Where Does the Glucose that the Body Makes Come from?
When carbohydrates are restricted completely, the body still has a small requirement for glucose (although this decreases over time) and the body has to find something to make glucose out of. That something is lactate and pyruvate (produced from glucose metabolism), glycerol (from fat metabolism) and some amino acids. It’s the amino acid use that can be problematic since they have to come from somewhere.
Now, if no food is being consumed (e.g. total starvation), that somewhere is generally muscle t
ue (the body will also break down liver proteins); the body will readily break down body protein to scavenge the amino acids it needs to produce glucose. In doing so, the muscle released alanine and glutamine (produced in the muscle from the breakdown of leucine and the branch chained amino acids, so you know) which can be converted to glucose in the liver. This process goes by the unwieldy name of gluconeogenesis which just means the production of new glucose.
Protein losses during total starvation are extremely high to start, gradually decreasing as the brain switches over to using ketones for fuel (this reduces the body’s glucose requirements which means less protein has to be broken down to make glucose). Even so, during complete starvation there is always some loss of body protein. Over long periods of time, this goes from harmful (because function is compromised from muscle loss) to downright fatal. Especially as folks get extremely lean and body protein breakdown increases.
In this context, an under-appreciated fact of liver and protein metabolism (but discussed in detail in The Protein Book) is that over half of all ingested amino acids are broken down in the liver in the first place. A good portion of those can be used to make glucose and this is especially true when carbohydrates are restricted.
Switching from starvation to dieting, this is fundamentally a big part of why protein requirements go up when folks are dieting, more of the ingested protein is being used in the liver to make glucose, meaning that more total protein has to be ingested to make sure there is sufficient amounts to support things like protein synthesis in skeletal muscle.
I don’t want to discuss this in detail here (since this article is about carbohydrates) but the topic is covered to some degree in nearly all of my books. My original Ketogenic Diet had a thorough examination of protein sparing on a diet and, of course The Protein Book discusses how protein requirements change during dieting in detail.
I’d also note that, as long as protein intake is sufficiently high (e.g. the diet is covering the increased breakdown of protein in the liver and elsewhere), the amount of carbohydrates which are truly required is still zero; this is the basis of my Rapid Fat Loss Handbook approach: eliminate all non-essential nutrients (including carbohydrates) and provide only those that are essential (in this case large amounts of high-quality protein and essential fatty acids) to generate the largest deficit and maximum fat loss per day.
But, let’s assume that you don’t just want to eat massive amounts of protein, how many carbohydrates are needed to limit (or prevent) protein loss on a diet?