The good news is that it's harmless, provided you don't faint, hit your head on something hard/sharp & fatally fracture your skull, or do it regularly. Stop and sit/lie down until the feeling passes. What's happening?
What isn't happening, is a Somogyi rebound. A Somogyi rebound happens when too much insulin is injected and it's bad because it results in rebound hyperglycaemia.
Did you know that there is insulin, insulin and insulin? According to Insulin: Degradation, "It has been estimated that an insulin molecule produced endogenously by the pancreatic beta cells is degraded within approximately one hour after its initial release into circulation (insulin half-life ~ 4–6 minutes)". According to Insulin: As a medication, injected insulin lingers in the blood for hours for fast-acting and days for slow-acting. This makes an overdose of injected insulin dangerous, as it can linger for long enough to cause fatal hypoglycaemia unless medical help is obtained.
What might be happening, is this: On a ketogenic diet, muscle glycogen stores are "trickle-charged" from blood glucose. High-intensity exercise oxidises muscle glycogen at a very rapid rate. See It's all in a day's work (as measured in Joules). Muscles can oxidise carbs at a rate of 4g per minute.
If muscle cells run out of stored glycogen, they become exquisitely sensitive to insulin and glucose importing processes are up-regulated. There is only ~4.5g of glucose in the blood at any given time, topped-up by the liver & kidneys and oxidised by the brain & red blood cells at a rate of less than 4g per hour (if keto-adapted). If muscles suddenly take up glucose at a rate of 4g per minute, blood glucose level can fall rapidly.
As dropping dead due to running for your life on very little or no food is bad, the body has several mechanisms for raising blood glucose.
From Blood Glucose, Insulin & Diabetes, "When BG falls to ~3.3mmol/L, the pituitary gland kicks-in and secretes ACTH (adrenocorticotropic hormone) which stimulates the release of cortisol from the adrenal cortex. Cortisol further stimulates gluconeogenesis in the liver & kidneys. When BG level falls to ~2mmol/L, the pituitary secretes GH (Growth Hormone) which has an anti-insulin effect." Adrenaline secretion increases, as adrenaline shifts fuel utilisation away from carbohydrate and towards fat. A sudden increase in the levels of adrenaline, cortisol & GH in the blood make you sweaty, dizzy & feel faint, so you stop exercising.
If it happens regularly, chronically-elevated adrenaline & cortisol levels result in you "falling apart" after about two years. See Survival of the Smartest (part 2) - Dr Diana Schwarzbein.
How come it's harmless? Glycogen-depleted & exquisitely insulin-sensitive muscles act as a natural blood glucose limiter because they draw in glucose so readily. There's no rebound hyperglycaemia.
This also reduces the "Dawn Phenomenon", another plus point for low-carb diets for people with T1D.
See also Can very-low-carb diets impair your mental faculties.
11 comments:
Interesting. How does depleting glucose stores on a ketogenic diet compare with doing so on a non-ketogenic diet? Wouldn't it just be a case of burning a bit more glucose to get to the same place?
I think that muscle glycogen stores will be more full on a non-ketogenic diet and therefore take longer to deplete.
A marathon runner who starts a race with full muscle glycogen stores depletes them completely before the end of the race and "hits the wall", though not as hard as someone doing high-intensity exercise.
Yeah, the marathoner's wall is apparently hypoglycemia. But is that the same wall one hits in high intensity effort?
I used to run the 400m in high school, and there one hits a wall also, at about 300m.
With high-intensity exercise, there's also lactate accumulation & electrolyte imbalance to cope with. See Why do muscles hurt after exercise?
The dip in BG due to high-intensity exercise is rapid but it goes away just as rapidly, so it won't show up on a finger-prick BG meter.
Yeah, the Cori cycle, and other things.
Did you know that the kidneys can also turn lactate, pyruvate, glycerol and AAs into glucose? See Liver and Kidneys Synthesize Glucose.
Did you also know that the body has 4 ways to produce ATP anaerobically?
1) Substrate-level phosphorylation in glycolysis.
2) Substrate-level phosphorylation in Krebs (produces GTP). (GTP+ADP=>GDP+ATP)
3) From PhosphoCreatine & ADP using creatine kinase. (PC+ADP=>C+ATP)
4) From ADP using adenylate kinase (ADP+ADP=>AMP+ATP)
From Medical Biochemistry at a Glance.
I am reposting in case my post was lost:
According to Wiki, marathon runners usually deplete their glycogen stores by 20 miles if they do not get refueled with carb-containing drinks of jellies. I suspect they take good care of their gl.stores through carbo-loading and avoiding exercises before the race.
When I did the low-carbing and tried to brake my 2 year long plateau with almost daily exercise, I usually "hit the wall" in 1.5 hour. My usual fasting BG was 84 - 87 mg/dl(4.7 - 5 mmol/L)
After I started a ketogenic diet 1 year ago, I discovered, I had a trouble to last longer then 45 min of intense cardio - if I persisted , I would get a migraine. I don't do it much lately. It is mostly yoga and walking now. Last time my fasted BG was 4.0. I have to get my own measuring device and get more information for observations.
I have to admit that I believe that a human body must have other pathways of putting a glycogen into a storage . I remember reading an article in the "Outdoor" magazine last year about a physiology of sled dogs during a race. They were fed a diet consisted of 70% fat and the rest protein. During the race they manage to synthesized glycogen. We are different, of course. But their example proves it should be carbohydrate and protein independent was of producing a glycogen.
It sounds like you get a migraine instead of a "funny turn" when you overdo it.
Protein can be turned into about 50% glucose. If the sled dogs have high endurance fitness and are not running flat-out, they could be burning mostly fat for energy. This would spare their muscle glycogen stores, which are continually being "trickle-charged" from blood glucose.
Interesting, this possibly explains what I've sometimes experienced.
I've been LC (or possibly VLC) for about 18months.
Once a week I a game of five-a-side around middday. I eventually ended up playing this fasted, as it was more comfortable.
However, sometimes towards the end of the game (around 50 mins out of 60), I'd end up feeling lightheaded and with a 'cotton wool' / fuzzy feeling at the back of my head.
I had been guessing I was operating with little muscle glycogen, as for most of the game my sprint ability would be poor.
Hi Nige,
I don't think this is what happened to me because it sometimes happened just when I was getting up in the morning or woke up in the middle of the night, and when it was prompted by exercise it was stuff like Pilates, nothing too vigorous.
I don't think it's harmless. Yeah, harmless if it passes there's no evidence of any lasting damage, but, and I think my situation was hypoglycemia, what if I *do* pass out?
I want to at least get my ambulance ride ;-)
Hi CarbSane,
Drats! I don't know what you experienced. I've had hypoglycaemia, as part of an insulin stress test (10units of fast-acting IV insulin) on my pituitary. My BG fell to 1.5mmol/L (27mg/dL), but all that happened was that I apparently became confused. I didn't notice anything out of the ordinary other than some shaking & sweating.
If you do faint, would your hubby know what to do?
Nige.
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