I am going out on a limb here and postulating my own theory as to why so many nutritionists push the 'Essential Carbs' mantra. I have studied the Krebs / citric cycle that controls how our basic cells function, and the pathways involved. There are two basic pathways, one is for carbs and the other is for fat. Our body has a default mode of operation for each path that it tends to use for everyday activities, but there are crossfeeds that it can use so we can adapt to changing environments and diets,
The carb pathway takes carb and starch and makes glucose. This then gets taken to the muscle tissue where it is either stored for future use, or used as immediate energy. It also feeds the nerves and braincells. This pathway needs insulin to open the gates into the muscle cells, so if insulin is missing or there is IR present then neither storage or energy use can proceed properly. If IR prevents glucose entering the cell, then the body will use any stored glucose instead just like a battery. So losing weight is often a pointer to diagnosis of diabetes. The muscle cells default to glucose as fuel, but they can be forced to use fat as an alternative fuel when glucose levels are low. Nerve cells and braincells cannot use fat, but can use ketones that are a by-product of fat burning as a backup fuel. So when we use low carb diet to drop our bgl to acceptable levels, then we actually force our body into secondary backup mode, which is a protection mechanism normally only triggered by starvation. This is why nutritionists throw up their hands in horror whenever we mention LC diet - We are intentionally starving our bodies by cutting essential carbs, Now said nutritionists struggle with this concept and assume that backup mode is an inferior operating mode, and that we should not be able to fully function without a minimum level of carb intake - the fact that the body successfully adapts to this mode, and is quite happy with low blood sugars is not a concept they can easily take on board - Its dangerous to do that AAAGH.
The fat pathway is equally problematic for them. By default fats get transported around the body in the blood stream - since fat is not blood soluble, it gets packaged into cholesterol packets - more fat in blood the more cholesterol lorries shifting it. But that leads to a high TC value, so that is dangerous !!!!!. Now fat gets transported to the liver and normally misses the muscle cells. In the presence of insulin fat gets stored in the fat cells surrounding the essential organs in the midriff and form a protective layer round these organs. This fat is blubber, and can supply energy for central heating of organs, and as a shock absorber. More fat + insulin creates more fat cells, more blubber. But the body has control mrchanisms in place to dump excess fat without storing it and only increases fat storage when there is extra insulin around with high bgl as well. The other mechanism that comes into play is that these fat stores can be raided when bgl is low to provide fuel for a fat burning session such as will occur when blood glucose drops and exercise depletes the glucose stores. A keto diet mimics this heavy exercise mode, and burns off midriff fat as well, so this is where the 'walkies' part of the Prof comments come into play. However IMO if an HC diet is advocated, then it takes much more exercise to dislodge the stored glucose and thus a strenouous workout is necessary, But keto diet makes the job easier by restricting the glucose pathway in the first place.
Finally we should reach a pont in our keto diet where glucose stores are depleted, the bgl is low, the cholesterol is steady but not necessarily low, and we have reduced the adipose fat stores. This state of equilibrium is the point where the muscle devourering phase is suspected of occurring, Any demands for energy such as sudden exercise will not find any backup stores since these are all depleted, so the body will suffer fuel starvation and start looking for other sources of burnable material. Muscle tissue is one possibility, but not the only one. The body has other crosspathways to use, it can convert dietary protein into glucose, and it can adjust demand by shutting down non essential functions in the same way it can drop body temperature to keep the inner core warm when in a cold environment. It has many such emergency measures it can use before it starts to devour itself, But real starvation could theoretically follow, but is an unlikely scenario. It is not a normal course of action that an LC dieter would use even when intermittent fasting. The simplest way of dealing with this is to increase the fat intake to supply fuel to meet increased demand, (or to increase protein intake but that is not such an efficient process).
I am lucky in that my HCP team are suitably impressed by my med reducing, bgl dropping, weight stabilising LC diet and now fully support my efforts. I was part of the original NHS team that devised DAPHNE and DESMOND as a user representative, but I in my ignorance had to support Eatwell#1 Sorry Guys and Gals, I know better now,