The Role of Fat in a Healthy Diet

Fat has gotten a bad reputation over the last few decades due to a misconception that all types of fat have a negative impact on health, such as increasing the risk for chronic conditions like heart disease. Now scientists are switching the focus from total fat to type of fat because type of fat may have the greatest impact on health. Research in this area has prompted government agencies and health organizations to revise dietary recommendations for fat intake to emphasize the quality of fat in the diet rather than the quantity of fat.

Studies Discussing the Potential Benefits of Healthy Fats

Many studies have been conducted to investigate the effects of different types of fatty acids on the body and then make appropriate consumption recommendations to the general public. While much is still unknown about fatty acids and their physiological impact, a growing body of evidence has shown that consumption of certain types of fatty acids may play a positive role in reducing risk of developing cardiovascular disease and type 2 diabetes.

Cardiovascular Disease

Saturated and trans fatty acids have both been shown to negatively impact blood lipid profiles-raising LDL and total cholesterol and in turn, increasing risk of developing cardiovascular disease.³
Clinical studies show that replacing saturated fat with mono- and polyunsaturated fat can reduced LDL and total cholesterol without a reduction in HDL cholesterol, helping to reduce risk of developing cardiovascular disease. Recently, USDA's Nutrition Evidence Library found that a significant improvement in heart health was seen when 5% of the daily energy consumed from saturated fats were replaced with mono- or polyunsaturated fatty acids.²

Type 2 Diabetes

People with type 2 diabetes are at greater risk for developing heart disease. Therefore, the American Heart Association recommends that people with type 2 diabetes follow a heart healthy diet that is low in saturated fat and high in MUFAs and PUFAs to help maintain normal cholesterol levels. In addition, some research has shown that both mono- and polyunsaturated fatty acids may decrease the risk of developing type 2 diabetes and increase insulin sensitivity among individuals who already have type 2 diabetes, especially when these unsaturated fats replace saturated and trans fatty acids in the diet. In a review of 12 studies, USDA's Nutrition Evidence Library found that saturated fatty acids impaired glucose metabolism and decreased insulin sensitivity; when saturated fatty acids were replaced by mono- and polyunsaturated fatty acids, both glucose metabolism and insulin resistance improved.² These studies showed that these improvements were seen after 5% of energy from saturated fatty acids were replaced with mono- and/or polyunsaturated fatty acids. At this time, more research is needed to understand the mechanism behind this relationship. These findings led the 2010 Dietary Guidelines Advisory Committee to recommend that Americans consume less than 10% of calories from saturated fat, and to attempt to decrease intake to 7% of calories over time. The advisory committee also recommended that as saturated fat intake is reduced, the energy from saturated fat should be replaced by unsaturated fat, so that fats and oils comprise 20-35% of the total diet.

Recent Research

A recent study sponsored by Frito-Lay investigated the effects of three different diets (low fat, high fat, and high PUFA with the high fat diet containing higher amounts of saturated and trans fats than the high PUFA diet) on the blood lipid profiles of a group of 33 individuals. This study investigated whole food substitution in the form of snacks; the base diet in each of the three phases was the same and was based on an American Heart Association Step 1 diet. Each participant in the study was placed on the three diets in three different phases, with each diet phase lasting 25 days. Each phase was isolated by a predetermined "wash-out" period between the phases which lasted between four and eight weeks. Each diet phase included snacks that made up about 12-15% of the participants' calorie needs. During the low-fat phase, snacks included foods such as fat-free cookies, cereal bars, and crackers; the high fat diet snacks included candy bars, buttered popcorn, and high-fat cookies and crackers; the high PUFA snacks included corn and tortilla chips fried in corn oil.³

As illustrated below, all three interventional diets reduced LDL and total cholesterol; however, a greater reduction in these lipid levels was seen from the low fat and high PUFA diets. Triglycerides were reduced to a greater extent on the high PUFA diet when compared to the high fat and low fat diets.

Research suggests that LDL particle size is linked to increased risk of CVD, with small, dense particles (pattern B) being associated with greater risk than larger, more buoyant particles (pattern A). While the results were not statistically significant, data from this study suggests that subjects following the low-fat diet were 3.5 times more likely to have pattern B LDL particles than those following the moderate fat, high-PUFA diet. While more research is needed to understand the effects of the diet on LDL density patterns, a study by Krauss et al. found similar results between pattern A and pattern B LDL particle sizes.⁵

Another study⁴ examined the previous study's same data set, but tracked participants' blood lipid response in association to their C-Reactive Protein (CRP) levels, and found that people with an intermediate or high CRP may reap the greatest benefits from consuming a moderate fat, high PUFA diet. Overall, a high PUFA diet produced the most improved blood lipid profile in regards to cardiovascular risk among the participants in this study.³

Heart Health Claim for Unsaturated Fats

Fatty Acid Profile of Oils and Fats
(In descending order by PUFA and MUFA content)

References

1. Dietary Guidelines for Americans Advisory Committee Draft Report. June 2010.
2. Appel LJ, Sacks FM, Carey VJ, Obarzane E, Swain JF, Miller ER 3^rd, Conlin PR, Erlinger TP, Rosner BA, Laranjo NM, Charleston J, McCarron P, Bishop L; OmniHeart Collabrative Research Group. Effects of protein, monounsaturated fat, and carbohydrate intake on blood pressure and serum lipids: results of the OmniHeart randomized trial. JAMA. 2005 Nov 16;294(19):2455-64.
3. St-Onge MP, Aban I, Bosarge A, Gower B, Hecker KD, Allison DB. Snack chips fried in corn oil alleviate cardiovascular disease risk factors when substituted for low-fat or high-fat snacks. Am J Clin Nutr. 2007 Jun;85(6):1503-10.
4. St-Onge MP, Zhang S, Darnell B, Allison DB. Baseline Serum C-Reactive Protein is Associated with Lipid Responses to Low-Fat and High-Polyunsaturated Fat Diets. The Journal of Nutrition. 2009;139(4):680-683.
5. Krauss RM, Blanche PJ, Rawlings RS, Fernstrom HS, Williams PT. Separate effects of reduced carbohydrate intake and weight loss on atherogenic dyslipidemia. Am J Clin Nutr.2006;83:1025-31.