Feeding butter with elevated content of trans-10, cis-12 conjugated linoleic acid to lean rats does not impair glucose tolerance or muscle insulin response

Abstract

Background: Numerous studies have investigated the effects of isolated CLA supplementation on glucose homeostasis in humans and rodents. However, both the amount and relative abundance of CLA isomers in supplemental form are not representative of what is consumed from natural sources. No study to date has examined the effects of altered CLA isomer content within a natural food source. Our goal was to increase the content of the insulin desensitizing CLAt10,c12 isomer relative to the CLAc9,t11 isomer in cow's milk by inducing subacute rumenal acidosis (SARA), and subsequently investigate the effects of this milk fat on parameters related to glucose and insulin tolerance in rats.

Methods: We fed female rats (~2.5 to 3 months of age) CLA t10,c12 -enriched (SARA) butter or non-SARA butter based diets for 4 weeks in either low (10% of kcal from fat; 0.18% total CLA by weight) or high (60% of kcal from fat; 0.55% total CLA by weight) amounts. In an effort to extend these findings, we then fed rats high (60% kcal) amounts of SARA or non-SARA butter for a longer duration (8 weeks) and assessed changes in whole body glucose, insulin and pyruvate tolerance in comparison to low fat and 60% lard conditions.

Results: There was a main effect for increased fasting blood glucose and insulin in SARA vs. non-SARA butter groups after 4 weeks of feeding (p < 0.05). However, blood glucose and insulin concentration, and maximal insulin-stimulated glucose uptake in skeletal muscle were similar in all groups. Following 8 weeks of feeding, insulin tolerance was impaired by the SARA butter, but not glucose or pyruvate tolerance. The non-SARA butter did not impair tolerance to glucose, insulin or pyruvate.

Conclusions: This study suggests that increasing the consumption of a naturally enriched CLAt10,c12 source, at least in rats, has minimal impact on whole body glucose tolerance or muscle specific insulin response.

Figure 1

Insulin stimulated glucose uptake in isolated soleus muscle. Data are presented as mean plus standard error. Dashed line represents average basal (unstimulated) glucose uptake in all groups. *significantly different from basal (unstimulated) glucose uptake (p < 0.05). SOL strips were equilibrated for 30 minutes in gassed KHB (0.1% fatty acid free BSA) at 30C containing 8 mM glucose and 32 mM mannitol, in the absence or presence of insulin (10 mU/mL). Strips were washed twice (10 min each) with glucose-free KHB (4 mM pyruvate, 36 mM mannitol). After washing, SOL strips were incubated for 20 min (insulin-stimulated, 10 mU/mL) or 40 min (basal) in KHB (4 mM pyruvate, 8 mM 3-O-[³H] methyl-D-glucose (800 μCi/mmol), 28 mM [¹⁴C]mannitol (60 μCi/mmol)].

Figure 2

Representative blot for Akt serine phosphorylation in incubated soleus muscle under basal and insulin stimulated conditions. Order of Lanes: low control butter (basal), low control butter (insulin), high control butter (basal), high control butter (insulin), low SARA butter (basal), low SARA butter (insulin), high SARA butter (basal), high SARA butter (insulin).

Figure 3

Calculated area under the curve for glucose, insulin and pyruvate tolerance tests. Data are presented as mean plus standard error. *significantly different from control (low fat butter) group (p < 0.05). Glucose tolerance test. Rats were fasted for 6 hours. A glucose bolus (2 g glucose/kg body weight) was administered by intraperitoneal injection and blood glucose concentrations were evaluated at 15, 30, 45, 60, 90 and 120 minutes post injection. Insulin tolerance test. Rats were fasted for 3 hours. Blood glucose levels were determined at 10, 20, 30, 45, 60, 90 and 120 minutes after an intraperitoneal insulin injection (0.75 U insulin/kg body weight). Pyruvate tolerance test. Rats were fasted for 6 hours. A pyruvate bolus (2 g pyruvate/kg body weight, adjusted to pH 7.35) was administered by intraperitoneal injection and blood glucose levels were evaluated at 15, 30, 45, 60, 90 and 120 minutes post injection.