|Image 1: Is this man temporary insulin resistant? Probably, otherwise he would hardly have made it to the finish line.|
When I come to think about it, the notion that "insulin resistance is just for the friends of Ronald McDonald" and the "health conscious" part of the population was immune to it, makes a particularly good starter not only because the number of "skinny fat" people with insulin resistance appears to be ever increasing, but also due to the fact that it implies that insulin resistance is unhealthy... now, I got you hooked, ha?
Insulin resistance nothing for healthy athletes like you!?
Now let's take you as an example: Let's imagine you are a 39y old man who realized a few years ago that his gut became bigger and bigger - not obese, yet but not as aesthetic as you wanted it to be. In view of the fact that the SuppVersity was not around in those days you fell for the mainstream idea that jogging would probably be the best way to get back and shape and began to work on your conditioning. Somehow you got hooked (I always say that you need to find something that you enjoy doing!) and decided that you want to run your first marathon, this year. You have been running >40km per week consistently and feel well prepared, today, the day of your run. And in fact, you made it post the finish line! The next day you go to your Dr to get a check up (this is obviously idiotic, but it is part of the story ;-) and the Dr says: "I am sorry Mr SuppVersitReader, but we will have to put you on metformin, you are pre-diabetic."
|Figure 1: Glucose disposal (µmol/kg/min) and lipid oxidation (mg/kg/h) during euglycemic clamp before and after "your" marathon (data based on Tuominen. 1996)|
Now, the whole story is obviously somewhat far-fetched and the -12% dumb in glucose disposal in the euglycemic clamp condition, during which glucose and insulin are infused and the uptake of the former in response to the latter is measured (cf. figure 1) would not qualify as pre-diabetes, but if it helped me to drive home the main message of this blogpost, i.e. that your cells do not refuse to respond to insulin without good reason, it did serve its purpose.
Temporary, exercise-induced insulin resistance paves the way for "fat-burning machines"
Although it appears logical that your body tries to maintain blood glucose levels, this phenomenon, which by the way came to be known as the Marathon Paradox, does yet appear to violate the physiological principles we discussed in some of the previous installments of the Intermittent Thoughts (cf. "The mTOR/AMPK Seesaw"). After all, the exercise induced depletion in muscle glycogen should increase the expression of AMPK (which responds to the increase in "used ATP" = ADP) and thusly improve not reduce glucose uptake. In the presence of >3x elevated free fatty acid levels (at the end of the marathon those were even >7x over baseline!) the effective mitochondrial powerplants within the oxidative muscle fibers of an endurance athletes will yet gear up so that they produce enough energy to fuel the acute energy demands. Therefore the total glucose disposal is reduced not so much because the amount of glucose that would be sucked into the glycogen stores of the muscle was reduced, but simply because less of it is burned as fuel - in the study at hand, the decreased whole body glucose disposal is mostly a result of the -43% reduced rate of glucose oxidation at almost identical rates of non-oxidative glucose disposal.
|Image 2: While it may help, temporary insulin resistance alone is not enough to run a marathon. It must be complemented by an adequate ability to burn fat for fuel if you want to make it to the finish line.|
Gradually increasing diet-induced insulin resistance is the slippery slope to type II diabetes
Unfortunately, the vast majority of the population has found another, more convenient way to decrease glucose disposal and increased blood lipid (specifically triglycerides) levels than exercise: Food! The revelation that even this unquestionably detrimental form of insulin resistance originates from a by all means well-meant physical response of your body will unfortunately have to be postponed to the next installment of the Intermittent Thoughts. After all the Sunday work of the past weeks, I figured I deserved less screen time this weekend ...
In the meantime, I hope that those of you who live in the north-eastern part of Europe have made good use of the fat storing effects of insulin and have equipped themselves with a nice padding of insulating adipose tissue that is keeping you warm even when the temperature falls below -30°C ;-)