by Virginia Messina, MPH, RD
Defining optimal fat intake for clients is an ever-growing challenge for dietetics professionals. No longer can we counsel clients to "eat less fat" or "limit saturated fat" and leave it at that. Research into the effects of different types of fat has given us greater insight into the complex tapestry that defines an optimal diet. And with that insight, we are probably left with more questions than answers. The following discussion grew from debates in the vegetarian internet forum sci-veg. VN DPG members who were involved in this discussion on "what is optimal fat intake" felt it would be helpful to organize some of the thoughts and questions in an effort to clarify the debate. This article is the result. It is not meant to answer questions about optimal fat intake or to serve as a review of the literature. Rather, it aims to frame the debate by outlining some of the research that challenges the idea that low fat diets are optimal.
Studies indicate that vegetarian diets are not particularly low in fat. Best estimates are that average fat intake among lacto-ovo-vegetarians is 32 to 34% of calories and among vegans 29 to 30% of calories (1). This is, of course, somewhat lower than average omnivore intake. Intakes of saturated fat and cholesterol, however, are considerably lower in vegetarians than in omnivores. There is reason to expect that fat intake of vegetarians could decrease with the interest in low-fat eating and the growing number of low-fat and nonfat vegetarian products on the market.
Among some segments of the vegan population, there is the perception that very low fat diets-10 to 15% of calories-are ideal for disease prevention. A number of prominent vegetarian spokespersons from both the medical and nutrition fields do, in fact, recommend fat intakes in this range and often suggest that ideal diets involve avoidance of all vegetable oils and higher fat plant foods such as nuts, seeds, avocado, olives, and full-fat soyfoods. The mainstream acceptance of comprehensive health programs like the Ornish Program, which has been shown to be effective in reversing heart disease and which includes a very low fat near-vegan diet, has given some strength to the opinion that very low fat diets are ideal(2). Attention to low-fat diets has also led to misconceptions among some vegetarian consumers regarding effects of fat on health. For example, some consumers believe that all fat raises cholesterol. This has helped to create a dichotomous attitude towards fat that seems to be fairly prevalent among consumers in general. "All fat is bad and therefore avoiding all fat is good."
Research does not necessarily support the idea that optimal diets must be low in fat. More interesting though, is the fact that some research suggests that diets higher in certain plant fats may be the better choice, at least for certain individuals.
Some of the findings that challenge very low fat diets include the following:
Diets high in carbohydrate and low in fat reduce both LDL and HDL cholesterol, sometimes producing no improvement or a worsening in TC:HDL ratio (3). The significance of low HDL levels in vegetarians and in people consuming low-fat diets continues to be a subject of debate. On the one hand, it has been suggested that low HDL does not matter when total cholesterol is low (4). However, some research suggests that the TC:HDL ratio is predictive of heart disease risk at all levels of total cholesterol (5).
Some research shows that HDL cholesterol levels stay depressed for as long as the low-fat diet is consumed and that it is not a transient effect as has been suggested (6).
While replacing fat with carbohydrate reduces both LDL and HDL cholesterol, replacing saturated fat with unsaturated fat lowers mainly LDL cholesterol (3).
Monounsaturated fatty acids may inhibit the oxidation of lipoproteins (7).
Concerns have been raised over the ratio of essential fatty acids in some vegetarian diets. Vegetarians, and in particular, vegans, consume little if any of the long chain omega-3 fatty acid docosahexanoic acid (DHA) which is found predominantly in fish oil and eggs. Although the omega-3 fatty acid linolenic acid can be converted to DHA, linoleic acid can interfere with the conversion process. Diets that are either too high in linoleic acid or too low in all fats may not provide optimal ratios of linoleic to linolenic acid to allow for optimal conversion of linolenic acid to DHA (8).
Higher ratios of omega-3 to omega-6 fatty acids have been associated with reduced risk for breast cancer (9).
Some high fat foods that are routinely discouraged in very low fat diet patterns are associated with protective effects against coronary heart disease. For example, the Adventist Health Study found that nut consumption is associated with protection against both fatal and nonfatal coronary heart disease and with decreased risk of death from all causes (10).
Compared to a high carbohydrate diet, a diet high in monounsaturated fats (45% fat from predominantly olive oil), was associated with a significant reduction in triglyceride and VLDL cholesterol levels in people with non-insulin dependent (Type 2) diabetes. It was also associated with mild improvements in HDL cholesterol levels, glycated hemoglobin levels, and fasting plasma glucose levels (11).
Epidemiologic studies suggest that very low fat intake raises risk for hemorrhagic stroke (12).
Monounsaturated fats have been linked to decreased risk for breast cancer (13).
Findings such as these suggest that diets that include some added fats and/or higher fats may offer advantages over very low fat menus. However, there is clearly a need for balance in recommendations. Diets high in polyunsaturated fats may be associated with certain cancers (14). And some research suggests that single, very high fat meals raise heart disease risk (15). Higher fat diets also may increase caloric intake and therefore are hypothesized to lead to problems in weight management, although clear data that reductions in fat intake alone are associated with long term weight loss are lacking.
Overall, however, the research suggests that we should not necessarily assume that very low-fat vegetarian diets are the ideal. In addition, there are many unanswered questions about their appropriateness for certain segments of the vegetarian population such as young children. A reasonable amount of dietary fat is required for the absorption of fat-soluble vitamins and health-promoting phytochemicals. While experts are not in agreement about optimal levels of dietary fat, a reasonable goal for most consumers (provided that saturated fat intake is kept low) would be a fat intake between 15 and 30% of calories, the level recommended in the World Health organization guidelines.
Finally, some might argue that focus on strict reduction of all types of fat in the diet makes it more difficult for clients to adopt healthy plant-based diets. Diets that include some added fats such as vegetable oils high in monounsaturated fats and in omega-3 fatty acids and higher fat foods such as nuts, nut butters, full-fat soyfoods and avocados may make it easier for consumers to plan realistic menus and to reap the many benefits of plant-based diets.
1. Messina MJ and Messina V. The Dietitians Guide to Vegetarian Diets. Gaithersburg, MD: Aspen Publications, 1996.
2. Ornish D, Brown SE, Scherwitz LW, et al. Can lifestyle changes reverse coronary heart disease? Lancet 1990;336:129-133.
3. Mensink RP, Katan MB. Effect of dietary fatty acids on serum lipids and lipoproteins: a meta-analysis of 27 trials. Arterioscler Thromb 1992;12:911-920.
4. Masarei JR, Rouse IL, Lynch WJ, et al. Vegetarian diets, lipids and cardiovascular risk. Aust NZ Med 1984;14:400-404.
5. Kukita H, Imamura Y, Hamada M, et al. Plasma lipids and lipoproteins in Japanese male patients with coronary artery disease and in their relatives. Atherosclerosis 1982;42:21-29.
6. Sandstrom B, Marckmann P, Bindslev N. An eight month controlled study of a low-fat high fibre diet: effects on blood lipids and blood pressure in healthy young subjects. Eur J Clin Nutr 1992;46:95-109.
7. Sola R, La Ville AD, Richard JL, Motta C et al. Oleic rich diet protects against the oxidative modification of high density lipoprotein. Free Rad Biol Med 1997;22:1037-45.
8. Emken, EA, Adlof, RO, Gulley, RM. Dietary linoleic acid influences desaturation and acylation of deuterium-labeled linoleic and linolenic acids in young adult males. Biochimica et Biophysica Acta 1994;1213:277-88.
9. Simonsen N, Strain JJ, van't Veer P, et al. Adipose tissue omega-3 fatty acids and breast cancer in a population of European women. Am J Epidemiol 1996;143:S34.
10. Fraser GC, Sabate J, Beeson WL, et al. A possible protective effect of nut consumption on risk of coronary heart disease: the Adventist Health Study. Arch Intern Med 1992;152: 1416-24.
11. Garb A, Bonanome A, Grundy SM, et al. Comparison of high carbohydrate diet with a high monounsaturated fat diet in patients with non-insulin dependent diabetes mellitus. N Eng J Med 1988, 319:829-34.
12. Gillman MW, Cupples LA, Millen BE, et al. Inverse association of dietary fat with development of ischemic stroke in men. JAMA 1997;278:2145-50.
13. Wolk A, Bergstrom R, Hunger D, et al. A prospective study of association of monounsaturated fat and other types of fat with risk of breast cancer. Arch Intern Med 1998;158:41-45.
14. Rose DP. Dietary fatty acids and cancer. Am J Clin Nutr 1997;66(suppl):998S-1003S.
15. Plotnick GD, Corretti MC, Vogel RA. Effect of antioxidant vitamins on the transient impairment of endothelium-dependent brachial artery vasoactivity following a single high-fat meal. JAMA 1997;278:1682-86.
Virginia Messina operates Nutrition Matters, Inc. from her office in Port Townsend, Washington. Ginny and and Mark Messina are co-authors of the book The Dietitians Guide to Vegetarian Diets.