Insulin resistance is a state where some tissues in the body become resistant to the effects of insulin and they require more of it to achieve the same effect.

Insulin resistance is effectively the first step on the road to type 2 diabetes. When we are insulin resistant, glucose from the starch in our meals goes higher and lasts longer; more damage is done by the excess glucose and insulin. Fat spends more time locked up in our fat cells and less time being used for fuel.

There is no scientific consensus on what causes insulin resistance. One theory is that it is caused by being overweight. But since there are a lot of normal weight people with insulin resistance and since our metabolic health can be improved before we lose much if any weight, there is likely more to the story.

Some research suggests that insulin resistance starts with fat in our livers from excess consumption of sugar and starch, which leads to elevated insulin and blood glucose, which in turn causes other tissues to become resistant to insulin. (Overfeeding fat can also leave fat deposited in the liver, but we are far more likely to overdo it with sugar and starch than with fat.) It could also be caused by high intake of starches and sugars that lead to chronically elevated insulin. When tissues are constantly exposed to a hormone they can downregulate their receptors for it. Over 50 million American adults are estimated to have non-alcoholic fatty liver disease. Many more have at least some excess fat in their livers.