Fat: Lower is Better

In September, the National Academy of Sciences’ Institute of Medicine (IOM) released a report with new dietary guidelines for Americans in the hope of reducing the risk of chronic diseases. According to Joanne Lupton, who chaired this panel of experts, “We established ranges for fat, carbohydrates and protein because they must be considered together. Fat should make up 20 to 35% of calories, protein 10 to 35% of calories and carbohydrate from 45 to 65% of calories. According to Professor Lupton, “Studies show that when people eat very low levels of fat, combined with very high levels of carbohydrates, high-density lipoprotein (HDL) concentrations, or “good” cholesterol, decreases.” Conversely, high-fat diets can lead to obesity, and its complications, if caloric intake is increased as well, which is often the case.”Figure 1 above shows that as dietary fat is increased, from just below to well above the new fat guideline (20 to 35%), calorie intake increases. This same study showed increasing weight on the highest fat content diet and reducing weight on the lowest fat content. Those eating the 30 to 35% diet saw no significant change in body weight.1The data from Figure 1 shows that increasing the percentage of fat in the diet from just below to about the new upper limit range promoted a significant increase in energy intake by about 265 calories per day. Over one year, an extra 265 calories a day could promote a body weight gain of about 25 pounds. The IOM?is correct that higher levels of fat in the diet often promote weight gain.The tendency for calorie intake to increase as fat is added or drop if fat is removed from foods is consistent with most clinical trials and epidemiological evidence. Most studies show a positive correlation between body weight and the percent of calories consumed as fat in adults and children.2, 3, 4 With weight gain often comes insulin resistance and the “metabolic syndrome,” which can lead to heart disease and diabetes.Is Less Than 20%?Dangerous?There have been many short-term studies showing that HDL drops on a very-lowfat (VLF) diet. This is most likely to happen when the subjects eating the low-fat diet are required to eat the same number of calories as those on a higher-fat diet. The IOM apparently believes that this drop in HDL on a VLF diet would somehow increase the risk of coronary artery disease.There are two reasons to believe this is not the case. First, there is growing evidence that the drop in HDL that results from restricting dietary fat intake does not lead to a permanently lower HDL. This is because reducing dietary fat usually leads to a lower calorie intake and weight loss. A lower body weight usually leads to an increase in HDL. A study by Thuesen found that when a group of men with high cholesterol were placed on a VLF, near-vegetarian diet for three months, their calorie intake decreased and they lost about 16.5 lbs on average. Changes in blood lipids on this VLF, high-carbohydrate diet are shown below:5The Thuesen study demonstrated that when a VLF diet was fed ad libitum (until full) to patients at high risk of coronary artery disease that HDL levels do not necessarily drop on average after just three months. In fact, blood lipids overall usually improve dramatically.The second reason a drop in HDL is not of concern on a VLF, high-carbohydrate diet is that research has shown that cholesterol is cleared out of the arteries faster on a lowfat diet than one higher in fat.6 Another study that examined the mechanism responsible for the drop in HDL when dietary fat was reduced noted that the decrease in HDL “should not be viewed as a negative outcome with regard to coronary heart disease risk.”7 This means that it is likely that the amount of cholesterol transported back to the liver from the arteries is not impaired on a VLF, high-carbohydrate diet even if the HDL level ends up somewhat lower.Indeed, other studies have shown that in many patients with advanced artery disease that a VLF diet often leads to regression of atherosclerotic plaque.8Bottom Line:Most research suggests that the most effective strategy for promoting weight loss, and avoiding diabetes, CAD?and cancer, is to keep dietary fat below 20% of total calories. There is no credible research linking diets with less than 20% calories from fat with an increased risk of heart disease.By James J. Kenney, PhD, RD, FACN.References:1. Am J Clin Nutr 1987;46:886-922. J Nutr 2002;132:2488-913. Am J Clin Nutr 1992;55:818-224.Intern J Obesity 2002;26:200-75. Am J Clin Nutr 1986;44:212-96. J Clin Nutr 1999;85:144-517. Arterioscler Thromb Vasc Biol 1999;19:918-248. JAMA 1998;280:2001-7