On the surfing trip to Devon we watched The Devil Wears Prada on DVD. The best line for me was the fashion waif screaming at the heroine, with the deepest insulting angst:
"and you eat CARBS"
Well, it made me laugh. What made me think a little more was the same character talking about her latest diet, I paraphrase only slightly:
"My new diet doesn't of course include any food. I simply don't eat until I am about to pass out from hypoglycaemia and then swallow a small cube of cheese"
This was used to maintain the skeletal look so prized in the world of high fashion. Assuming the whole of the film is utterly true to life (except perhaps the too good then fashion corrupted heroine), I started to think about the physiology here, and about the physiology of that life threatening illness, anorexia nervosa.
Now, anyone of us relatively normal people on a lowish carb diet will never become hypoglycaemic. If we don't eat we just convert the stored triglycerides in our adipose tissue to non esterified fatty acids in our blood stream and use these to fuel our muscles. The glycerol from the triglycerides is half a glucose molecule, we can join two of them together to make glucose. Filling muscles with palmitic or stearic acid makes them insulin resistant enough to spare glucose and so maintain an adequate plasma glucose concentration to keep our brains working. Brain tissue cannot suck glucose out of plasma. It gets it by diffusion down a concentration gradient. You need at least 3.0mmol/l in your plasma unless you are in deep ketosis, when you can get by on a shade less.
But fashion waifs clearly can get hypoglycaemic if the script of the film is true. They are comparable to, or thinner than, anorexic patients. Here are the patient details of a group of ten anorexic patients. Note the % body fat in the anorexics is 4.1%, but we don't get the range. Some of these women will have body fat percentages well below 4%. Blood glucose averages below 4.0mmol/l and insulin is low. Click to enlarge.
They are not insulin resistant by HOMA-R guesstimate. They are starving yet they are exquisitely insulin sensitive. No one thought to measure their non esterified fatty acids.
What would happen if a healthy human being, such as the women in the control group with a body fat around 20%, were to eat nothing until they needed that cube of cheese? They would release NEFA from their fat, become insulin resistant and keep their blood glucose at physiological levels. They might LIKE some cheese (me too), but eating it to raise blood glucose is not needed.
Fashion models and anorexics do fast. But they have no significant body fat and their silicone implants (you have to put something in the bust of the clothes) will not release NEFA. So they can't release enough palmitic acid from their non existent adipose tissue to induce insulin resistance, so will be hypoglycaemic enough to faint if they were to move their residual muscles enough to soak up their blood glucose.
They have similar body fat % to patients with lipodystrophies such as Berardinelli-Seip syndrome which deprives people of all adipose tissue, but there the similarity ends. BS patients eat but can't store energy in fat cells so dump it in their muscles (plus anywhere else they can put it!) and become so intensely insulin resistant they become diabetic. Anorexia patients have no fat but lots of empty adipocytes and empty muscle cells, all aching for calories of any sort, glucose included.
The fashion waifs have no glycogen in their liver and no protein in their muscles (you know the look, where upper arm is thinner than the elbow, lovely). Dropping in some saturated fat from the cheese will allow transient NEFA production and the protein will allow a spike of gluconeogenesis. Blood glucose under these circumstances then rises to a level which transiently restores a semblance of brain function.
So there ARE situations where saturated fat (plus some protein) can be used to raise blood sugar. I would strongly suggest no one ever gets in to such dire straits!