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How Does Training Status Affect Glycemic Index

Mettler S et. al. The influence of the subjects’ training state on the glycemic index.  Eur J. Clin Nutr (2007 ) Jan;61(1):19-24.

Objective:To determine the glycemic index (GI) dependence on the training state of healthy adult males.Subjects and design:Young, adult males of normal body mass index and normal glucose tolerance were tested twice with a 50 g reference glucose solution and twice with a breakfast cereal containing 50 g of available carbohydrates in a randomized order. Ten subjects were sedentary (SE), 12 were moderately trained (MT) and 12 were endurance trained (ET). Blood glucose, insulin and glucagon were measured.Results:The GI differed significantly between SE and ET subjects (P=0.02, mean difference: 23 GI units, 95% CI=3-42 GI units). The GI of the MT subjects was intermediary, but did not differ significantly from the SE or ET subjects. The insulin index did not differ significantly between the groups (P=0.65).Conclusion:The GI of the commercially available breakfast cereal depended on the training state of the healthy males. The training state is the first reported factor influencing the GI that is subject specific rather than food specific.European Journal of Clinical Nutrition advance online publication, 12 July 2006

My comments: For readers who aren’t familiar with the concept, the glycemic index (GI) is a measure of how a given food affects blood glucose levels.  It was introduced over 25 years ago as a more accurate measure of foods (as opposed to earlier schemes that simply used simple versus complex carbs) for diabetics and has been researched extensively since that time.

To determine GI, first a standard food is given.  This used to be 50 or 100 grams of pure glucose but now 50 grams of white bread is used as the standard.  Blood glucose is tracked over time and the area under the curve for blood glucose is given a value of 100.  Other foods are then tested (again, 50 grams of digestible carbs are given) and the ratio of area under the curve to the test food gives the GI. So a food that has 75% of the area under the curve is given a GI of 75, a food which has 20% of the area under the curve is given a GI of 20.  A food that gave 120% of the area under the curve has a GI of 120; you get the idea.

While I’m at it, I want to mention a related/similar concept called the insulin index (II) which is functionally identical to the GI but looking at insulin response.  Although research into the II seems to have stopped almost immediately after it started, it will come up in this week’s review.

Over the years, an absolutely tremendous number of foods have been tested and GI lists can be found in a variety of places (one of THE most thorough sources for GI information is Rick Mendosa’s amazing GI site).  GI becomes interesting because the results aren’t always what you expect.

Some ‘simple’ carbs have a much lower GI than other ‘complex’ carbs. For example, sucrose (table sugar) has a medium high GI (about 70) while some GI measurements for potatos are much higher.  Full fat ice cream has a low GI (due to the fat content) and fructose (fruit sugar) has an extremely low GI (about 20) because of how it is metabolized by the liver.

However, the use of the GI is still debated in terms of its utility.  Factors such as how the food is prepared, the presence of other nutrients (fiber, fat, protein), and the effects of previous meals all impact on GI.  For non-diabetics, it’s especially questionable how relevant the GI actually is.  Bodybuilders and athletes often use GI as a proxy for insulin response under the assumption that low GI means low insulin which is good.

Up until this point, essentially all of the research into GI has suggested that the impact of a given food is independent of the individual, having only to do with the food itself.

However, this week’s research review brings that into question.  Following up on an earlier paper (by the same group) which found a difference in GI between sedentary and endurance trained individuals, it sought to measure the GI in three different groups of individuals: sedentary folks, moderately trained individuals (defined as aerobic exercise 2-3X/week on average) and trained (aerobic exercise 4X/week on average with some being competitive endurance athletes). Only males were studied so its unknown if these results apply to females.

A standard GI test was done with 50 grams of glucose and then a meal of (seriously) Kellog’s Special K with partial skim milk providing 50 grams of carbohydrate, 14 grams of protein, 4.6 grams of fat and 1.7 grams of fiber was given and blood glucose and insulin response was measured.  GI and II were calculated for all groups.

And, as with the previous study, the endurance trained group showed a significantly lower glycemic response for both the glucose and cereal meal as well as an average GI response that was 23 points (with a range of 3-42 units) less for the cereal meal (GI was about 80 for sedentary, 65 for moderately trained and 57 for endurance trained).  This was nearly identical to the previous study which found a 25 point difference.  The moderately trained group was square in the middle.

Although the absolute insulin response was different across groups (highest in sedentary, medium in moderately trained, lowest in endurance trained), the insulin index was not different between the groups.  Nor was the absolute insulin response different between the glucose and cereal trial, the same amount of insulin was released despite the difference in GI.

Unfortunately, the researchers couldn’t pin down a mechanism for this response although it’s clearly related somehow to training status, probably insulin sensitivity (how well skeletal muscle responds to the hormone insulin).  Given the differences in absolute insulin response (although the insulin index was no different), this would seem the logical conclusions.

So what does this paper tell us?  First and foremost, in endurance trained individuals, choosing foods based on glycemic index may be that much less relevant; regular endurance training will decrease the effective GI significantly.  Additionally, endurance training reduces the absolute amount of insulin released in response to a given carb load. Given the effect of regular training on insulin sensitivity, this makes a certain level of sense.

An unanswered question by this paper is whether regular resistance training will have the same effect.  While weight training can certainly improve insulin sensitivity, there is also evidence that extremely heavy lifting (which causes significant muscle damage) can negatively impact on insulin sensitivity. So the question is still unanswered.

Perhaps more interestingly is how little endurance training is needed to have at least some impact on both glycemic and insulin responses. Even two to three days/week had an impact although a greater frequency had a larger impact.  Strength/power athletes (including bodybuilders) might want to include a moderate amount of aerobic work (2-3X/week at low intensity) solely for that purpose, to improve glycemic and insulin responses to carb intake.


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