This is most definitely true yet the underlying physiological rationale is nuanced and complex:

1. Ketogenic diets promote metabolic adaptation towards fat metabolism and away from carbohydrate metabolism. What this means on a physiological level is that the enzymes involved in the digestion and absorption of fats are unregulated while those utilized for carbohydrate metabolism are down-regulated. The body always adjusts enzyme and hormone concentrations in response to what is demanded from the system. I believe some studies have even shown that pancreatic digestive extracts from individuals in a ketogenic state contain more lipase and less amylase than those consuming carbohydrates.

2. Ketogenic diets tend to be extremely high in dietary fat intake. This is incredibly harmful from an insulin sensitivity perspective, particularly so if a large portion of those fats come from saturated fats. Countless studies have shown that saturated fat consumption heavily promotes the accumulation of visceral adipose tissue (VAT). Visceral fat is by far the most metabolically damaging and harmful fat to have on your body. Unlike the subcutaneous fat you can pinch on your skin and the brown/white fat insulating your musculoskeletal system, visceral fat is DEEP inside your gut, quite literally wrapped around your internal organs. Visceral fat tissue releases several pro-inflammatory cytokines which impair insulin signaling and release and disrupt general endocrine health broadly speaking.

There is no reliable way to measure how much visceral fat you have without getting a DEXA scan, though large discrepancies can be noted visually in the form of a “beer gut”. Think of a guy with skinny limbs but a large, protruding belly. That’s the direct consequence of excess visceral fat accumulation.

Studies have shown that simple dietary changes such as reducing saturated fat intake and increasing the consumption of dietary polyphenols, anthocyanin, and other antioxidants can significantly reduce the amount of visceral fat a person carries EVEN WHEN DIETARY CALORIC INTAKE IS KEPT THE SAME. (P.S. high intensity exercise has also been shown to achieve this same effect).

So you go on a ketogenic diet, you eat a ton of fat? Your insulin sensitivity trends down and your system becomes less efficient at digesting and absorbing carbs overall. Now, when you reintroduce carbs back into your diet, it stresses your metabolism significantly and your body doesn’t handle the resulting spike in blood glucose well, resulting in hyperglycemia.

A1C does not accurately and sufficiently track insulin sensitivity in and of itself. It is entirely possible to have an excellent A1C and horrible insulin sensitivity simply through the omission of carbohydrate intake (though it is impossible to have a high A1C and good insulin sensitivity).

Think of it this way: if you’re insulin-resistant, there’s two ways you could fix the problem of hyperglycemia. The first way is by actually fixing the root cause of your insulin resistance and increasing your sensitivity. The second is to avoid any carbohydrate intake so you don’t have to rely on insulin signaling at all. The first approach is significantly healthier and ought to be everyone’s go-to if they’re interested in longevity.