In a new and exciting study published in the Journal of the American Medicine Association, researchers explored whether the human body responds better to a low-fat diet or a low-carbohydrate diet. This is a question hotly debated among fitness and nutrition enthusiasts as well as everyday people hoping to achieve or maintain a healthy weight. In an attempt to answer this question, investigators recruited overweight adults and observed them for one year while the study participants followed either a low-fat or low-carbohydrate diet.
The authors of this study reference previous research that suggests gene variation, or unique characteristics of a person’s DNA, plays a role in whether a person will succeed when following either a low-fat or a low-carbohydrate diet. In an attempt to provide further clarity to this theory, the investigators studied the gene profiles of each participant in order to observe a pattern of genetic predisposition to either diet. Genotype analysis among the study participants showed that 40 percent had a low-fat genotype, 30 percent had a low-carbohydrate genotype, and the remaining 30 percent had a “neutral” genotype, meaning their genotype pattern had not been shown in any previous studies to predispose them to any particular diet. Since the trial was randomized, the participants weren’t assigned a diet based on genotype. In the low-fat diet group, 42.6 percent of participants had the low-fat genotype and 27.2 percent had the low-carbohydrate genotype; in the healthy low-carbohydrate group, 37.5 percent had the low-fat genotype and 31.9 percent had the low- carbohydrate genotype.
The randomized, clinical trial recruited 609 adults, all with a body mass index (BMI) between 28 and 40 (a BMI of 28 is considered overweight and 40 is considered extremely obese). Researchers assigned the diets randomly; 305 participants were told to follow a healthy low-fat diet, and 304 were told to follow a healthy low-carbohydrate diet. To aid the groups in following their assigned diets, the participants attended 22 diet-specific intervention classes taught by registered dietitians over the course of the 12-month study.
At the start of the study, the participants were instructed to reduce their normal daily
fat or carbohydrate intake by 20 grams (g). So, for example, someone in the low-fat group
who normally consumes 50g of fat per day would decrease their intake to 30g. Then, the participants were instructed to increase their intake of carbohydrates or fats by 5 to 15 grams per day each week until they reached a level that they believed they could consume indefinitely. This particular instruction is important to note: the participants were encouraged to eat a reduced fat or carbohydrate diet, but they were not told to eliminate these nutrients completely or to consume an extremely low amount (such as on the currently popular ketogenic diet, or the Atkin’s diet of past popularity). Further, they were told to find a diet that they believed they could maintain indefinitely, going against the idea that a diet is an extreme, unsustainable ordeal that we endure only until we reach our goal weight.
And for those of you out there who can’t stand the thought of counting calories, this study gets even more interesting. The researchers did not, at any time, instruct the participants to restrict their calories. However, the participants in both groups, in addition to their group-specific guidelines, followed three rules: “1) maximize vegetable intake; 2) minimize intake of added sugars (sugar found in food naturally, such as the sugar in fruit, was not off-limits), refined flours, and trans fats; and 3) focus on whole foods that were minimally processed, nutrient-dense, and prepared at home whenever possible.” In terms of physical activity, both groups were told to follow the federal recommendations outlined in the 2008 Physical Activity Guidelines for Americans (these guidelines were updated in 2015, and can be accessed at https://health.gov/dietaryguidelines/2015/guidelines/ appendix-1/).
Results. Nutrition changes. The study participants were measured on the third, sixth, and final month of the year-long study. Despite not being instructed to intentionally decrease their calorie intake over the course of the year, results show that calorie intake among both groups dropped from around 2,200 calories at baseline to about 1,500 to 1,700 calories throughout the study. The participants also showed notable changes to their nutrient intake over the study period. At baseline, both groups were consuming roughly 29g of saturated fat per day. Participants in the low-fat group decreased their saturated fat intake by roughly half, ending the year with an average daily consumption of 18.2g. The low-carbohydrate group decreased their saturated fat consumption to 28g of daily saturated fat. For reference, the American Heart Association (AHA) recommends a daily saturated fat intake of 5 to 6 percent of your total calorie intake. On a 2,000 calorie per day diet, this would be roughly 13g of saturated fat.3 The low-carbohydrate group showed a greater decrease in consumption of added sugars than the low-fat group. Both groups started the study consuming about 49 to 52 grams of added sugar per day. At the end of the study, the low-fat group was consuming an average of 33.1g a day, while the low-carbohydrate group was consuming 22.8g per day. The AHA recommends limiting daily consumption of added sugars to 25g for women and 36g for men.4 The key word here is added sugars; sugar in fresh fruit and unsweetened dairy products (e.g., plain yogurt) does not count as added sugar.
Biometric changes. In addition to measuring weight loss among the groups, the investigators analyzed a full biometric profile for each participant. The good news: both groups lost roughly the same amount of weight. The healthy low-fat diet resulted in an average 11-pound weight loss among participants, and the healthy low-carbohydrate group enjoyed an average weight loss of 13 pounds. Results revealed that both diets improved lipid profiles and lowered blood pressure, insulin, and glucose levels. One of the only notable differences between the groups that the investigators pointed out was the change in low-density lipoprotein (LDL) cholesterol, which is generally regarded as the undesirable cholesterol that increases risk for heart disease and stroke.7 The low-fat group saw a decrease in LDL cholesterol, while the low-carbohydrate group experienced an increase in LDL cholesterol. This makes sense when considering the greater consumption of saturated fat shown among the low-carbohydrate group; saturated fat is known to increase LDL, and limiting saturated fat is recommended as a method of lowering cholesterol.5
Genotype patterns. The investigators in this study observed no significant difference in weight change among participants who were matched with their hypothetically “ideal” diet based on their genotype pattern versus those who were mismatched.
Conclusion. In this year-long study, the authors observed no significant difference in weight change between a healthy low-fat diet versus a healthy low-carbohydrate diet. In addition, they observed no notable relationship between genotype patterns and a low-fat or low-carbohydrate diet. Both groups experienced a weight loss of at least five percent of their body weight by eating a diet of nutrient-dense, minimally processed whole foods that were low in added sugars, trans fats, and refined flours. Studies such as this help to take some of the guesswork out of losing weight and living a healthy, balanced life. The results of this study suggest there isn’t one perfect way to lose weight, that the same amount of weight can be lost on a healthy low-carbohydrate diet or a healthy- low fat diet. Even better, both of these diets can support your health in ways that go beyond the number on the scale. If you’re interested in losing weight by following one of these diets, be sure to first consult your doctor if you have any special dietary needs or health concerns.
1. Gardner CD, Trepanowski JF, Del Gobbo LC et al. Effect
of low-fat versus low-carbohydrate diet on 12-month weight loss in overweight adults and the association with genotype pattern or insulin secretion: the DIETFITS randomized clinical trial. JAMA. 2018 Feb 20;319(7):667– 679.
2. Qi Q, Bray GA, Smith SR et al. Insulin receptor substrate
1 gene variation modifies insulin resistance response
to weight-loss diets in a 2-year randomized trial: the Preventing Overweight Using Novel Dietary Strategies (POUNDS LOST) trial. Circulation. 2011 Aug 2;124(5):563– 71.
3. American Heart Association. Saturated Fat. 2017 Mar. https://healthyforgood.heart.org/eat-smart/articles/ saturated-fats. Accessed April 2018.
4. American Heart Association. Added Sugars. 2017 Feb. http://www.heart.org/HEARTORG/HealthyLiving/ HealthyEating/Nutrition/Added-Sugars_UCM_305858_ Article.jsp#.WsuSAbbMyRc. Accessed April 2018.
5. Mayo Clinic. Top 5 lifestyle changes to improve your cholesterol. 2015 Jan. https://www.mayoclinic.org/ diseases-conditions/high-blood-cholesterol/in-depth/ reduce-cholesterol/art-20045935. Accessed April 2018.