Weight training is known to increase insulin sensitivity, which is one reason it’s highly recommended for those who have type 2 diabetes and other abnormalities in insulin metabolism. The problem is, eccentric muscle contractions, which generally involve a lowering of weight during exercise, damage muscle fibers so much that muscle is temporarily made insulin resistant. That prolongs the replenishment of depleted muscle glycogen stores, making extra rest time between training mandatory.
On the other hand, eccentric exercise also leads to augmented insulin-induced muscle protein synthesis. A 1996 study involving rats found that four days of weight training led to elevated insulin-mediated muscle protein synthesis. The authors of that study later examined how both concentric and eccentric (raising and lowering of weights) affects both muscle protein synthesis and insulin-mediated glucose uptake into muscle.1
Once again the subjects were rats. The animals did a squatlike exercise with weights during four training sessions using progressively increased resistance. They did 50 reps per session, and the workouts were separated by 48 hours. The results showed that the rats exhibited significantly decreased insulin-mediated glucose uptake into muscle following exercise. Insulin’s effect on promoting muscle protein synthesis wasn’t affected.
Although the exact mechanism for the paradoxical effect of increased insulin-mediated muscle protein synthesis coupled with decreased muscle glucose uptake after weight training wasn’t disclosed, the researchers suspect that it may involve a decrease of GLUT-4, a glucose cellular carrier that does decrease after eccentric weight-training exercise. The significance is that GLUT-4 is vital for replacing diminished muscle glycogen stores following exercise, and the process may be impaired after weight training.
The good news is that the effect may only occur in beginners. Experienced weight trainees apparently become accustomed to that glucose uptake effect, and with experience it simply doesn’t occur. That’s underscored by many human-based studies showing increased muscle glucose uptake and improved insulin sensitivity with ongoing weight training. IM
1 Fluckey, J.D., et al. (1999). Attenuated insulin action on glucose uptake and transport in muscle following resistance exercise. Acta Physiol Scand. 167:77-82.