This study was conducted to evaluate the potential negative health effects relating to insulin sensitivity of naturally occurring dietary CLAt10,c12, given the studies showing that supplemental doses of this isomer have a negative impact on insulin resistance in humans [3–6, 18]. To our knowledge, this is the first study to examine the metabolic consequences of consuming naturally altered CLAt10,c12 content in a food source. To that end, we produced a 60% high fat diet with the fat component composed almost entirely of butter produced from milk collected from dairy cows suffering from SARA, a condition that can sometimes result from the practice in the dairy industry of feeding a high grain, low forage diet to milk-producing cows [12–14]. This butter represents, in reality, what is probably the most naturally enriched source of CLA t10,c12 available for human consumption. In this study, we examined the impact of shorter (4 weeks) and longer (8 weeks) term consumption of a diet highly enriched in CLA t10,c12 in lean healthy rats. Specifically, we examined the effects of i) 4 weeks consumption of low (10% of total kcal) and high amounts (60% of total kcal) of control (non-SARA) and SARA butter based diets on plasma glucose and insulin, as well as insulin signaling and glucose uptake in isolated rodent skeletal muscle, and ii) 8 weeks consumption of high amounts of control and SARA butter (60% of total kcal) on whole body glucose, insulin and pyruvate tolerance. The second intervention was longer in duration (8 vs. 4 weeks) in order to increase the opportunity to observe any potential detrimental effect of the CLA t10,c12 -enriched diet if such an effect existed.
Results from the present study indicate that within a relatively short period of time (4 weeks), there was a significant main effect for treatment (SARA vs. non-SARA) on blood glucose and plasma insulin (p < 0.05). That is, blood glucose and plasma insulin were greater in the SARA vs. non-SARA conditions, regardless of the amount of fat/butter in the diet. However, this effect was small in magnitude. Furthermore, based on the individual groups, there were no significant differences in fasting glucose or insulin concentrations. This was also confirmed in the second, longer term (8 week) intervention following which no significant change in fasting blood glucose was detected. Furthermore, the ex vivo stimulation of muscle glucose uptake and phosphorylation of Akt in response to a maximal insulin concentration was not affected by consuming butter for 4 weeks with a greater content of CLAt10,c12. Finally, tolerance tests (glucose, insulin and pyruvate) were performed after consuming 8 weeks of the low fat, and various high fat diets. Calculated AUC demonstrated significant impairment, or worsening, of insulin and pyruvate tolerance tests in the LARD fed animals, as would be predicted. SARA butter fed animals also demonstrated an impaired insulin tolerance test relative to the low fat fed group, but glucose and pyruvate tolerance were not adversely affected.
Collectively, our results suggest that in contrast to the findings of studies feeding higher amount of purified CLAt10,c12, the consumption of a diet containing naturally elevated amounts of CLAt10,c12 poses relatively little risk in terms of impaired muscle insulin response and whole body glucose tolerance. This may be due to several reasons including the relatively lower consumption of CLA in the current study (0.18 to 0.55 g CLA/100 g diet) compared to previous rodent studies using supplemental CLA (1.5 g/100 g diet). Furthermore, the relative abundance of all CLA isomers increased in the SARA butter such that the ratio between CLAc9,t11 and CLAt10,c12 was approximately 11:1, which is considerably greater than the 1:1 ratio of supplemented CLA. In addition, the duration of CLA consumption may also be a factor. Other studies have reported effects of CLAt10,c12 in mice, at similar concentrations to ours (0.5%) over a longer time course (6 months), including increases in muscle mass  and decreases in whole body insulin response . In the present study, one of the limitations was the availability of our custom manufactured butter, which necessitated a somewhat shorter time course of feeding. Therefore, we cannot be certain as to whether a longer period of feeding CLA might not have demonstrated a clearer negative outcome.
High dairy intake has been associated with a reduced risk for type 2 diabetes [21, 22] without increasing the risk of cardiovascular disease . A recent study demonstrated that high consumption of low fat dairy lowered fasting insulin by 9% and insulin resistance by 11% in obese individuals . An obvious limitation of our current study is that the sole source of dietary fat was butter. Few studies focus on butter as the sole source of dietary fat. One study demonstrated that healthy men consuming a high fat, butter-based diet supplemented with 5.5 g/day of mixed isomer CLA showed elevated markers of lipid peroxidation compared to butter alone, but no significant differences in fasting insulin, glucose or insulin resistance; however, there was no non-butter control used in this study . In rats, the addition of milk to a diet high in sucrose was able to improve insulin sensitivity . Dairy fat is complex in composition and contains numerous bioactive components that may potentially impact insulin sensitivity. For example, butter contains the short chain fatty acid butyric acid, which when supplemented in the diets of obese, high fat-fed mice improves fasting glucose, insulin and insulin tolerance . In the current study, we specifically attempted to increase the content of the CLA t10,c12 isomer relative to that of the CLAc9,t11 isomer. However, it is possible that changes in other components of butter may have influenced the outcome. Indeed, the lack of any clear negative metabolic consequence to increased CLAt10,c12 may be due in part to compensatory effects due to altered amounts of other components, including other CLA isomers which were all observed to increase in the SARA butter. While this may make interpretation more difficult in terms of cause and effect, it does point to the need to study the effects of altered CLA content in the context of natural foods, and not merely isolated CLA isomer supplements.
Finally, it should be acknowledged that the choice of model will likely have significant bearing on the outcome. The majority of rodent studies examining the impact of CLA supplementation have utilized lean mice and obese fa/fa Zucker rats. In the current study, we did not use mice as they demonstrate a rather unique lipoatrophic response to CLA supplementation [7–9]. It is also mice which consistently show impairments in insulin sensitivity with CLA supplementation [7, 8, 20]. Although we plan to examine the impact of feeding our altered butters on fa/fa Zucker rats, we decided to first examine the effect of naturally altered CLA on lean rats, as they are a less “extreme” model. Less information is available regarding the effects of CLA supplementation on lean rats. However, in the Zucker rat, a commonly used rodent model of obesity and insulin resistance, CLA supplementation is generally observed to improve insulin sensitivity and inflammation [28, 29]. Therefore, it is possible the general lack of negative consquences to feeding high amount of SARA butter in the current study is due to the choice of lean rats as a model i.e. that mice are more susceptible to the effects of CLAt10,c12 than are lean rats. Nonetheless, rats have been used by many laboratories including our own to study the effects of high fat diets on insulin sensitivity and in our opinion, represent a valid choice of species.