Your 48-year old patient has a family history of coronary artery disease (CAD), and you’ve recently noticed that her low-density lipoprotein cholesterol (LDL-C) has gone up. When you explain that this is a risk factor, she asks what kind of nutritional advice you have for her, and specifically, which oils to use for cooking. Should she stick with sunflower, check out canola or opt for olive?
Trial and error
Soon after it was recognized in the 1980s that saturated fat was a substantial culprit in causing CAD, researchers set out to study the benefits of polyunsaturated fatty acids (PUFA). Randomized controlled trials were performed, finding that PUFA reduced the incidence of heart disease — but, surprisingly, they also showed excess mortality due to cancer when this class of fatty acid was raised in the diet. As a result, recommendations across Western nations now suggest that no more than 10% of calories should come from PUFA. With saturated fat generally pegged at less than 10%, the only major fatty acid left to promote is monounsaturated fat.
Indeed, the recommendations for total fat intake have gone from 30% to 35% of calories in the new Dietary Reference Intake Guidelines. As monounsaturated fat is the predominant fatty acid of olive oil, its popularity has gone hand in hand with the proposed health benefits of the Mediterranean diet.
Another contestant for the most beneficial cooking oil is canola. Evidence is now emerging from previously communist countries of the former Eastern Block that heart disease is falling rapidly in places where butter used to be subsidized and is now replaced by the cheaper canola oil and its products. This oil has the advantage of having both monounsaturated fat and a vegetable omega-3 fatty acid called alpha-linolenic acid. Randomized controlled trials have shown that diets high in this fatty acid prevent recurrence of cardiovascular events in high-risk people.
Keep it cool
Ideal fats are therefore olive oil, as a rich source of monounsaturated fats, and canola, with the added benefit of omega-3 fatty acids. Other vegetable fats including safflower, sunflower and corn oil are rich in PUFA of the omega-6 type; they should be restricted to no more than 10% of dietary calories despite their effect of lowering LDL-C. Tropical oils such as palmitic acid may be used to “thicken” margarines. Palmitic acid raises LDL-C but has some offsetting advantage in also increasing HDL-C — so in moderation and as part of a balanced diet its negative impact is likely to be small, especially in individuals with a healthy body weight.
Whatever the nature of the fat they choose, remind your patients that high-temperature cooking is very much discouraged. Not only may it form potentially toxic substances in the fat but the foods themselves will also undergo reactions during browning that create compounds such as advanced glycation end products and acrylamide. These have been linked to renal damage, cardiovascular disease and cancer in rodents. Oils should be added to foods after cooking if both the taste and health benefits are to be enjoyed.
David Jenkins, MD, PhD, FRCPC is Director of the Risk Factor Modification Centre at St. Michael’s Hospital and a professor of medicine and nutritional sciences at the University of Toronto.
Proportions of world vegetable oil consumption in 2004:
(United States Department of Agriculture)