Saturated Fat — Friend Or Foe?
It seems that over the past five years dietary advice has been turned on its head—saturated fat, as the latest nutrition dogma to be challenged, appears to be joining previously questioned dietary concerns related to salt and cholesterol. Recent publications in high-profile scholarly journals such as the Annals of Internal Medicine and the BMJ seem to support a change in the way we view this long-vilified nutrient. Dietary guidelines of the World Health Organization recommend that saturated fat be limited to less than 5% to 10% of our daily caloric intake to reduce the risk of heart disease and stroke. But should this be the case? Or are these findings more reflective of a need to update our approach to providing nutrition guidance to the public?
The systematic review and meta-analysis published in the BMJ ("Intake of Saturated and Trans Unsaturated Fatty Acids and Risk of All Cause Mortality, Cardiovascular Disease, and Type 2 Diabetes: Systematic Review and Meta-Analysis of Observational Studies") found no association between saturated fat and health outcomes in studies, but the certainty in the null associations for saturated fat and the health outcomes is "very low." Though the review found no association between saturated fat and the health outcomes, I would caution against using the findings to support calls to increase the allowable amount of saturated fat in the diet. The study could not confidently rule out an increased risk of death from heart disease with higher amounts of saturated fat. This is similar to well-designed studies of the past that reported eating less saturated fat and more polyunsaturated fat from vegetable oils reduces "bad" cholesterol and an individual's chances of developing or dying from heart disease.
There were some limitations of the data due to study design and the large amount of variability among past studies. In most of these studies, the investigators relied on people to recall and report what they ate over a number of years, and then followed these individuals for years to see what diseases they developed. What they found was that people who said they ate less saturated fat weren't any less likely to develop heart disease than those who ate more. This finding directly relates to the statistical analysis of these studies; as modeled, the association of saturated fat with the health outcomes reflected a replacement of saturated fat with carbohydrate. In most studies, this carbohydrate was largely "refined starches" such as white flour, refined grains, and sugar. From a cardiovascular risk perspective, this swap probably wasn't much better for the heart.
The finding underscores that simply telling people to not eat saturated fats, such as those found in red meat, butter, cheese, and ice cream, isn't likely to be helpful if we do not provide alternatives. Dietary advice needs to reflect evidence from both observational studies and randomized trials, and data from randomized trials show that replacing saturated fats with healthier fats such as olive, canola, sunflower, soy, and corn oils as well as nuts, seeds, and fish lowers heart disease risk.
Converging evidence shows that no one nutrient or food is responsible for heart disease, stroke, type 2 diabetes, or early death. Replacing saturated fats with unsaturated plant fats is an important step toward better health, but it's important to note that even healthful foods such as nuts and avocados contain a small amount, so dietary advice must consider the whole diet. Dietary patterns consistently associated with good health, such as the Mediterranean diet, plant-based diets, or the DASH diet, tend to be low in saturated fat, but their healthfulness is not due solely to the fact that they are low in saturated fat. These diets combine a number of foods that are highly nutritious, such as whole grains, fruits, legumes, vegetables, and nuts, and avoid foods that contain refined starch and sugar and processed trans fats.
— Russell de Souza, RD, ScD, the lead author of the BMJ study, is a registered dietitian and assistant professor in the department of clinical epidemiology and biostatistics at McMaster University in Hamilton, Ontario, Canada.