High Fat Diets and Insulin Resistance

Does fat cause diabetes?

Disclaimer: We all have a bias in this debate. Sometimes I agree more with the low-carb crowd, but other times I can’t stand the low-carb dogma. I can’t be perfect in this analysis, but I will at least explain my rationale.

This topic doesn’t need a long introduction. The big debate that has persisted in nutrition for decades is simple: Does the high consumption of dietary fats induce insulin resistance, per se. The “per se” part is the most important nuance in this question. I think we can say with a good amount of certainty that overeating is a direct cause of insulin resistance in humans, regardless of dietary composition. Sure, that view is probably overly simplistic in reality; but in general, the excess nutrient intake will result in bad things happening. This should be fairly undeniable at this point.

However, what’s more complicated is whether the problems are due to fat, or carbs. Dietary guidelines have argued for decades (as we all know) to lower fat intake because fat is calorically dense and fat is what makes you fat and gives you heart disease and fills your liver with fat and blah, blah, blah. You all know the drill. Funny, though, that despite these strict guidelines, the evidence is still considered weak.

IMO, I think the evidence says that neither is necessarily a problem. The body is pretty capable of adapting to macronutrient ratios to where it can handle a high-carb load about as well as a high-fat load, ceteris paribus. Remember, fats are a complex group of nutrients. Some can be bad, others can be good. Some are neutral. It depends on context anyway.

Enter Lundsgaard et al (2019).

TL;DR: They took 18 humans and placed them on a eucaloric high-PUFA or high-SFA diet for 6 weeks and measured changes in several muscular and hepatic variables (insulin sensitivity, fat oxidation, DNL, etc.). Respiratory exchange ratio (RER) decreased after both diet interventions (indicating more whole-body fat oxidation), fatty acid transporters increased, fat oxidation increased, DNL markers decreased, glycogen dropped a little, and just to make sure this was controlled well, caloric intake and body mass did not change. So, the effect on insulin sensitivity? Surprisingly, the hepatic insulin resistance index decreased after both interventions, but mostly due to lower basal glucose. Most things stayed the same.

To put the cherry on the cake, they repeated a similar protocol in mice and had the same outcomes!

The short summary? High-fat eating, per se, doesn’t alter insulin sensitivity. There may be some beneficial effects (lower inflammation markers and diglycerides), but I wouldn’t place my bets on any of that just yet.

Certainly, though, fat itself doesn’t cause insulin resistance. Overeating does. Is it easier to overeat if you eat a lot of fat? Yes, if ad libitum. If you watch your calories, you should be fine. Weight loss and benefits on keto are real, but only IIFYM.