The prevalence of overweight and obesity is unacceptably high in most affluent societies and seems to be increasing. Excess body weight contributes to major health problems, including hypertension, noninsulin-dependent diabetes and coronary heart disease as well as some types of cancer (NIH Technology Assessment Conference Panel 1993).

Current methods used to treat obesity are relatively ineffective in the long term (NIH Technology Assessment Conference Panel 1993), and greater emphasis needs to be placed on the prevention of excess weight gain. Two major factors thought to contribute to obesity are a sedentary lifestyle (Gortmaker et al. 1990, Tryon et al. 1992) and diets high in fat (Rolls and Shide 1992, Swinburn and Ravussin 1993). However, there is evidence that all dietary fats do not increase body fat stores equally. In experimental animal models, diets high in saturated fats are more likely to cause excess fat gain than diets high in polyunsaturated fats (Meservey and Carey 1994, Pan et al. 1994, Parrish et al. 1991, Shimomura et al. 1990). There is little information on the effects of monounsaturated fats on body fat storage. Oils rich in monounsaturated fatty acids make up an increasing part of our fat intake. For example, canola oil (low erucic acid rapeseed oil) currently accounts for more than 35% of the soft oils and 17% of all fats and oils in the Australian food supply (Burden 1995).

Current health messages for the prevention of heart or other chronic diseases emphasize the importance of decreasing total dietary fat and replacing some of the saturated fats with monounsaturated fat and complex carbohydrates (WHO 1991). Regular physical activity also is encouraged. The effect these recommendations could have on the prevention of overweight and obesity needs to be investigated. The following experiment was conducted to compare the effects of common food fats high in saturated fat or high in monounsaturated fat on energy balance and body composition in mice with and without increased voluntary physical activity.

Materials and Methods

Table 1. Composition of and low fat, beef fat or canola oil diets [View Table]

Ten mice from each diet group were housed individually in standard mouse cages without access to activity wheels (nonexercise mice). Thirteen mice from each diet group were housed individually in larger rat cages equipped with activity wheels, giving them the opportunity to voluntarily increase their physical activity (exercise mice). The use of the activity wheels was monitored by bicycle odometers. The mice were maintained with their respective dietary and exercise regimens for 8 wk.

At the end of the study, mice were killed with an overdose of pentobarbital, and the retroperitoneal abdominal fat pad was removed and weighed as an additional measure of adiposity (Bell et al. 1995). Carcass energy was calculated from LBM and body fat using the values of 4.99 kJ/g for LBM and 39.16 kJ/g for fat (Graham et al. 1990), assuming 73.2% moisture in LBM. Energy expenditure was calculated by subtracting changes in carcass energy content from total energy intake during the 8-wk study. Energy efficiency was calculated by dividing the increase in carcass energy (kJ retained) by the MJ of energy consumed during the study.

The experimental protocols used in this study were approved by the Animal Experimentation Ethics Committee of Curtin University of Technology.

RESULTS

Table 2. Body fat and lean body mass of mice fed low fat or high fat diets containing beef fat or canola oil that did or did not exercise voluntarily1 [View Table]

Table 3. Energy intake, energy expenditure and food efficiency in mice fed low fat or high fat diets containing beef fat or canola oil that did or did not exercise voluntarily1 [View Table]

Energy intake, energy expenditure and carcass energy. Exercising mice consumed between 10 and 23% more energy than their nonexercising counterparts. This greater energy intake was offset by the significantly greater energy expenditure in the exercising mice (Table 3). Diet did not significantly affect energy intake in the nonexercising mice. However, the exercising mice fed beef fat consumed more energy than the mice fed the low fat or the canola oil diet (Fig. 2).

All mice with exercise wheels had high voluntary activity, and there was no measurable difference among diet groups in average distance run per day (low fat, 14.9 ± 1.6 km; beef fat, 14.5 ± 0.9 km; canola oil, 14.6 ± 0.9 km).

Energy efficiency was significantly lower in the exercising mice than in their nonexercising counterparts. In the nonexercising mice, energy efficiency was significantly lower in the low fat and the canola oil groups compared with the beef fat group.

DISCUSSION

Romieu et al. (1988) demonstrated that among 141 women there was a significant positive correlation between body mass index (BMI, kg/m²) and total fat intake and between BMI and saturated fat intake; however, there was no correlation between BMI and polyunsaturated fat intake. Research with animals has demonstrated that those fed diets containing high levels of saturated fat had greater amounts of body fat than those fed diets high in polyunsaturated fats (Meservey and Carey 1994, Parrish et al. 1991, Shimomura et al. 1990, Takeuchi et al. 1995). However, other research groups have reported little (Hill et al. 1992) or no difference (Awad et al. 1990) in body fat between rats fed saturated fat and those fed polyunsaturated fat. The composition of the polyunsaturated fat may influence fat storage. Polyunsaturated fats containing (n-3) fatty acids may be of particular interest. Rats fed diets containing fish oil, a rich source of very-long-chain (n-3) fatty acids, store less body fat than rats fed lard (Hill et al. 1993) or rats fed beef tallow or olive oil (Su and Jones 1993).

The results of the few studies conducted on the effect of dietary monounsaturated fats on body fat are conflicting. In one study, rats fed olive oil had body fat similar to that of rats fed beef tallow, and both had significantly more body fat than rats fed fish oil (Su and Jones 1993). In another study, rats were subjected to food restriction and then refed diets containing different fat sources. The rats refed olive oil or fish oil gained more weight and body fat than rats refed polyunsaturated safflower oil or lard (Dulloo et al. 1995). In the present study, mice fed the canola oil gained less body fat than mice fed beef fat.

The mechanisms by which dietary fats may differ in their potential to contribute to obesity are not fully understood. However, there are several possible explanations. Leyton et al. (1987) demonstrated that rats oxidize fatty acids at different rates, with common fatty acids found in human diets oxidized in the following order: oleic and -linolenic > linoleic > palmitic and stearic. Similar differences exist in humans: Jones et al. (1985) reported that adult males oxidize oleic acid and linoleic acid more rapidly than stearic acid. Furthermore, increasing dietary linoleic acid decreases the -oxidation of saturated fatty acids (Emken 1994). This would suggest that common saturated fatty acids are more slowly oxidized and therefore more likely to be stored than monounsaturated or polyunsaturated fats.

Another possible explanation is that the composition of dietary fat affects cell membrane composition and hence cell membrane function and metabolic rates (Pan et al. 1994). These researchers have demonstrated that high levels of saturated fat in the diet of rats alter membrane fatty acid composition and result in decreased metabolic rate. Takeuchi et al. (1995) also demonstrated that whole-body oxygen consumption in rats was lower after an isoenergetic meal containing lard than after meals containing safflower oil, high oleic safflower oil or linseed oil. Furthermore, the rats fed the lard-based diet accumulated greater amounts of body fat than rats fed the oil-based diets.

Although it is possible that the high oleic acid in the canola oil was more rapidly oxidized and therefore did not contribute to body fat gain in the canola oil-fed mice as did the more saturated fatty acids in the beef fat-fed mice, it also is possible that other fatty acids in the canola oil may have contributed to the lower body fat in these mice. Canola oil contains about 10% -linolenic acid, 18:3(n-3), and Pan and Storlien (1993) have demonstrated that -linolenic acid will decrease weight gain in rats fed high fat diets. The decrease in weight gain is directly related to the amount of (n-3) fatty acid incorporated into the tissues. Diets high in fat produce insulin resistance, which is prevented by very-long-chain (n-3) fatty acids or by 18:3(n-3) when there is no excess competition from (n-6) fatty acids for conversion to very-long-chain (n-3) fatty acids.

Voluntary exercise prevented obesity in all mice fed diets providing 40.8% of energy from fat. Regular physical exercise is associated with lower body fat in both humans (Blair 1993, Tryon et al. 1992, Williamson et al. 1993) and animals (Bell et al. 1995, Meservey and Carey 1994). The exercise was sufficient to increase energy intake and increase energy expenditure by an average of 20% over the 8 wk of the study. The exercise is best characterized as low to moderate intensity and of long duration because mice ran an average of 14 km/d. Low intensity exercise uses fat as a major source of fuel and increases the release of fatty acids from adipose tissue stores (Newsholme et al. 1993). The use of fat as a fuel increases with increasing duration of the exercise (Saltin and Åstrand 1993).

A further benefit of exercise is that muscles that are exercised regularly have an enhanced capacity to oxidize fatty acids (Romijin et al. 1993, Saltin and Åstrand 1993). Research in both humans (Froidevaux et al. 1993) and animals (Chang et al. 1990) has demonstrated that subjects with a greater capacity for fatty acid oxidation are less likely to accumulate excess body fat. In the present study, in which all mice had free access to food, the exercising mice had significantly less body fat than the nonexercising mice despite the fact that the exercising mice consumed significantly more energy than the nonexercising mice. Although the type of diet still influenced the percentage of body fat in the exercising mice, all exercising mice had low body fat, with fat levels that were less than or similar to those of the low fat-fed, nonexercising mice. The low fat diet is the diet recommended for laboratory mice (AIN 1977 and 1980); thus the low fat-fed, exercising mice represent what would be considered the normal body composition for females of this strain of mice.

This study demonstrates that the type of dietary fat as well as the level of dietary fat influences the amount of body fat stored. Mice fed canola oil, which is a rich source of oleic acid and contains appreciable amounts of -linolenic acid, accumulated less body fat than mice fed beef fat, which is high in saturated fatty acids. Increased voluntary exercise resulted in lower body fat stores in all mice, regardless of the level or kind of fat in the diet, and prevented diet-induced obesity even in mice fed high levels of saturated fat.