CLA has been shown to have an inverse relationship with adiposity. A reduction in body fat mass has been found in growing animals such as mice, rats, pigs, and cattle as well as adult humans. In fact, when 6-week-old male ICR mice were supplemented with 0.5% CLA plus 5% corn oil to their diet for 32 days, a 57% reduction in body fat and a 5% increase in lean body mass was observed compared with their respective controls.13 In the same study, 6-week-old females were fed a 0.5% CLA plus 5% corn oil-supplemented diet for 28 days and showed a 60% reduction in body fat and a 14% increase in lean body mass compared with their respective controls. The control male and female mice were fed a diet with 5.5% corn oil. It is interesting to note the similarity of body fat reduction, but there was a nearly threefold difference in lean body mass between the male and female mice; perhaps male and female mice metabolize and incorporate CLA differently. In a separate study, a 50% reduction in adipose tissue mass was observed when 8-week-old female ICR mice were fed a diet containing 0.5% of a CLA mixture for 4 weeks.14 The adipose tissue reduction was sustained after the CLA was removed from the diet. When AKR/J mice were fed a semipurified diet supplemented with 2.46 mg/kcal of CLA mixture for 6 weeks, a significant reduction in adipose tissue deposition (43 to 88%) independent of the high-fat (45 kcal%) and low-fat (15 kcal%) diet composition was observed.15
To determine whether a metabolite of CLA could explain the reducing effect of CLA on body weight and adiposity, the effect of CLA and conjugated linolenic acid (CLNA) on body fat in male Sprague-Dawley rats was determined.16 CLNA is a highly unsaturated conjugated fatty acid and is expected to affect lipid metabolism. The 4-week-old male rats were fed a purified diet with either 1% CLA or CLNA for 4 weeks. Peri-renal and epididymal adipose tissue weight was reduced in both the CLA and CLNA groups; however, the effects were heightened within the CLNA group. These results suggest that CLNA and CLA may work differently in reducing adipose tissue weight.
In studies involving human subjects, research findings have been mixed. A study supplementing varying amounts of CLA from 1.7 to 6.8 g/day to overweight and obese humans for 12 weeks showed a reduction in body fat mass, measured by DEXA, for the CLA group.17 In people with type 2 diabetes mellitus, supplementation with CLA (mixture of c9t11 CLA and t10c12 CLA isomers) or placebo was provided at 8.0 g/day. Supplementing for 8 weeks with the CLA mixture resulted in reduced body weight.18 Further, it was noted that the t10c12 CLA isomer was more significantly associated with the decreased body weight than the c9t11 CLA isomer, suggesting that t10c12 CLA may be the bioactive isomer responsible for weight loss. Another study, involving weanling ICR rats, also found that the t10c12 CLA isomer was associated with reduced body fat, enhanced body water, enhanced body protein, and enhanced body ash, whereas the c9t11 and t9t11 CLA isomers did not affect these parameters.19
In addition to reducing adipose tissue mass, the t10c12 CLA isomer has been linked to increased insulin resistance in men who have symptoms of the metabolic syndrome.2627 A CLA mixture also appeared to cause hyperinsulinemia in C57BL/6J mice24 that was accompanied by severe adipose tissue ablation and decreased leptin levels. The effects of the CLA mixture on adipose tissue depletion were reversed by continuous leptin infusion. In a follow-up study, decreasing the amount of a CLA mixture from 1 to 0.1 g/100 g diet, while increasing the amount of total fat in the diet from 4 to 34 g/100 g diet, did not lead to lipodystrophy, while fat mass was modestly reduced.25 Insulin resistance was present in the group fed the 1 g CLA/100 g diet, but not present in the 0.1 g CLA/100 g diet group.
In men who were supplemented with 3.4 g/day of a CLA mixture, purified t10c12 CLA isomer, or olive oil placebo, the t10c12 CLA isomer exerted an increase in insulin resistance that correlated with increased urinary isoprostane levels, suggesting an increase in oxidative stress in these same individuals. Previously, in a study from the same group, 60 abdominally obese men supplemented with 3.4 g/day of the t10c12 CLA isomer, a CLA mixture, or equal amounts of olive oil became more insulin resistant when supplemented with t10c12 CLA than did people supplemented with the CLA mix or olive oil control.27 These results are significant in the clinical usages of various isomers of CLA as a dietary supplement.
In contrast to the above studies, women supplemented with 3 g/day of CLA had no significant difference in body composition compared with the placebo.28 Seventeen women were supplemented with either a 3 g/day capsule of a CLA mixture or a sunflower oil placebo while being confined to a metabolic suite for 94 days. Their diet and activity were held constant. It is possible that 3 g/day is not enough to elucidate a change in body composition in women.
In 1994, it was proposed that CLA acted as an in utero anabolic stimulus for rats, since it appeared to enhance weight gain in rats.29 Eight-week-old female Fisher rats were fed a nonpurified diet and allowed to mate with a male counterpart. Immediately after mating, the females were separated and fed a mixture of either 0.5 g CLA/100 g or 0.25 g CLA/100 g or corn oil mixed with a semipurified diet. Diets with CLA did not affect the weight of dams but increased the weight of the pups in the CLA-fed groups. Conversely, Poulos et al. later performed a similar study in which pregnant Sprague-Dawley rats were provided diets with 6.5 g/100 g soybean oil and 0.5 g/100 g CLA or 7 g/100 g soybean oil.30 The maternal treatment continued until day 21 of lactation, at which time the pups were weaned. The pups were assigned control or CLA diets until 11 weeks of age. No difference was found in the number of pups per litter, weights of whole litters, litter weight gain, litter efficiency, or food intake of the dams fed. Parametrial fat pad weight and retroperi-toneal pad weight were less in dams fed CLA, and pups from dams fed CLA were significantly heavier at weaning than the pups from the control dams. Heavier gastrocnemius and soleus muscles and longer tail lengths, which are markers of skeletal growth, were found at 11 weeks of age in male pups than in dams fed CLA. These results suggest that CLA treatment in relation to body composition may be dependent on the sex and age of the animal as well as the duration of feeding.
Summary of the Role of Conjugated Linoleic Acid in Energy Expenditure and Food Intake
Action with CLA
West et al., 199815 West et al., 200032
J, Energy intake in AKR/J mice « No change in energy intake in AKR/J mice f Energy expenditure
« No change in energy expenditure in Syrian hamsters
Bouthegard et al., 200231
Human Models Zambell et al., 200028
« No change in energy expenditure or intake in women
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