafood by replacing some meat and poultry, choosing low-fat or fat-free dairy foods and eating more vegetables, fruits and whole grains.

Kathleen Zelman
Kathleen Zelman, MPH, RDN, is the nutrition director of WebMD.