I've spent my day reading all about insulin resistance and insulin response. It's a very fascinating subject.
What prompted this was that I saw arguments on some forums that insulin has a short half-life, therefore it cannot have an adverse impact on tissue. I think that this is quite frankly, codswallop. Not because it's been proven one way or the other, but because the amount of time (or exposure) is not the only thing that determines damage. Frequency is just as important to take into consideration. Temporary high blood sugar spikes could possibly damage your internal organs. If it happens once, it might not be a problem, but when it happens repeatedly every day, over the course of time, it would come as no surprise if organ damage developed. Insulin does indeed have a short half-life, it is estimated to be between 9 and 16 minutes in humans.
However, this does not mean that insulin does it's job in this amount of time, nor does it mean that insulin is reduced completely in that amount of time. By definition, a half life is the amount of time it takes half of the thing in question to decay. Because it is always half, if you understand basic math, you will realize that it's exponential as to how long it takes for something to be cleared completely1 from a system.
The math is for n, the number of half lives elapsed, the percentage remaining is 100/(2^n). This does not tell you how much is left after n cycles, the quantity being dependent upon the initial amount produced or in the case of insulin, secreted. It takes five half-lives for complete decay to be achieved. This means that after a meal insulin is in your blood at higher than basal levels for at least 45 to 75 minutes based on the 9 to 16 minute half-life. However, in real life, it may be much longer. Insulin remained elevated in subjects after a meal for more than 120 minutes in this study. If you are eating high carb meals with snacks in-between, how often do you think your insulin levels are elevated?
In a healthy person, perhaps this isn't enough time to do damage, even with frequency (although I doubt it, which may be why almost everyone becomes insulin resistant at some point, even if it's when they're old). However, a lot of people become insulin resistant when young. Insulin resistance means that your cells are unable to use the glucose in your blood. Insulin secretion will continue in waves so long as glucose levels in the blood are raised. If glucose levels remain elevated long enough, your pancreas' β-cells may lose their ability to produce enough glucose and this is by definition diabetes.
So is it just carbs? Not completely. If you are insulin resistant, and trying to lose weight, one reason why it's very important to make sure that on a low carb or paleo diet that a good quantity of your diet is from fat, is because protein can cause an insulin response just like carbohydrates. So get the 'low fat' crap out of your head! One study showed that consuming a steak could cause plasma insulin levels to rise nearly 100 pmol/l above fasting levels, whereas cod fillet only caused a rise of 50 pmol/l. This may very well be because steak has about twice the protein as cod fillet. According to caloriecount.about.com, one ounce of steak has between 7.7 and 8.6g of protein (depending upon the type of steak). The same amount of cod fillet has 4.8g of protein. The study's authors say the difference in insulin response could also be attributed to carnosine, a dipeptide which is found in beef but not cod. Regardless, lean meat can cause an insulin response.
Keep in mind that protein is vital for rebuilding muscles, and if you're weightlifting you may need more than someone who's not.
Now, what do all these studies mean? Not a damned thing if you ask me. Honestly, the people funding the study influence the results. The data can really be interpreted to mean anything. And I do mean anything. We now know that cholesterol doesn't cause heart disease, but rather inflammation and homocysteine probably do. And yet there are dozens of studies about the nefarious effects of cholesterol, most of them poorly done, and the data manipulated to say what the researchers wanted to be said.
So, what causes insulin resistance? It seems no one is very sure, and there may be multiple causes. Some say it's chronically elevated insulin due to elevated blood glucose from over consumption of carbohydrates. Some of it is probably genetic. Whatever causes it, eating low carb or paleo is beneficial, as it reduces inflammation and makes insulin resistance unlikely. Indeed, though it is anecdotal, many people have noticed improved glucose tolerance after eating low carb for a long time.
1 Insulin is never completely cleared from our system. We secrete insulin in small amounts every few minutes, even when we haven't eaten.