Daily insulin output is much more closely related to total caloric intake than to carb intake. Studies with type 1 diabetics have found that doubling daily carb intake (from about 150 to 300 grams) only increased daily insulin requirements by about 25%, suggesting that a non-diabetic will release similarly "moderately increased" amounts of insulin but not in any way proportional to carb intake, given a constant daily caloric intake.
(This changes if someone starts to use significant amounts of ketones as ketones can be produced and burned in the absence of insulin).
Also, there are three ways to increase carb intake, 1) eat more fructose containing foods like sugar, and sugar containing plants, 2) eat low GI starches like high gluten wheat, oats, legumes and 3) eat high GI starches like white rice, potatoes, plantains, yams, or glucose polymers (branched dextrin for example).
The issues with those are that, #1 raises daily fructose load, and the liver can only process about 25 grams of fructose normally a day (plus more with high activity levels, #2 low GI starches are low GI because they contain inflammatory substances which raise cortisol, and require more insulin as a result, and #3 high GI starches can cause greater blood sugar excursions in the high range, as well as late post prandial hypoglycemia, or at least mild low blood sugar (60-75) which tend to cause hunger as well as, guess what? Increased levels of stress hormones.
So the problem with carbs is not the carbs, it is that all three sources of carbs 1) fructose containing, 2) low GI "pro-inflammatory" like wheat and legumes and 3) high GI starches can lead to metabolic syndrome by raising stress hormone levels and or causing liver insulin resistance (EXCESS fructose turns to liver triglycerides).
Now even though total daily insulin output only rises a little as the percentage of carbs in the diet rise (at a constant caloric intake) the post meal insulin spike will be greater after higher carb meals. Also, eating carbs tends to upregulate muscle glycogen storing enzymes, and glucose burning enzymes in muscles. It also tends to result in clearing of fatty acids from muscle cells which can result in greater insulin sensitivity-So a high carb, low fat diet CAN RAISE insulin sensitivity as long as calories are not excessive.
But the key to carbs strategically is to avoid a) excess fructose, b) inflammatory starches and , c) post prandial stress hormone release due to low blood sugar after an insulin surge.
A question I have then is, DO we want to elicit a big insulin spike similarly to what happens with protein pulsing for AA levels to trigger anabolism, and then let insulin levels return to baseline? There are good reasons that suggest this might be the case. One is that elevated insulin levels block GH and so it is beneficial to have periods in the day, especially early during sleep, when insulin levels are very low. Also you can't burn fat with high insulin levels.
The solution seems to me to be using low inflammatory, high GI carbs around training, but always following it up with something that will stabilize blood sugar on the way down to prevent low blood sugar levels 2-4 hours post prandial (which itself causes a release in "anti-inflammatory" but catabolic stress hormones). In a way, we are trying to walk the narrow path between pro-inflammatory and pro-stress hormone (which are actually anti-inflammatory, but catabolic). The inflammatory part is bad because inflammation results in release of anti-inflammatory cortisol.
Anyway, I think the main reason that carbs cause gains then is not raw insulin levels, but in an insulin spike being an anabolic trigger, and secondarily that carbs bring water into muscle cells and cause volumization. If, however, you have been on a calorie deficit, muscles will pull in fatty acids just as well as they will pull in glucose and I know many bodybuilders will use "fat loading" to revolumize after a period of depletion.
Top athletes tend to have muscles loaded with glycogen AND fatty acids, and yet they tend to be insulin sensitive all at the same time.
Anyway, I have been a low-moderate carb proponent for a long time, but I have come to believe that the main issues with carbs are fructose, and high inflammatory, LOW GI whole grains and beans that all contain KNOWN pro inflammatory compounds, in fact it is the presence of those inflammatory compounds that makes them low GI, by inflaming the cells lining the gut and slowing absorption.
So most issues can be dealt with by consuming low inflammatory, low fructose carbs like white rice and potatoes again with a strategy to consume them when they are going to get into muscles, and a strategy to prevent rebound "low" blood sugar which itself causes inflammation.
Anyway, just wanted to point out that high fat diets tend to require more insulin per gram of carbs due to muscles loading with fatty acids so insulin is not much lower on a low carb diet than on a high carb diet (calories being equal), but rather blood sugar and insulin peaks and valleys are more pronounced with higher carbs, while they are more stable with high fat.
(I wanted to clarify one thing). If someone has been eating lowing carb (20-30%) and high fat consistently, they WILL need a lot more insulin if they suddenly bump up carbs, for the first 2-3 days while their muscles switch over from fatty acid based to glycogen based energetics). I don't think it is a good idea to alternate high fat and high carb days anymore, but rather to go from high fat to semi-fast (like pulse fasting) THEN to high carb.