Hyperinsulinemia and insulin resistance : What comes first ?

Background

1)	Classical explanation :
Classical explanation of diabetic pathophysiology states that obesity induced insulin resistance develops first and is followed by compensatory hyperinsulinnemia. Further insulin resistance leads to prolonged, increased secretary demand on beta cells leading to subsequent secondary beta cell failure, giving rise to hyperglycaemia and diabetes^2^.

2)	 Neurobehavioral origin hypothesis :
The Neurobehavioral origin hypothesis suggests that insulin resistance mediates a shift from muscle dependent (soldier) to brain dependent (diplomat) strategies of making a livelihood. If nutrient limitation affects intrauterine development, brain development is the least affected among all the organs^4,5^. As a result, in IUGR babies muscle weight is poor but the brain is relatively well developed. Such a person is more likely to be a successful diplomat rather than a soldier and insulin resistance is adaptive for such an individual^3^. Since insulin is involved in brain development and cognitive functions, higher levels of insulin are needed. As insulin is having strong anti-lipolytic effect, hyperinsulinnemia is followed by subsequent excess fat accumulation. Also compensatory insulin resistance is needed to avoid hypoglycemia. This hypothesis predicts a reverse order of pathophysiology i.e. primary hyperinsulinnemia followed by compensatory insulin resistance^3^

Objective-
To determine in diabetes whether hyperinsulinnemia develops first or insulin resistance develops first.

Methods :
We searched literature for studies that investigated directly or indirectly the sequence of development of hyperinsulinnemia and insulin resistance in humans and animal models from an early stage. Meta-analysis was conducted on published data.

Results-
1)	In low birth weight neonates in humans as well as in rat models, hyperinsulinnemia is found at very early stage.^6^
2)	Development of insulin resistance is preceded by hyperinsulinnemia in mice, rats as well as in humans.^7, 8^
3)	In normoglycaemic hyperinsulinemia state if insulin production is suppressed insulin sensitivity increases rapidly maintaining the normoglycaemic state.^9,10^
4)	Beta cell expansion beginning in intrauterine life is independent of glucose, Insulin and Insulin receptors.^6^


Conclusion-
All the four lines of evidence indicate that hyperinsulinnemia precedes insulin resistance supporting the predictions of neurobehavioral origin hypothesis over the orthodox view.



References :
1)	DeFronzo RA, Ferrannini E (1991). Diabetes Care 14:173-194
2)	Kruszynska YT, Olefsky JM (1996). J Investig Med 44: 413-428.
3)	Watve MG, Yajnik CY (2007). BMC Evolutionary Biology.7: 61-74.
 4) Winick M, Rosso P, Waterlow JC (1970). Exp Neurol, 26:393-400.
 5) Winick M. (1969) J Pediatr,74:667-679.
 6) Chakravarthy MV et.al. (2008) Diabetes, 57:2698-2707.
 7) Ramin A et. al. (1998) J Clin Endo and Met, 83 :1911-1915.
 8) Hansen BC (1990) Am J Physiol Regul Integr Comp Physiol 259: 612-617.
 9) Stanley L (1981) Life Sciences, 28: 1829-1840.
 10) Ratzmann KP et. al. (1983) Int J Obes, 7 : 453-458

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