Body Composition Analysis

Chemical carcass analysis sets the benchmark for determination of body composition and the COEC still uses this method to validate new instruments and techniques. It is also useful for animals that have been killed and frozen. The carcasses/samples are dried (for water content), fat is extracted, and combusted to quantify ash content.

Dual energy x-ray absorptiometry (DXA) uses two different X-rays to measure fat, soft-lean tissue, and bone in living animals. Animals are anesthetized (Isoflurane) and imaged with the front and back legs extended away from the body. The COEC DXA machines can measure small (12-80g) and larger (250g-120kg) animals. A typical scan takes approximately five minutes.

Quantitative magnetic resonance (QMR) is used to measure water content, fat, and fat-free mass in vivo with no need for anesthesia. The core can use QMR to measure body composition in groups of 10 fruit flies, tissue samples (1g-10g), mice (15-100g), and rats (up to 900g). Scans take approximately two minutes for mice and rats, up to nine minutes for fruit flies.

Micro computed tomography can be used to identify and quantify regional distribution of body fat and lean tissue. Mice, or other small animals, are anesthetized during the scan, which takes 15-25 minutes. Unlike, DXA which gives a two dimensional image, microCT provides a three dimensional reconstruction of the mouse. Different tissues can be identified and outlined based on differences in density. This allows analysis of tissue composition. For example, fat content of the liver can also be determined based on the lower density of the tissue with higher amounts of fat. Although labor intensive, subcutaneous and visceral fat can be identified, as well as individual fat pads.

Energetics

Energy balance studies are conducted using the TSE Systems PhenoMaster/Labmaster animal monitoring system (Bad Homburg, Germany). This system allows us to collect data on oxygen consumption, carbon dioxide production, food consumption and locomotor activity in 8 animals at once. The entire unit is housed in an environmental chamber (Powers Scientific), allowing precise control of ambient temperature and photoperiod. Data are obtained using the TSE software.

Methods

Mice are acclimated to the metabolic cage for two days prior to all measurements. Measurements are then taken for 22 hours or longer depending on the experimental design. Energy expenditure is calculated using 1-minute samples. Total energy expenditure is determined by calculating the average hourly energy expenditure over 22 hrs and then multiplying by 24. Resting energy expenditure is calculated by averaging the three lowest 18 consecutive-minute periods of energy expenditure, with at least one hour between each period. Locomotor activity is determined with infrared sensor pairs arranged in a grid pattern for horizontal (x, y level) activity. Movement is monitored continuously and reported as total counts every 9 minutes and expressed as counts/24 hrs. Food intake is monitored using hanging baskets attached to force transducers. Data on food intake is acquired every 9 minutes, and hence meal size, duration, and timing can be acquired. The automated food access system can be programmed to restrict access to food by time of day, duration of feeding, or by the amount consumed. All cages contain a running wheel, which allows for exercise energy expenditure to be measured. The wheel can be always available, always locked, or controlled to allow only a certain time for running, or a specific distance run. There is a suspended tube in the cage connected to a force transducer, which allows for measurement of body weight, whenever the mouse is in the cage.

The E-Mitter transponder/receiver system is used to measure core body temperature and consists of battery-free transponders that are implanted into the animal’s abdominal cavity and linked to a receiver base on which that the cage rests. The transponders transmit information on core body temperature via the base to a computer. Locomotor activity is assessed as the mouse moves over the top of the receiver. Thus activity and body temperatures can be measured continuously while the animal is in its home cage.

Spontaneous exercise can be measured during and independently of indirect calorimetry using running wheels. For investigators interested in the effect of forced exercise, the core has a system of running wheels on a rolling bed, for which the speed and time of the exercise can be determined and changed by the investigator.

Digestive efficiency can be assessed by measuring food intake and weight of feces produced and then measuring the energy content of samples of food and feces using a bomb calorimeter. By measuring the weight of food consumed and the energy content of that food, energy intake can be calculated. The weight and energy content of the feces can be measured to give an estimate of energy lost, and together these give an indication of how much energy the animals are absorbing from the food.

Body Composition Measurements

  • 3 DXA systems to measure in vivo body composition in animals from 12-100g and 250g to 120kg
  • 2 QMR systems to measure in vivo body composition in mice and rats (or similar sized animals)
  • 1 Imtek microCAT II computed tomography system
  • 1 bomb calorimeter for the measurement of the energy content of food and feces
  • 2 drying ovens
  • 12 Soxhlet fat extractors
  • 1 muffle furnace
  • 3 industrial blenders

Indirect Calorimetry/Metabolism Measurements

  • 8-cage TSE indirect calorimetry system with oxygen and carbon dioxide analyzers, infra-red beam break activity system, running wheels, and automated feeders
  • 16-cage acclimation system for the indirect calorimetry system

Activity and Energetics Measures

  • 32 wheel running cages: 16 for rats, 16 for mice
  • 20 walking wheel forced exercise system (Lafeyette Instruments)
  • 12 receiver bases; 12 PDT-4000 (rats) and 20 G2 (mice) implantable transponders to measure temperature and activity in home cages
  • 1 rectal probe for measurement of rectal body temperature in mice

Environmental Variables and Energetics Assessment

  • 3 environmental chambers to measure indirect calorimetry of mice at different temperatures and light cycles