Weight Loss and Maintenance in Obesity-Prone Rats

The goal of this work is to characterize the metabolic state of obese rats before and after weight reduction and prolonged periods of weight maintenance. In particular, this work attempts to identify metabolic barriers to maintenance of a reduced body weight in obesity prone rats after a period of dietary obesity.

Principle Investigator: (Click on name for biographical information)
James O. Hill, Ph.D.

Contact information:(Click on name for biographical information)
Paul S. MacLean, Ph.D.
Paul.MacLean@uchsc.edu
(303)315-1508

Staff and/or Affiliated Personnel:(Click on name for biographical information)
Janine Higgins, Ph.D.
Ginger Johnson, M.S.
Neal Beeman, M.S.
Brooke Fleming, B.S.

Program Description:
Background

Obesity is reaching epidemic proportions and threatens both the health and quality of life of people around the world1. There has been a considerable effort to prevent people from becoming obese by promoting exercise and proper nutrition. Likewise, a concerted effort has been made to treat those who are already obese. The treatment of obesity involves two stages: 1) producing weight loss by creating a negative energy balance; and 2) preventing weight regain by preventing a positive energy balance. The first stage has proven to be much easier than the second stage in the treatment of obesity. There are numerous examples of success in producing weight loss in obese subjects²,³, and most successful weight-reduction programs involve lowering energy intake and increasing physical activity¹. It is the maintenance of weight loss that is the greatest barrier to long-term successful treatment of obesity⁴,⁵. Success or failure in long-term weight maintenance must depend upon an interaction of metabolism and behavior. However, despite studies of human obese subjects, there is no agreement on the extent to which the widely recognized lack of success in maintenance of weight reduction is due to metabolic vs behavioral factors^6-9. Some of the controversy may be due to the inability to prospectively examine people in the pre-obese, obese, and weight-reduced states. It is difficult to prospectively study the "reduced obese" state in humans. Not only is there is no effective way of predicting those who are susceptible to obesity so that they can be studied in the pre-obese, obese, and obese-reduced metabolic states without exorbitant expense, but it is also rare for human subjects to achieve successful long-term weight loss. Because of these limitations, we may not be able to learn everything we need to know about the barriers to weight loss maintenance from human studies.

Rodent Model of Human Obesity

Our approach is to use an animal model of obesity that we have developed and characterized over the past several years^10-14. This model has many similarities to human obesity and should help us to characterize the metabolic barriers to successful weight maintenance. This model identifies animals that are more and less susceptible to obesity (obesity prone, OP; obesity resistant, OR) by examining weight gain in response to one week of feeding a high fat diet in the early stages of development. This predictive tool provides a way to specifically select OP animals so that they can be studied in pre-obese, obese, and obese-reduced conditions.

Current Directions

Our observations in the National Weight Control Registry indicate that those who are successful at maintaining a significant amount of weight loss do so by consuming a low-fat diet and engaging in large amounts of physical activity⁴. Additionally, NWCR subjects report that weight maintenance becomes easier after maintaining a lower weight for 3-5 years¹⁵. These observations suggest that NWCR subjects may be using cognitive behavioral strategies to overcome a metabolic susceptibility to weight regain in an environment that promotes obesity and that this metabolic susceptibility may decline with time. Unfortunately, there is no metabolic data on NWCR subjects to support or refute this hypothesis. Because of the difficulties in examining these issues in a well-controlled human study, we are currently characterizing the metabolic state of pre-obese rats, obese rats, and obese-reduced rats at various stages and conditions of weight maintenance.

It is our contention that the metabolic state in the reduced-obese state will be such that to virtually ensure weight regain and that adaptations in the metabolic state with prolonged weight maintenance will attenuate the susceptibility to regain weight.

The aspects of their metabolic state that are being determined are daily energy expenditure, fuel utilization, body composition, and insulin sensitivity. Energy expenditure of both the light and dark cycle as well as the concomitant utilization of metabolic fuels (carbohydrates, fats, and proteins) are determined with an open indirect calorimetry system. Body composition is being determined by dual-energy x-ray absorptiometry (DEXA). Whole body and tissue-specific sensitivity to insulin action are being estimated with the hyperinsulinemic-euglycemic clamp technique. A weight regain promoting metabolic state could be seen as a low rate of energy expenditure, poor utilization of fat as a fuel source, and adipose tissue that is highly sensitive to insulin action. If we identify perturbations in these parameters, we will pursue a physiological explanation for these metabolic abnormalities by examining the metabolic-related hormonal milieu and the expression of metabolic-related genes in pertinent tissues.

Future Directions

We also plan on examining how a regular exercise regimen will alter the metabolic state of weight-reduced rats and whether this addition will alter the metabolic state throughout an extended period of weight maintenance.

Specialized Techniques and Facilities
Indirect Calorimetry

This program is the primary supporter and user of the JFK Indirect Calorimetry Satellite Facility (JFK ICSF). This satellite facility provides temporary housing of rats in the vicinity of an open indirect calorimetry system. The use of indirect calorimetry allows us to estimate metabolic rate (energy expenditure) and respiratory exchange ratio (relative measurement of fat and carbohydrate oxidation). This is accomplished by utilizing a sophisticated system whereby the amount of oxygen consumed and carbon dioxide produced by an animal is precisely measured over a period of time. The system consists of cages (chambers), air pumps, air flow controllers, valves, and gas analyzers, and is computer controlled to simultaneously measure oxygen and carbon dioxide in an individual chamber, recording data from each of the four chambers in a sequential manner. The system operates as follows. Air taken from a common source is pulled through each chamber via separate intake lines. Rats are placed in four chambers (1 rat/chamber). These cylindrical chambers are designed to collect food spillage, feces, and urine for the determination of energy intake, unabsorbed calories, and nitrogen excretion. The air exiting each chamber is then passed through a cooling unit for dehumidification, a requirement of the gas analyzers. The air then passes through a flow controller/pump system (1 per line) that maintains a constant air flow through each chamber (1 L/min). The air then passes to a manifold containing 4 switch valves. At predefined intervals, the computer sends a signal to close a specified valve corresponding to a particular chamber, resulting in the shunt of air from that chamber to the analyzers. The time of measurement, carbon dioxide concentrations, oxygen concentrations, flow rate, vCO2, vO2, RQ, and MR (Weir equation) are calculated and stored with software designed and written by researchers here at the Center for Human Nutrition. This procedures acquires and records thousands of data points per variable over a 24 hr period.

Rat Clamp Hyperinsulinemic-euglycemic Clamp

This technique allows us to determine whole body insulin sensitivity and to estimate the sensitivity of some specific tissues to insulin action¹⁶. Catheter extensions are inserted into carotid and jugular cannulae so that animals can move freely in the experimental cage. These extensions will exit the cage through the top of the cage in such a manner that prevents the rat from tangling or chewing on them during the procedure. Following a rest period, a primed, continuous infusion of insulin is begun and followed by a variable rate glucose infusion to clamp plasma glucose at basal levels. Arterial blood samples are drawn at regular intervals in order to monitor blood glucose levels and to properly adjust the glucose infusion rate. Tracers of glucose and 2-deoxyglucose are injected late in the procedure for determining the decay rate and tissue-specific glucose uptake. The amount of glucose infused at this high level of insulin is used to indicate the body's sensitivity to insulin action. Tracer uptake into the tissues allows us to estimate each tissue's sensitivity to insulin action.

Rat Dexa Dual-Energy X-ray Absorptiometry (DEXA)

DEXA provides a non-invasive method for determining fat mass and fat free mass in body compositional analyses. In rats, chemical analysis is generally perceived to be the gold standard in body compositional analysis, DEXA and other non-invasive methods are preferred when repeated measurements in longitudinal study designs are required. In our studies, we use body composition estimates to normalize our metabolic data (energy expenditure) from indirect calorimetry. We use the Lunar DPX-IQ with their current small animal software. This approach has been compared to chemical analysis, and we employ recommended adjustments in the calculations for fat free mass and fat mass¹⁷.

Program Support:

We appreciate the continued support over the past several years from the National Institutes of Health (2R01DK038088-17).

Relevant References:

Clinical Guidelines on the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults--The Evidence Report. National Institutes of Health. Obes Res 1998; 6 Suppl 2:51S-209S.
Sheppard L, Kristal AR, Kushi LH. Weight loss in women participating in a randomized trial of low-fat diets. Am J Clin Nutr 1991; 54:821-8.
Westerterp KR, Verboeket-van de Venne WP, Westerterp-Plantenga MS, Velthuis-te Wierik EJ, de Graaf C, Weststrate JA. Dietary fat and body fat: an intervention study. Int J Obes Relat Metab Disord 1996; 20:1022-6.
Klem ML, Wing RR, McGuire MT, Seagle HM, Hill JO. A descriptive study of individuals successful at long-term maintenance of substantial weight loss. Am J Clin Nutr 1997; 66:239-46.
Wadden TA, Vogt RA, Foster GD, Anderson DA. Exercise and the maintenance of weight loss: 1-year follow-up of a controlled clinical trial. J Consult Clin Psychol 1998; 66:429-33.
Shah M, Geissler CA, Miller DS. Metabolic rate during and after aerobic exercise in post-obese and lean women. Eur J Clin Nutr 1988; 42:455-64.
Larson DE, Ferraro RT, Robertson DS, Ravussin E. Energy metabolism in weight-stable postobese individuals. Am J Clin Nutr 1995; 62:735-9.
Wyatt HR, Grunwald GK, Seagle HM, et al. Resting energy expenditure in reduced-obese subjects in the National Weight Control Registry. Am J Clin Nutr 1999; 69:1189-93.
Amatruda JM, Statt MC, Welle SL. Total and resting energy expenditure in obese women reduced to ideal body weight. J Clin Invest 1993; 92:1236-42.
Chang S, Graham B, Yakubu F, Lin D, Peters JC, Hill JO. Metabolic differences between obesity-prone and obesity-resistant rats. Am J Physiol 1990; 259:R1103-10.
Commerford SR, Pagliassotti MJ, Melby CL, Wei Y, Gayles EC, Hill JO. Fat oxidation, lipolysis, and free fatty acid cycling in obesity-prone and obesity-resistant rats. Am J Physiol Endocrinol Metab 2000; 279:E875-85.
Pagliassotti MJ, Shahrokhi KA, Hill JO. Skeletal muscle glucose metabolism in obesity-prone and obesity- resistant rats. Am J Physiol 1993; 264:R1224-8.
Pagliassotti MJ, Knobel SM, Shahrokhi KA, Manzo AM, Hill JO. Time course of adaptation to a high-fat diet in obesity-resistant and obesity-prone rats. Am J Physiol 1994; 267:R659-64.
Pagliassotti MJ, Pan D, Prach P, Koppenhafer T, Storlien L, Hill JO. Tissue oxidative capacity, fuel stores and skeletal muscle fatty acid composition in obesity-prone and obesity-resistant rats. Obes Res 1995; 3:459-64.
McGuire MT, Wing RR, Klem ML, Hill JO. Behavioral strategies of individuals who have maintained long-term weight losses. Obes Res 1999; 7:334-41.
Podolin DA, Gayles EC, Wei Y, Thresher JS, Pagliassotti MJ. Menhaden oil prevents but does not reverse sucrose-induced insulin resistance in rats. Am J Physiol 1998; 274:R840-8.
Feely RS, Larkin LM, Halter JB, Dengel DR. Chemical versus dual energy x-ray absorptiometry for detecting age- associated body compositional changes in male rats. Exp Gerontol 2000; 35:417-27.

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