For me it was carbs and sugar, while especially sweeteners have not been a problem at all.
it turns out it's more than a century old; see https://en.wikipedia.org/wiki/Ketogenic_diet#Diet
Though now the disadvantage is that things like low carb wraps are not a normal store item, and neither monkfruit not alluose are allowed as food in the EU.
But I guess by far the biggest part was actually that I've been working from home since 2008 and love cooking ;)