Nutrition plays an important role in the prevention of chronic illnesses such as diabetes and heart disease. The Seven Countries Study, one of the earliest and most influential epidemiological studies, spanned the second half of the last century and showed a strong correlation between consumption of saturated fat and incidence of coronary heart disease. This result established a “low-fat” dietary dogma that has gone largely unchallenged over decades and remains entrenched today . It is therefore surprising that a new study from Tulane University published in the Annals of Internal Medicine, as well as other recent articles, should call that dogma into question. Their primary finding: dieters who avoid carbohydrates in favor of fat—even saturated fat—lose more weight and improve their health more than dieters on a low-fat diet . Since its debut, the news has stirred up discussion among medical professionals as well as in the popular press about what type of diet is healthiest .
Current trends in diet optimization
Calories come from three sources: protein, fat and carbohydrates, collectively known as macronutrients. Protein is found primarily in animal products, as well as beans and legumes. Fat is divided into three categories based on chemical structure. From healthiest to least healthy, these are unsaturated, saturated, and trans fats. Vegetable oils and nuts are high in unsaturated fats, and animal products such as meats and cheeses are high in saturated fats. Trans fats are most often found in highly processed foods. (Look for the words “partially hydrogenated oils.”) Grain-based products such as breads and cereals, as well as sweetened and refined foods such as cookies and crackers, are high in carbohydrates.
All three macronutrients play a role in supplying certain essential molecules that cannot otherwise be synthesized by the body itself. A balanced diet should contain energy in the form of calories from each of these three sources. The key question is: “How many calories should we be getting from each group?” The American Heart Association recommends eating nutrient-rich foods, such as fruits and vegetables, and limiting fat . On the other hand, certain diet plans, such as the Atkins Diet, South Beach Diet and others, recommend the restriction of one calorie source in favor of the others to promote weight loss. However, weight loss does not always lead to improvements in health, complicating the overall picture. Nutritional studies evaluate both weight and “metabolic profile,” which includes examining blood sugar, cholesterol and triglyceride levels, with the goal of determining an optimal diet for both weight control and overall health.
Cutting carbs helps with short-term weight loss and increases “good” cholesterol…
One reason for conflicting findings in the field of nutrition, as in other scientific fields, is that differences in the design of a study and the outcomes being tested can strongly affect the conclusions. In the Tulane study, 148 obese men and women without heart disease or diabetes were assigned to one of two groups: one which followed a low-carbohydrate diet and one which followed a low-fat diet. The low-carbohydrate group ate less than 40 g/day of digestible carbohydrate (total carbohydrate minus total fiber) – for reference, a slice of bread contains about 20 g of carbs. The low-fat group ate less than 30% calories from fat and 7% calories from saturated fat (for a 2000 calorie diet, that’s 67 g and 15 g, respectively). Importantly, total calories were not restricted, and both groups received regular dietary counseling during the year-long study.
Figure 1 ~ Dieters in the low-carbohydrate group consumed a higher percentage of calories from fat (40.7% total calories) compared to the low-fat group (29.8% total calories) . They obtained 13.4% of their calories from saturated fat, more than double the 5-6% recommended by the American Heart Association . Data shown are from dietary recalls at 12 months.
Every three months, researchers monitored the effects of the diets by calculating changes in total weight, fat mass and waist circumference. To evaluate the changes in metabolism and heart disease risk, they also measured levels of compounds known to affect the risk of heart disease, including HDL (“good” cholesterol), LDL (“bad” cholesterol), and triglycerides. The researchers found that, while the two diets led to comparable changes in waist circumference, participants in the low-carbohydrate group lost significantly more weight as a percentage of their starting weights and had a significantly greater increase in non-fat (lean) mass at 12 months compared to the low-fat group. Total and LDL cholesterol did not change in either group, but HDL levels and the ratio of HDL to total cholesterol were both increased in the low-carbohydrate group versus the low-fat group, showing a small benefit to heart health associated with a low-carbohydrate versus a low-fat diet.
Figure 2 ~ Cholesterol, an essential molecule in our bodies, is carried in lipoprotein particles, two of which are especially relevant to human health: low-density lipoprotein (LDL or “bad”cholesterol) and high-density lipoprotein (HDL or “good” cholesterol.) Both are made in the liver, but they have opposite effects on cardiovascular health. When the LDL levels in the blood are too high, LDL can accumulate and form plaques in arteries. These plaques increase the risk of heart attack and stroke. HDL lowers the risk for heart disease by removing cholesterol from the arteries and carrying it back to the liver, where it can be broken down.
…but what do these results mean?
Figure 3 ~ Summary table of findings. Both the low-fat and low-carbohydrate group lost weight during the 12-month period, but the low-carbohydrate group also improved multiple parameters of their metabolic profiles. Green arrows indicate an improvement in a specific parameter, such as body weight or triglycerides, at the end of the study, with double arrows indicating a larger change. Dashes indicate no change.
In summary, participants lost weight on both the low-carbohydrate and low-fat diets, but those on the low-carbohydrate diet lost more weight, in addition to larger improvements in certain aspects of their metabolic profiles. Given these results, the researchers concluded that the dangers of fat, including saturated fat, may not be as high as previously thought, and that restricting carbohydrates leads to effective weight loss.
Although the study contained a relatively large sample size (148 participants) and had a high completion rate, it, like other nutrition studies, has a number of limitations. The first of these is the short time frame – subjects were monitored for only 12 months. Weight loss and subsequent regain over time are very common for dieters, and a 12-month time period doesn’t necessarily allow measurement of this common effect . Second, the patients recruited were free of chronic disease, so their results may not apply to other populations, such as to people looking to lose weight after a heart attack or a diabetes diagnosis. Third, patients received intensive dietary counseling (weekly for the first month, then biweekly), a service which, by itself, has been shown to increase weight loss . Many US dieters do not have access to this level of counseling, meaning that these diets may not be as effective in the general population. The study is also limited because the researchers only measured changes in weight and cholesterol levels. To truly determine whether a diet can prevent disease, researchers must monitor participants over a period of many years to determine how their diets influence their rates of diabetes, heart attack, stroke and death. Heart disease and diabetes are very complicated, multifactorial diseases, and it is impossible to say how much a small increase in HDL or decrease in LDL, as seen in this study, will reduce a person’s disease risk. This article raises interesting questions about fat and its role in nutrition, but such studies are very preliminary and should be extended in length and to other populations.
Mary E. Gearing is a graduate student in the Biological and Biomedical Sciences Program at Harvard University.
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