During the first 3 mo of the study, the group showed a small but significant gain in body weight (5.5 ± 0.88%, t = −6.3, df = 17, P < 0.0001; ). There were large differences in weight gain between individual monkeys, with several monkeys gaining no weight during this experimental period, whereas others gained up to 13% of their initial weight in 3 mo. Body fat also increased significantly in the group, from 15.9 ± 3.0 to 18.87 ± 3.4% of total body mass during this time period (z = −3.17, P = 0.002; ). Initial body fat was not correlated with weight (r = −0.19, P = 0.45) or fat gain (r = −0.17, P = 0.51), and there was no difference in weight gain or fat gain between the monkeys who were in the leanest vs. fattest quartile at the beginning of the study (t = 0.33, df = 8, P = 0.75; t =−1.3, df = 8, P = 0.24 for weight and fat gain, respectively). In addition, the age of the monkeys was not correlated with weight gain (r = −0.25, P = 0.33) or fat gain (r = 0.05, P = 0.86), and there were no differences in the amount of weight or fat gained between the youngest and oldest monkeys (t = 0.04, df = 8, P = 0.97; t = −0.11, df = 8, P = 0.92 for weight and fat gain, respectively).
Metabolic parameters across 3 mo of weight gain
Initially, a multivariate regression analysis determined that when food intake, basal metabolic rate, and activity were used as independent variables, body weight gain could not be significantly predicted (R2 = 0.27, F3,12 = 1.50, P = 0.27). However, activity was a better predictor of weight gain (P = 0.07) than food intake (P = 0.73) and basal metabolic rate (P = 0.87). Food intake was not significantly changed during the 3-mo period (t = 1.04, df = 17, P = 0.31; ). Although there were considerable individual differences in the amount of calories consumed by individual animals (a 5-fold difference, ranging from 411 to 2,210 kcal/day), the food intake of individual monkeys was consistent over this 3-mo time period, such that the initial food intake of each individual was highly correlated with food intake after 3 mo (r = 0.95, P < 0.0001; ). However, food intake did not correlate with the amount of weight gained by each monkey during this time period (r = 0.15, P = 0.56). In addition, there was no difference in the weight gain of the quartile of monkeys that ate the most compared with that of the quartile of monkeys that ate the least (t = −0.20, df = 8, P = 0.85; ). To begin to look at whether food intake accurately predicts calorie absorption, we measured the caloric content of the stool in the two monkeys eating the most and the two eating the least calories. Mean caloric content of the four stool samples was 1.70 ± 0.28 kcal/g, and the caloric content of the stool samples was correlated with food intake such that the monkeys that ate the most excreted more calories per gram of stool (r = 0.95, P = 0.046, 2.1-fold difference between monkeys with the highest and lowest food intake).
Fig. 1 A: food intake for individual monkeys remained stable. There was a correlation between food intake at study initiation and after 3 mo (r = 0.95, P < 0.0001). B: however, the quartile of monkeys eating the most food showed the same percent change (more ...)
Daily energy expenditure significantly increased from 21.3 ± 2.3 to 28.7 ± 2.9 kcal/h (t = −3.46, df = 17, P = 0.003; ) over the first 3 mo of the experimental period. Initial daily energy expenditure was correlated with final daily energy expenditure (r = 0.68, P = 0.002; ). Moreover, the change in daily energy expenditure correlated with weight gain (r = 0.47, P = 0.05) such that the monkeys that gained the most weight increased their daily energy expenditure the most. There was a sixfold difference in daily energy expenditure between individual monkeys. However, initial daily energy expenditure did not correlate with weight gain (r = −0.35, P = 0.16), and although percent weight gain was somewhat higher in the quartile of monkeys with the lowest daily energy expenditure (7.4% weight gain) compared with the quartile of monkeys with the highest daily energy expenditure (4.0% weight gain), this was not a significant difference (t = 2.08, df = 8, P = 0.07; ). Once total energy expenditure was adjusted for lean body mass by regression analysis, there was only a 2.6-fold difference in energy expenditure between individual monkeys. However, adjusted daily energy expenditure did not correlate with weight gain (r = −0.04, P = 0.89), and there was no difference in weight gain between monkeys in the quartile with the highest adjusted daily energy expenditure compared with the quartile of monkeys with the lowest adjusted daily energy expenditure (t = 0.04, df = 6.1, P = 0.97).
Fig. 2 A: correlation between daily energy expenditure at study initiation and after 3 mo (r = 0.68, P = 0.002). B: the change in body weight over 3 mo between the monkeys in the top and bottom quartiles of energy expenditure was not significantly different (more ...)
The average basal metabolic rate was 291 ± 19 kcal/day and ranged from 172 to 406 kcal/day. On average, basal metabolic rate accounted for 61% of total daily energy expenditure, ranging from 47 to 83% of total energy expenditure in individual monkeys. Basal metabolic rate did not correlate with weight gain (r = 0.08, P = 0.75), and weight gain was not different between the quartile of monkeys with the highest basal metabolic rate and the quartile of monkeys with the lowest basal metabolic rate (t = 0.40, df = 8, P = 0.70). In addition, basal metabolic rate adjusted for lean body mass with the use of regression analysis was not correlated with weight gain (r = −0.04, P = 0.89), and there was no difference in weight gain between the quartile that had the highest adjusted basal metabolic rate and the quartile with the lowest adjusted basal metabolic rate (t = 0.04, df = 6.1, P = 0.97).
The mean thermic effect of a 108-calorie meal was 19.9 ± 3.2 kcal and ranged from 8.5 to 59.3 kcal (a 7-fold difference) between individual monkeys. There was no significant difference in the weight gain in the monkeys with the highest thermic effect of the meal and the monkeys with the lowest thermic effect of the meal (t = −1.81, df = 8, P = 0.11).
There was an eightfold difference in activity between the most active and most sedentary monkey (), with the most sedentary monkey displaying a mean of 92,110 ± 7,873 activity counts per day and the most active monkey displaying 770,446 ± 110,476 activity counts per day. The number of activity counts per day did not change significantly during the 3-mo period (t = 1.15, df = 15, P = 0.27; ), and each monkey's daily activity level (counts/day) was consistent over time such that the number of activity counts per day initially recorded for each monkey was highly correlated with the number of activity counts per day recorded after 3 mo (r = 0.79, P < 0.0001; ). There was a significant correlation between the number of daily activity counts and weight gain such that the most active monkeys gained less weight than the least active monkeys (r = −0.52, P = 0.04). The quartile of monkeys that were most active gained significantly less weight during the 3-mo period than the quartile of monkeys that were least active (t = −2.7, df = 8, P = 0.03; ). To follow up this initial finding, we measured activity over an additional 6 mo and found that the number of activity counts per day remained stable (F1,16 = 1.13, P = 0.30) and that the number of activity counts initially recorded for each monkey was highly correlated with the number of activity counts recorded for that monkey after 9 mo (r = 0.85, P < 0.0001). There was a 10-fold difference in the number of activity counts recorded during the day between individual monkeys, and 96% of total daily activity occurred during daylight hours. Interestingly, although nighttime activity accounted for only 4% of total daily activity, there also was a 10-fold difference in nighttime activity. Nighttime activity was positively correlated with daytime activity such that the monkeys that were the most active during the day were also the most active at night (r = 0.57, P = 0.02; ).
The most sedentary monkey (A) was 8 times less active than the most active monkey (B).
Fig. 4 A: there was a significant correlation between physical activity at study initiation and after 3 mo (r = 0.79, P < 0.0001). B: the quartile of monkeys that had the lowest physical activity had significantly greater weight gain than the quartile (more ...)
Nighttime activity was significantly correlated with daytime activity (r = 0.57, P = 0.02).
Activity counts correlated strongly with activity-associated energy expenditure (adjusted for body mass) during the time periods from both 0200 to 0300 (r = 0.80, P < 0.0001; ) and 0600 to 0700 (r = 0.74, P = 0.001; data not shown). The regression equation for calculation of activity-associated energy expenditure (AEE) was similar at both times of day [0200–0300: AEE = (number of activity counts × 0.000025) + 0.71; 0600–0700: AEE = (number of activity counts × 0.000021) + 0.54]. On average, 0.045 ± 0.006 kcal were expended per kilogram of body weight per 1,000 activity counts. Average activity-associated energy expenditure was 109 ± 14 kcal/day and ranged from 24 to 206 kcal/day. On average, 18 ± 3% of total energy was expended by physical activity, with physical activity accounting for 8–43% of total daily energy expenditure in individual monkeys. The quartile of monkeys that expended the most calories due to activity gained significantly less weight than the quartile of monkeys that expended the least amount of energy due to activity (t = −2.85, df = 4.6, P = 0.04).
Correlation between activity counts and activity-associated energy expenditure measured simultaneously from 0200 to 0300 (r = 0.80, P < 0.0001).
Further analysis of the activity data revealed that there was an inverse correlation between the number of daily activity counts and the number of minutes that the monkeys were completely inactive such that the least active monkeys were inactive more than the most active monkeys (r = −0.51, P = 0.046). However, there was no correlation between the number of minutes that the monkeys were inactive and weight gain (r = 0.39, P = 0.13).