Obesity rates have been increasing in the USA and around the world for decades (1). As obesity rates climb, there has been a long-running debate about the possible role of the ratio of fat, protein, and carbohydrate [FAT/PRO/CHO] might be playing in promoting excessive calorie intake and promoting obesity. While a few fad diet book authors have even suggested that calories don’t count, there has long been a consensus that no matter what the ratio of FAT/PRO/CHO in the diet, weight gain only occurs when total calorie intake exceeds energy expenditure. There have been many popular diet books proclaiming that either dietary fat or dietary carbohydrate is the primary dietary factor causing people to consume more energy and gain more weight, and some have even suggested that carbohydrate calories promote weight gain even if calorie intake is controlled for. The claim that only fat or carbohydrate calories promote excessive calorie intake and obesity lacks credible scientific evidence, so if the ratio of FAT/PRO/CHO impacts body weight, then it must be doing so by somehow leading people to consume more or fewer calories. Certainly, most doctors and clinical nutritionists have seen people lose weight on diets that severely limit either fat or carbohydrate intake. This no doubt could occur because such diets lead to a reduced calorie intake. Could it be that for some people the ratio of FAT/PRO/CHO impacts satiety such that some people will become less hungry and spontaneously consume fewer calories on a diet in which fat or carbohydrate calories are limited?
Does FAT/PRO/CHO Impact Satiety/Kcal?
Let’s explore the results of the DIETFITS trial, which was a 12-month weight loss diet study that examined the impact of changing the dietary FAT/CHO ratio. After one year there was no significant difference in weight change between a healthy low-fat diet vs a healthy low-carbohydrate diet. Researchers also looked at the genotype of their subjects and found that neither a genotype pattern suggesting a tendency toward insulin resistance nor baseline insulin secretion levels were associated with the dietary effects of a higher or lower fat or carbohydrate intake with body weight loss. Dr. Gardner concluded: “In the context of these 2 common weight loss diet approaches, neither of the 2 hypothesized predisposing factors was helpful in identifying which diet was better for whom" (2).
Dietary sugar has been blamed for years as a main cause of obesity and factor promoting type 2 diabetes mellitus (DM). However, a recent review of current scienti?c evidence does not support the conclusion that dietary sugars per se are detrimental to human health. This does not mean that a diet high in refined sugar and especially sugar-sweetened drinks is not an important factor in promoting obesity and type 2 DM. However, cutting out whole fruits, which get about 90% of their calories from sugar, won’t aid weight control or promote type 2 DM (3).
The 2-year Pounds Lost study by Frank Sacks et. al. at Harvard University looked at the impact of cutting calories from fat, carbohydrate, or protein. Researchers randomly assigned 811 overweight adults to one of four diets; the targeted percentages of energy derived from fat, protein, and carbohydrates in the four diets were 20, 15, and 65%; 20, 25, and 55%; 40, 15, and 45%; and 40, 25, and 35%. The diets consisted of similar foods and met guidelines for cardiovascular health. The participants were offered group and individual instructional sessions for 2 years. The primary outcome was the change in body weight after 2 years in two-by-two factorial comparisons of low-fat versus high-fat and average-protein versus high-protein and in the comparison of highest and lowest carbohydrate content. After 6 months, study participants were assigned to each diet had lost an average of about 6 kg, which represented 7% of their initial weight. They then began to regain weight after 12 months. By 2 years, weight loss remained similar in those who were assigned to a diet with 15% protein and those assigned to a diet with 25% protein (3.0 and 3.6 kg, respectively); in those assigned to a diet with 20% fat and those assigned to a diet with 40% fat (3.3 kg for both groups); and in those assigned to a diet with 65% carbohydrates and those assigned to a diet with 35% carbohydrates (2.9 and 3.4 kg, respectively) (P>0.20 for all comparisons). Among the 80% of participants who completed the trial, the average weight loss was 4 kg; 14 to 15% of the participants had a reduction of at least 10% of their initial body weight. Satiety, hunger, satisfaction with the diet, and attendance at group sessions were similar for all diets; attendance was strongly associated with weight loss (0.2 kg per session attended). The diets improved lipid-related risk factors and fasting insulin levels. Dr Sacks et. al. concluded: “The composition of fat, carbohydrate, and protein in a healthy diet had no effect whatsoever on weight loss or regain. Nor did it affect hunger" (4).
There is no doubt that high-protein diets increase weight loss during energy restriction. Some have suggested that additional protein intake may improve weight maintenance after weight is lost. We investigated the effect of protein supplements from either whey with or without calcium or soy on weight loss maintenance success after weight loss compared with that of a control group. In a randomized, controlled, double-blinded trial, 220 participants aged 18-60 with body mass index from 27.6 to 40.4 were included. The study was initiated with an 8 week weight loss period followed by a 24 week weight maintenance period. During weight maintenance, participants consumed the following isocaloric supplements (45-48 g/d): whey and calcium (whey+), whey, soy, or maltodextrin (control). Data were collected at baseline, before and after the weight maintenance period (weeks 0, 8, and 32, respectively). Measurements included body composition, blood biochemistry, and blood pressure. Meal tests were performed to investigate diet-induced-thermogenesis and hunger levels. Compliance was tested using 24-hour urinary nitrogen excretion. The results for 151 participants who completed the weight maintenance period showed the control and 3 protein supplemented groups did not show any significant differences in body weight regained or fat mass regained, or any improvements in lean body mass during the weight maintenance phase. Changes in blood pressure and blood biochemistry were also not different between groups. Compared with the control, protein supplementation did result in somewhat higher diet-induced thermogenesis resting energy expenditure and a small reduction in appetite-sensation profile. However, protein supplementation did not result in improved weight maintenance success, or improved blood biochemistry after weight loss compared with the effects of normal dietary protein intake (5).
Bottom Line: The ratio of FAT/PRO/CHO independent of other dietary factors or calorie intake appears to have very little or no impact on the risk of developing obesity and/or successful weight loss and control. The ratio of FAT/PRO/CHO has little or no effect on satiety/kcal or on body weight or on body fat stores or on lean body mass.
This is not to say that diet composition cannot impact satiety per calorie. There are dietary factors that we know do increase or decrease satiety/kcal and likely are contributing to the obesity epidemic seen in the USA and increasingly worldwide. Dietary factors such as increased calorie density and/or beverage calories, and/or reducing the fiber/Kcal ratio all do appear to impact ad libitum calorie intake (or satiety/Kcal) and over time do lead to increased body weight and increased body fat stores. To lose weight and keep it off without being chronically hungry, one needs to focus on dietary factors such as these that likely do impact satiety/kcal rather than simply look for some magic ratio of FAT/PRO/CHO. It certainly now appears time to put to rest the mythology about the pros and cons of low-fat, high-protein, low-CHO and ketogenic diets and other dietary strategies that focus primarily of the ratio of FAT/PRO/CHO. If the goal is weight loss without chronic hunger and long-term weight control, then the focus should be on reducing beverage calories, reducing calorie density, and increasing dietary fiber intake rather than continuing the futile search for some magical ratio of FAT/PRO/CHO. Past studies that have suggested the ratio of FAT/PRO/CHO impacts body weight or composition have invariably failed to control for % beverage calories, calorie density, and/or fiber/Kcal. Demonizing sugar or fat per se without looking at other dietary factors makes no sense.
By James J. Kenney, PhD, FACN
- Zylke JW, Bauchber H. The unrelenting challenge of obesity. The unrelenting challenge of obesity. JAMA 2016;315:2277-8.
- Prinz P. The role of dietary sugars in health: molecular composition or just calories. Euro J Clin Nutr. 2019 - https://doi.org/10.1038/s41430-019-0407-z.
- Frank M. Sacks, M.D., George A. Bray, M.D., Vincent J. Carey, et.al. Comparison of Weight-Loss Diets with Different Compositions of Fat, Protein, and Carbohydrates. N Engl J Med 2009;360:859-73.
- Kjolbaek L, Sorensen LB, Sondertoft NB, et. al. Protein supplements after weight loss do not improve weight maintenance compared with recommended dietary protein intake despite beneficial effects on appetite sensation and energy expenditure: a randomized, controlled, double-blinded trial. Am J Clin Nutr.2017 Aug;106(2):684-697. doi: 10.3945/ajcn.115.129528.
Stephanie Ronco has been editing for Food and Health Communications since 2011. She graduated from Colorado College magna cum laude with distinction in Comparative Literature. She was elected a member of Phi Beta Kappa in 2008.