Improved Equations for Estimating the Resting Metabolic Rate

New models were developed for predicting the resting metabolic rate (RMR) with sufficient accuracy for use in epidemiologic studies and for weight control of individuals. For this purpose, the RMR of 213 women and 76 men was measured, and physical measurements were taken. The RMR was regressed on correlates of RMR, avoiding harmful degrees of collinearity by rejecting interregressor correlations exceeding r=0.5. For women, the best model (R₂=0.71) included the regressors age, race, weight, pulse rate, smoking, and body temperature. The best model for males (R2 = 0.81) included age, race, weight, blood pressure, smoking, time (of day the RMR was measured), and whether subjects had a meal prior to calorimetry. The models were cross validated internally and also validated using an external database. In both cases, the mean estimated RMR did not differ significantly from the measured RMR. The accuracy of the models was compared with four models reported in the literature, three of which overestimated the RMR by up to 17%. In conclusion, improved RMR prediction models have been developed, more accurate than existing models, rendering them suitable for application to epidemiological databases and for individual weight control programs.     

Gene or Size: Metabolic Rate and Body Temperature in Obese Growth Hormone‐Deficient Dwarf Mice

Objective: SMA1 mice carry a missense mutation in the growth hormone gene that leads to semidominant dwarfism and obesity. In this study, the basic thermal and metabolic properties of SMA1 mice were examined to detect metabolic alterations that can support the accretion of excess fat. Research Methods and Procedures: Basal and resting metabolic rates (RMRs) in wild‐type and SMA1 (sma1/+ and sma1/sma1) mice were determined by indirect calorimetry. Body temperature (T_b) was recorded using intraperitoneally implanted temperature‐sensitive transmitters, and body composition was determined by DXA. Results: SMA1 mice have proportionally lower basal and resting metabolic rates, higher body mass (BM)‐specific RMRs, and a higher lower critical temperature, and display a decrease in T_b by 0.4 °C in sma1/+ and 0.9 °C in sma1/sma1. Discussion: The analysis of gene effects on BM and energy expenditure in mouse mutants must consider the appropriate allometric relationship between BM and metabolic rate. With the exception of T_b, all metabolic alterations observed in SMA1 reflect reduced size.     

Association of African Genetic Admixture with Resting Metabolic Rate and Obesity Among Women

Objective: To investigate the role of genetic admixture in explaining phenotypic variation in obesity‐related traits in a sample of African‐American women (n = 145) and to determine significant associations between obesity traits and admixture genetic markers. Research Methods and Procedures: Associations between genetic admixture and BMI, resting metabolic rate, fat mass, fat‐free mass, and bone mineral density were tested using linear regression considering the estimation of admixture by 1) a maximum‐likelihood approach (MLA) and 2) a Bayesian analysis. Results: Both the conservative MLA and the Bayesian approach support an association between African genetic admixture and BMI. Evidence for the associations of African genetic admixture with fat mass and fat‐free mass was supported by the Bayesian analysis; the MLA supported an association with bone mineral density. When the individual ancestry informative markers that were used to estimate admixture were tested for associations with BMI, significant associations were identified in chromosomes 1, 11, and 12. Discussion: These results provide evidence supporting the application of admixture mapping methods to the identification of genes that result in higher levels of obesity among African‐American women. Further research is needed to replicate and further explore these findings.     

Simplified Resting Metabolic Rate—Predicting Formulas for Normal‐Sized and Obese Individuals

Objective: Resting metabolic rate (RMR) is known to be proportional to body weight and to follow allometric scaling principles. We hypothesized that RMR can be predicted from an allometric formula with weight alone as an independent variable. Research Methods and Procedures: An allometric, power‐law scaling model was fit to RMR measurements obtained from a cohort of patients being treated for weight loss. This, as well as many of the commonly used RMR‐predicting formulas, was tested for RMR prediction ability against a large publicly available RMR database. Bland‐Altman analysis was used to determine the efficacy of the various RMR‐predicting formulas in obese and non‐obese subjects. Results: Power law modeling of the RMR—body weight relationship yielded the following RMR‐predicting equations: RMR_Women = 248 × Weight^0.4356 ? (5.09 × Age) and RMR_Men = 293 × Weight^0.4330 ? (5.92 × Age). Partial correlation analysis revealed that age significantly contributed to RMR variance and was necessary to include in RMR prediction formulas. The James, allometric, and Harris‐Benedict formulas all yielded reasonable RMR predictions for normal sized and obese subjects. Discussion: A simple power formula relating RMR to body weight can be a reasonable RMR estimator for normal‐sized and obese individuals but still requires an age term and separate formulas for men and women for the best possible RMR estimates. The apparent performance of RMR‐predicting formulas is highly dependent on the methodology employed to compare the various formulas.     

Myotubes from Severely Obese Type 2 Diabetic Subjects Accumulate Less Lipids and Show Higher Lipolytic Rate than Myotubes from Severely Obese Non-Diabetic Subjects

About 80% of patients with type 2 diabetes are classified as overweight. However, only about 1/3 of severely obese subjects have type 2 diabetes. This indicates that several severely obese individuals may possess certain characteristics that protect them against type 2 diabetes. We therefore hypothesized that this apparent paradox could be related to fundamental differences in skeletal muscle lipid handling. Energy metabolism and metabolic flexibility were examined in human myotubes derived from severely obese subjects without (BMI 44±7 kg/m²) and with type 2 diabetes (BMI 43±6 kg/m²). Lower insulin sensitivity was observed in myotubes from severely obese subjects with type 2 diabetes. Lipolysis rate was higher, and oleic acid accumulation, triacylglycerol content, and fatty acid adaptability were lower in myotubes from severely obese subjects with type 2 diabetes compared to severely obese non-diabetic subjects. There were no differences in lipid distribution and mRNA and protein expression of the lipases HSL and ATGL, the lipase cofactor CGI-58, or the lipid droplet proteins PLIN2 and PLIN3. Glucose and oleic acid oxidation were also similar in cells from the two groups. In conclusion, myotubes established from severely obese donors with established type 2 diabetes had lower ability for lipid accumulation and higher lipolysis rate than myotubes from severely obese donors without diabetes. This indicates that a difference in intramyocellular lipid turnover might be fundamental in evolving type 2 diabetes.     

Metabolic Syndrome in Overweight and Obese Japanese Children

Objective: To determine the prevalence of and sex differences related to the metabolic syndrome among obese and overweight elementary school children. Research Methods and Procedures: Subjects were 471 overweight or obese Japanese children. Children meeting at least three of the following five criteria qualified as having the metabolic syndrome: abdominal obesity, elevated blood pressure, low high‐density lipoprotein‐cholesterol levels, high triglyceride levels, and high fasting glucose levels. Fasting insulin levels were also examined. Results: Japanese obese children were found to have a significantly lower prevalence (17.7%) of the metabolic syndrome than U.S. obese adolescents (28.7%, p = 0.0014). However, Japanese overweight children had a similar incidence (8.7%) of the metabolic syndrome compared with U.S. overweight adolescents (6.8%). Hyperinsulinemia in girls and abdominal obesity in boys are characteristic features of individual metabolic syndrome factors in Japanese children. Discussion: The prevalence of the metabolic syndrome is not lower in preteen Japanese overweight children than in U.S. overweight adolescents, although it is significantly lower in Japanese obese preteen children than in U.S. obese adolescents. Primary and secondary interventions are needed for overweight preteen children in Japan.     

Low Plasma Leptin Concentration and Low Rates of Fat Oxidation in Weight‐Stable Post‐Obese Subjects

Objective: A low resting metabolic rate for a given body size and composition, a low rate of fat oxidation, low levels of physical activity, and low plasma leptin concentrations are all risk factors for body weight gain. The aim of the present investigation was to compare resting metabolic rate (RMR), respiratory quotient (RQ), levels of physical activity, and plasma leptin concentrations in eight post‐obese adults (2 males and 6 females; 48.9 ± 12.2 years; body mass index [BMI]: 24.5 ± 1.0 kg/m²; body fat 33 ± 5%; mean ± SD) who lost 27.1 ± 21.3 kg (16 to 79 kg) and had maintained this weight loss for ≥2 months (2 to 9 months) to eight age‐ and BMI‐matched control never‐obese subjects (1 male and 7 females; 49.1 ± 5.2 years; BMI 24.4 ± 1.0 kg/m²; body fat 33 ± 7%). Research Methods and Procedures: Following 3 days of weight maintenance diet (50% carbohydrate and 30% fat), RMR and RQ were measured after a 10‐hour fast using indirect calorimetry and plasma leptin concentrations were measured using radioimmunoassay. Levels of physical activity were estimated using an accelerometer over a 48‐hour period in free living conditions. Results: After adjustment for fat mass and fat‐free mass, post‐obese subjects had, compared with controls, similar levels of physical activity (4185 ± 205 vs. 4295 ± 204 counts) and similar RMR (1383 ± 268 vs. 1430 ± 104 kcal/day) but higher RQ (0.86 ± 0.04 vs. 0.81 ± 0.03, p < 0.05). Leptin concentration correlated positively with percent body fat (r = 0.57, p < 0.05) and, after adjusting for fat mass and fat‐free mass, was lower in post‐obese than in control subjects (4.5 ± 2.1 vs. 11.6 ± 7.9 ng/mL, p < 0.05). Discussion: The low fat oxidation and low plasma leptin concentrations observed in post‐obese individuals may, in part, explain their propensity to relapse.     

Alcohol Consumption in the Severely Obese: Relationship with the Metabolic Syndrome

Objective: The aim of this study was to examine the association between the clinical and biochemical features of the metabolic syndrome and quantity and type of alcohol intake in the severely obese. Research Methods and Procedures: A cross‐sectional study was performed in 486 consecutive severely obese subjects. Data on alcohol consumption was collected by serial clinical interviews and a questionnaire. The relationship between alcohol intake and the clinical and serum chemistry features of the metabolic syndrome was analyzed by multiple statistical techniques. Laboratory measures included lipid profile, fasting blood glucose, hemoglobin A1c, and fasting serum insulin. An indirect index of insulin resistance was calculated using the log‐transformed fasting insulin and glucose product. Results: There were 486 subjects, 84% women, with a mean age of 40.6 ± 10 years (range, 16 to 71 years) and a body mass index of 45.3 ± 7 kg/m² (range, 34 to 77 kg/m²). Alcohol consumers (N = 276) showed a marked reduction in the adjusted odds ratio of type 2 diabetes (odds ratio = 0.29; 95% confidence interval, 0.16 to 0.55) compared with rare or nonconsumers (N = 210). There was a U‐shaped relationship between the amount and frequency of alcohol consumption and fasting triglyceride, fasting glucose, hemoglobin A1c, and index of insulin resistance measurements. Consumers of <100 g/wk had more favorable measures. The effect was attenuated when diabetics were excluded from the analysis. Timing of alcohol consumption did not influence outcome measures. Discussion: Light‐to‐moderate alcohol consumption is associated with a lower prevalence of type 2 diabetes, reduced insulin resistance, and more favorable vascular risk profile in the severely obese. We would propose that light to moderate alcohol consumption should not be discouraged in the severely obese.     

Diabetic Complications in Obese Type 2 Diabetic Rat Models

We overviewed the pathophysiological features of diabetes and its complications in obese type 2 diabetic rat models: Otsuka Long-Evans Tokushima fatty (OLETF) rat, Wistar fatty rat, Zucker diabetic fatty (ZDF) rat and Spontaneously diabetic Torii (SDT) fatty rat. Pancreatic changes with progression of diabetes were classified into early changes, such as islet hypertrophy and degranulation of β cells, and degenerative changes, such as islet atrophy and fibrosis of islet with infiltration of inflammatory cells. Renal lesions in tubuli and glomeruli were observed, and nodular lesions in glomeruli were notable changes in OLETF and SDT fatty rats. Among retinal changes, folding and thickening were interesting findings in SDT fatty rats. A decrease of motor nerve conduction velocity with progression of diabetes was presented in obese diabetic rats. Other diabetic complications, osteoporosis and sexual dysfunction, were also observed. Observation of bone metabolic abnormalities, including decrease of osteogenesis and bone mineral density, and sexual dysfunction, including hypotestosteronemia and erectile dysfunction, in obese type 2 diabetic rats have been reported.     

Effects of Sibutramine on Resting Metabolic Rate and Weight Loss in Overweight Women

Sibutramine, a monoamine re-uptake inhibitor, has recently been approved by the Food and Drug Administration as a weight loss agent. Sibutramine lowers body-weight in rodents by reducing energy intake and increasing energy expenditure. Sibutramine facilitates weight loss in human subjects, but it is not clear whether it acts on energy intake, energy expenditure, or both. The present study was a randomized clinical trial designed to assess the effects of sibutramine (at 10 or 30 mg/day) on body weight and resting metabolic rate (RMR). Forty-four overweight women were randomized to 1) placebo (n=15); 2) sibutramine at 10 mg/day (n=15) or, 3) sibutramine at 30 mg/day (n=14). All subjects were instructed to consume a 1200 kcal/day diet for 8 weeks while receiving drug or placebo. RMR was assessed by indirect calorimetry at baseline, at 3 hours after the first dose of drug (or placebo), and at the end of the 8-week weight-loss period. Sibutramine reduced body weight-relative to placebo, but there was no difference between weight loss on the two sibutramine doses. No significant differences in RMR between sibutramine and placebo were seen, either 3-hour post dose or after the 8-week weight-loss period. After the weight loss period, all groups were taken off medication and kept weight stable for another 4 weeks. RMR was measured again and was not different among groups. That there was no change in RMR when sibutramine was stopped further suggests that the drug does not directly affect RMR. In summary, while sibutramine was shown to be an effective weightloss agent over 8 weeks, we found no evidence that it increased RMR.     

Energy Expenditure in Lean and Obese Prepubertal Children

The relationship between energy expenditure and obesity was examined in prepubertal children. Consenting fifth graders underwent Tanner Staging, weight, height and skinfold measurements. Subjects were selected for further study to obtain equal numbers of girls and boys with a wide range of body composition. Weight, total daily energy expenditure (TDEE) by doubly labeled water (DLW), resting metabolic rate (RMR), and body composition were measured. Children were grouped into level of obesity based on tertiles of subscapular plus triceps skinfolds. The skinfold tertiles did quite well in grouping subjects by degree of obesity, as differences in percent fat in each tertile were significantly different. There were no differences in fat-free mass between the groups, while the highest tertile group weighed 14 kg more than the lowest. For DLW, energy expenditure was calculated using day 8 and day 9 urine samples as the final time point to examine precision. Mean energy expenditure using either day was nearly identical (2220 ± 400 vs. 2300 ± 370 kcal/d), with a CV of the difference of 5.5%. No differences in RMR, energy expended in activity, or TDEE between the three groups were observed. A reduction in RMR or TDEE could not explain differences in obesity in these prepubertal children. However, the fact that the heaviest children expended the same amount of energy in activity and had the same TDEE as the leanest, while weighing 14 kg more, indicates that the obese children had a reduced activity level.     

Fiber Intake of Normal Weight,Moderately Obese and Severely Obese Subjects

ALFEERI, MARGARET AH, JOCELINE POMERLEAU, D MICHAEL GRACE AND LORRAINE ANDERSON. Fiber intake of normal weight, moderately obese and severely obese subjects. Obes Res. The lack of dietary fiber may be a contributing factor in obesity. This study examined the fiber intake of three weight groups: normal (20.0≤BMI≤27.0), moderately obese (27.1≤BMI≤39.9) and severely obese (BMI≥40.0). Each group contained 50 subjects. Detailed 3-day food records were used to gather the nutritional data. Fiber intake in the normal weight group was 18.8 ± 9.3 grams, the moderately obese consumed 13.3 ± 5.8 grams of fiber and the severely obese 13.7 ± 5.7 grams. Total fiber intake in grams was found to be significantly higher in the lean group (p<0.05) and was positively associated with sex and education level with men and more highly educated individuals consuming more fiber. Using regression analysis total fiber in grams and fiber in g/1000 kcalories was inversely associated with BMI after adjusting for sex, age, education level and income (p<0.01). A high fiber diet may help to promote a negative energy balance by causing early satiety secondary to gastric distention. Dietitians and physicians need to emphasize the importance of a high fiber diet to their obese patients.     

笼养雌性褐马鸡的静止代谢率(RMR)

利用“中型陆生动物呼吸仪”,在不同季节的不同温度下,测定了笼养雌性褐马鸡(Crossoptilon mantchuricum)的静止代谢率(resting metabolic rate,RMR)。RMR与环境温度(Ta)之间可用等式RMR=A BTa CTa^2进行拟合,得到了不同季节下的特定温度(special temperature)：春季22.6℃,夏季25.9℃,秋季22.2℃,冬季20.7℃。同时得到以下结论：笼养雌性褐马鸡的RMR在冬季最高,夏季最低,春季与秋季介于二者之间。     

力竭运动锻炼和饥饿对南方鲇静止代谢率及体重的影响

在25℃饥饿和维持日粮(1.5%体重)条件下,测定了连续15d无氧运动锻炼(追赶至力竭)及随后5d撤消运动锻炼过程中南方鲇(Silurus meridionalis)静止代谢率(VO2rest)的变化。实验以饥饿和维持日粮条件下非运动锻炼组分别作为饥饿及摄食对照组。研究发现:饥饿和摄食对照组的VO2rest在实验过程中显著下降(P<0.05),饥饿锻炼组VO2rest在锻炼的4～15d显著上升(P<0.05),而摄食锻炼组的VO2rest相对稳定;当撤消锻炼后,饥饿和摄食锻炼组VO2rest迅速下降,与对照组无显著差异。研究表明,相对于对照组,4～7d的无氧运动锻炼导致VO2rest显著上升;而撤消锻炼后,15d的锻炼影响在3～5d内完全消除。实验还提示,与维持日粮处理比较,饥饿条件下无氧运动锻炼对VO2rest的影响更为显著。     

温度和饥饿对鲤鱼幼鱼静止代谢率及自发运动的影响

分别在15和25℃条件下对鲤鱼(Cyprinus carpio)幼鱼[体重(10.36±0.22)g,n=88]进行2周的饥饿处理(饥饿组)或持续投喂(对照组),然后对实验鱼的静止代谢率(resting metabolic rate,RMR)进行了测定,同时以活动期的胸鳍﹑尾鳍摆动频率,观察期(10 min)胸鳍﹑尾鳍平均摆动次数,和运动时间比为指标对实验鱼的自发运动进行了观察。研究发现:温度和饥饿均显著影响鲤鱼幼鱼的静止代谢率(P0.05);在15与25℃条件下饥饿分别导致静止代谢率下降了19%和20%;温度由15℃上升至25℃,对照组与饥饿组静止代谢率分别增加了83%和80%。在15与25℃条件下,饥饿对鲤鱼幼鱼的自发运动具有显著不同的影响(P0.05),且胸鳍和尾鳍的结果十分相似;在15℃饥饿组与对照组的胸鳍、尾鳍摆动频率(活动期,以下类同)均无显著差异,而饥饿组运动时间比显著低于对照组,导致饥饿组的胸鳍、尾鳍平均摆动次数(观察期,以下类同)均显著低于对照组(P0.05);而在25℃饥饿组的胸鳍、尾鳍摆动频率均显著高于对照组,运动时间比无显著差异,导致饥饿组的胸鳍、尾鳍平均摆动次数均显著高于对照组(P0.05)。研究结果提示,在15℃下鲤鱼幼鱼通过降低自发运动以降低能量消耗来适应饥饿,而在25℃条件下,鲤鱼幼鱼通过增加自发运动来增大获取食物的机率以应对饥饿,这可能与野外环境中不同温度下食物丰富度及鲤鱼幼鱼的生理状况有关,但静止代谢率并未呈现与自发运动相关联的变化。     

Racial Differences in Metabolic Predictors of Obesity among Postmenopausal Women

NICKLAS, BARBARA S., DORA M. BERMAN, DAWN C. DAVIS, C. LYNNE DOBROVOLNY, AND KAREN E. DENNIS. Racial differences in metabolic predictors of obesity among postmenopausal women. Ober Res. Objective: This study determined whether there are racial differences in resting metabolic rate (RMR), fat oxidation, and maximal oxygen consumption (VO,max) in obese [body mass index (BMI = 34±2 kg/m²)], postmenopausal (58±2 years) women. Research Methods and Procedures: Twenty black and 20 white women were matched for fat mass and lean mass (LM), as determined by dual energy X-ray absorptiometry. RMR and fat oxidation were measured by indirect calorimetry in the early morning after a 12-hour fast using the ventilated hood technique. VO₂max was measured on a treadmill during a progressive exercise test to voluntary exhaustion. Results: RMR, adjusted for differences in LM, was 5% higher in white than black women (1566±27 and 1490±26 kcal/day, respectively; p<0. 05); and fat oxidation rate was 17% higher in white than black women (87±4 and 72±3 g/day, respectively; p<0. 01). VO₂max (L/minute) was 150 mL per minute (8%) higher (p<0. 05) in white than black women. VO₂max correlated with LM in black (r = 0. 44, p = 0. 05) and white (r=0. 53, p<0. 05) women, but the intercept of the regression line was higher in white than black women (p<0. 05), with no significant difference in slopes. In a multiple regression model including race, body weight, LM, and age, LM was the only independent predictor of RMR (r² = 0. 46, p<0. 0001), whereas race was the only independent predictor of fat oxidation (r² = 0. 18,p<0. 05). The best predictors of VO,max were LM (r² = 0. 22, p<0. 05) and race (cumulative r² = 0. 30, p<0. 05). Discussion: These results show there are racial differences in metabolic predictors of obesity. Determination of whether these ethnic differences lead to, or are an effect of, obesity status or other lifestyle factors requires further study.     

Low Resting and Sleeping Energy Expenditure and Fat Use Do Not Contribute to Obesity in Women

Objective: Determine whether sleeping and resting energy expenditure and sleeping, resting, and 24‐hour fuel use distinguish obesity‐prone from obesity‐resistant women and whether these metabolic factors explain long‐term weight gain. Research Methods and Procedures: Forty‐nine previously overweight but currently normal‐weight women were compared with 49 never‐overweight controls. To date, 87% of the 98 women have been re‐evaluated after 1 year of follow‐up, without intervention, and 38% after 2 years. Subjects were studied at a General Clinical Research Center after 4 weeks of tightly controlled conditions of energy balance and macronutrient intake. Forty‐nine obesity‐prone weight‐reduced women were group‐matched with 49 never‐overweight obesity‐resistant controls. All were premenopausal, sedentary, and normoglycemic. Energy expenditure and fuel use were assessed using chamber calorimetry. Body composition was assessed using DXA. Results: At baseline, percent body fat was not different between the obesity‐prone and control women (33 ± 4% vs. 32 ± 5%, respectively; p = 0.22). Analysis of covariance results show that after adjusting for lean and fat mass, sleeping and resting energy expenditure of obesity‐prone women was within 2% of controls. Neither sleeping nor resting energy expenditure nor sleeping, resting, or 24‐hour fuel use was significantly different between the groups (p > 0.25). None of the metabolic variables contributed significantly to patterns of weight gain at 1 or 2 years of follow‐up. Discussion: The results suggest that when resting and sleeping energy expenditure and fuel use are assessed under tightly controlled conditions, these metabolic factors do not distinguish obesity‐prone from obesity‐resistant women or explain long‐term weight changes.     

Resting Metabolic Rate in African-American and Caucasian Girls

YANOVSKI, SUSAN ZELITCH, JAMES C REYNOLDS, ALLISON J BOYLE, JACK A YANOVSKI. Resting metabolic rate in African-American and Caucasian girls. Recent studies have found a lower resting metabolic rate (RMR) in African-American (AA) women with obesity as compared with Caucasian (C) women with obesity. It is unknown if this difference in RMR is seen in prepubertal girls or in those of average body weight. Therefore, we studied RMR in 21 AA and 24 C girls, ages 7–10, who were well matched for age, weight, body mass index (BMI), and pubertal status. All had BMI between 15% and 85% for age and race, based on data from the First National Health and Nutrition Examination Survey. Fat free mass (FFM) was measured by dual-energy X-ray absorptiometry. RMR was measured with a Deltatrac indirect calorimeter under controlled conditions after the subjects underwent an overnight fast. The slopes of the regression equations were similar for both groups (ρ=0.7). After adjustment for FEM, the AA girls had a significantly lower RMR than did the C girls (?92 ± 32 kcal/d, ρ=0.007 by analysis of covariance). This significance was maintained after exclusion of total body bone mineral content. These data suggest that normal-weight prepubertal AA girls may have reduced resting energy expenditure compared with C girls.     

小型哺乳动物的持续能量收支限制研究进展

综述了小型哺乳动物持续能量收支限制的研究概况和进展。最大持续能量收支在决定物种的地理分布、生存适应、繁殖成功等方面都具有重要意义,但在许多条件下受到限制。食物的丰富度,或者动物自身的摄食、消化和吸收能力似乎不是主要限制因素。持续能量收支可能被外周组织和器官消耗能量的能力限制,即"外周限制"假说;或者机体的散热能力所限制,即"热耗散限制"假说。动物也可能通过衡量季节性繁殖投资的价值,实现最大繁殖输出,即"季节性投资"假说。尽管这些假说得到了一些研究的证实,但仍未阐明持续能量收支限制的机理。本文对相关研究的发展方向进行了展望。