Effects of Low-Carb Diet on Metabolic Rate

Diets have a tendency to slow down your metabolism. When your metabolism slows down, you burn less calories, and you usually feel tired and run-down. These studies analyze the effect of low-carb dieting on the body's metabolic rate.

The effect of varying carbohydrate content of a very-low-caloric diet on resting metabolic rate and thyroid hormones.

Mathieson RA, Walberg JL, Gwazdauskas FC, Hinkle DE, Gregg JM
Metabolism 1986 May;35(5):394-398

Twelve obese women were studied to determine the effects of the combination of an aerobic exercise program with either a high carbohydrate (HC) very-low-caloric diet (VLCD) or a low carbohydrate (LC) VLCD diet on resting metabolic rate (RMR), serum thyroxine (T4), 3,5,3'-triiodothyronine (T3), and 3,5,3'-triiodothyronine (rT3). The response of these parameters was also examined when subjects switched from the VLCD to a mixed hypocaloric diet. Following a maintenance period, subjects consumed one of the two VLCDs for 28 days. In addition, all subjects participated in thrice weekly submaximal exercise sessions at 60% of maximal aerobic capacity. Following VLCD treatments, participants consumed a 1,000 kcal mixed diet while continuing the exercise program for one week. Measurements of RMR, T4, T3, and rT3 were made weekly. Weight decreased significantly more for LC than HC. Serum T4 was not significantly affected during the VLCD. Although serum T3 decreased during the VLCD for both groups, the decrease occurred faster and to a greater magnitude in LC (34.6% mean decrease) than HC (17.9% mean decrease). Serum rT3 increased similarly for each treatment by the first week of the VLCD. Serum T3 and rT3 of both groups returned to baseline concentrations following one week of the 1,000 kcal diet. Both groups exhibited similar progressive decreases in RMR during treatment (12.4% for LC and 20.8% for HC), but values were not significantly lower than baseline until week 3 of the VLCD. Thus, although dietary carbohydrate content had an influence on the magnitude of fall in serum T3, RMR declined similarly for both dietary treatments.

Comments: they split 12 women into two groups of 6. Half were given a 1000-calorie low-carb diet, the other half a 1000-calorie high-carb diet. The metabolic rate of the low-carbers declined by 12.4%, the metabolic rate of the high-carbers declined by 20.8%.

The effect of fat composition of the diet on energy metabolism.

van Marken Lichtenbelt WD, Mensink RP, Westerterp KR. Department of Human Biology University of Limburg, Maastricht, The Netherlands. Medline record in process.
Ernahrungswiss 1997 Dec;36(4):303-305

Animal and human studies show that polyunsaturated fatty acids (P) are oxidized more rapidly than saturated fatty acids (S). There are indications that diets high in P/S ratio result in a relatively high resting metabolic rate (RMR) and high diet induced thermogenesis (DIT). However, studies with human subjects are limited. The effect of dietary fatty acid composition on energy metabolism was studied in 6 male subjects, age 25-48 y. Two diets were supplied, each over a period of 14 days, in a randomized crossover design with a washout period of 14 days. P/S ratios of the diets were 0.19 and 1.67. On day 14, RMR was determined in the morning in fasting state by means of indirect calorimetry (ventilated hood), followed by a 4 hour measurement of the DIT after consumption of a standardized meal of 3.3 MJ with the same fatty acid composition as during the dietary period. The meal contained 46, 37, and 17 % energy as fat, carbohydrate, and protein, respectively. RMR after the period with the high P/S diet was significantly higher than after the period of the low P/S diet. The average difference (+/- SD) was 0.17 +/- 0.14 kJ/min or 3.6 +/- 2.7 % of RMR. The DIT was also higher in all subjects during a breakfast with a high P/S ratio. The average difference was 0.29 +/- 0.16 kJ/min, which is 22.1 +/- 12.6 % of DIT. The study showed that a prolonged food intake of a diet with a high P/S ratio results in a relatively high RMR and DIT. These results indicate the importance of dietary lipid profile in the treatment of obesity.

Comments: When you're sitting on the sofa, your body is using a small amount of energy. Of course, it's burning fat to get that energy. The rate at which it's burning fat is called your "resting metabolic rate." These guys gave a group of 6 men a diet high in saturated fat, then measured their resting metabolic rates. Then, they switched them to unsaturated fat, and measured their resting metabolic rates again. They discovered this: when they ate unsaturated fat, their resting metabolic rate was higher. In other words, they were burning fat faster when they were eating unsaturated fat instead of saturated fat. This probably also meant their energy levels were higher on unsaturated fat.

Effects of aerobic exercise and dietary carbohydrate on energy expenditure and body composition during weight reduction in obese women.

Racette SB, Schoeller DA, Kushner RF, Neil KM, Herling-Iaffaldano K. Department of Medicine, University of Chicago, IL 60637.
Am J Clin Nutr 1995 Mar;61(3):486-494

To test the benefits of aerobic exercise and dietary carbohydrate during reduced-energy feeding, 23 obese women (44 +/- 4% fat) were randomly assigned to either aerobic exercise (Ex) or no exercise (Nx), and to a low-fat (LF) or low-carbohydrate (LC) reducing diet (5.00 +/- 0.56 MJ/d) for 12 wk. Changes in body composition, postabsorptive resting metabolic rate (RMR), thermic effect of a meal (TEM), and total daily energy expenditure (TDEE) were measured by respiratory gas exchange and doubly labeled water. Significant effects of Ex included a greater loss of fat mass (Ex: -8.8 +/- 2.1 vs Nx: -6.1 +/- 2.3 kg, P = 0.008) and maintenance of TDEE (Ex: +0.07 +/- 1.23 vs Nx: -1.46 +/- 1.04 MJ/d, P = 0.004), due to a difference in physical activity (Ex: +0.75 +/- 1.06 vs Nx: -0.61 +/- 1.03 MJ/d, P = 0.006), which was not attributable solely to the Ex sessions. RMR in both groups decreased comparably (-0.54 MJ/d), and TEM (% of meal) did not change. Diet composition did not significantly influence body composition or energy expenditure changes, but a greater weight loss was observed after the LC than after the LF (-10.6 +/- 2.0 vs -8.1 +/- 3.0 kg, P = 0.037) diet. The addition of aerobic exercise to a low-energy diet was beneficial in the treatment of moderate obesity because of its favorable effects on body composition, physical activity, and TDEE.

Comments: They got 23 women, and split them into two groups: a low-carb group, and a high-carb group. In both groups, metabolism slowed down by the same amount. In both groups, exercise helped raise metabolism back to where it started.