Lifestyle Not Genetics Most Responsible for Insulin Resistance in Identical Twins

A unique study performed on identical twins suggests that lifestyle choices, not genetics, plays the biggest role in the development of insulin resistance.

Insulin resistance is both a precursor and a symptom of type 2 diabetes, a disease which effects approximately 21 million adults in the United States. Insulin resistance is a condition characterized by the bodies inability to control blood glucose levels with normal insulin levels. This often occurs when defects develop in genes which convert food to energy.The condition also greatly increases the risk of heart disease.

The study aimed to discover whether the food to energy genetic defects (given the medical title mitochondrial oxidative phosphorylation) that lead to insulin resistance were a result of genetics or lifestyle choices, such as inactivity and subsequent obesity. In order to test this, researchers analyzed identical twins (genetically the same), where differences in the two could supposedly be attributed only to lifestyle differences and environment.

24 identical twin pairs were studied, 13 being male pairs and 11 being female pairs. 14 of the pairs had one twin of normal weight and one twin being obese (“discordant”). These 14 pairs were the primary test group. The other 10 pairs had twins of similar weight (“concordant”). These 10 pairs were the control group.

For the discordant twins, the obese twin was observed to have lower capabilities than the normal weight twin for three major functions. The obese twin unsurprisingly had lower fitness levels, meaning lesser oxygen uptake capabilities, among other things. The obese twins displayed lower insulin sensitivity (the ability to convert glucose to energy). And obese twins had lower levels of healthy genes that convert food to energy. This last result leads to further weight gain in these individuals, and has been strongly linked to insulin resistance. The results suggest that poor eating habits and lack of exercise, which are the two primary unhealthy lifestyle choices that lead to obesity, are of major importance in the development of conditions that lead to type 2 diabetes (insulin resistance and decreased insulin sensitivity).

Genetics are also known to play a major role in the development of this disease, and as lead author Dr. Linda Mustelin notes, “although we found that the reduced transcript levels of genes encoding mitochondrial oxidative phosphorylation in obesity is influenced by environmental and acquired factors, it does not exclude the possibility that genetic factors contribute to regulation of mitochondrial oxidative metabolism.” Nonetheless, the study states that the “data suggest that physical inactivity may have contributed to the defects in mitochondrial oxidative phosphorylation described in type 2 diabetic patients and pre-diabetic subjects.” Most importantly, these results mean that in many cases, the development of type 2 diabetes is preventable with proper nutrition and exercise levels.

Source: Defeat Diabetes Foundation: Mustelin, Linda. Guilfoy, Christine. American Journal of Physiology-Endocrinology and Metabolism press release. June 2008.

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