A couple of weeks ago I wrote Running on Fat in which I said that glycogen was king. The current paradigm can be summed up as things go better with carbs. That is before during and after. Muscle glycogen depletion during exercise is the main factor in the onset of fatigue. If you want to exercise again in a hurry you need to get some carbs down in a hurry. The amount and type vary from paper to paper and there is an unresolved debate about the addition of protein. Overall though it is suggested that 6 to 12 grams of carbohydrate per kilogram within 30 minutes of completing an exhausting workout should have you ready to go again the next day.
Since my muscles are not being rewarded for their efforts with a jar of marmalade after every session I have been wondering how much of a disadvantage I’m putting them at. And it’s not that easy to find out.
Some short term research has been done putting normal (high carb) athletes on low carb diets for three weeks and watching their performance suffer. Hardly surprising that it goes down hill it takes a few weeks to sort out your fluid and electrolyte balance and adjust to ketosis.
The body can make it’s own glucose from fat and if it’s starving it can turn to protein. In the absence of ingested carbohydrate does glycogen replacement grind to a halt or does gluconeogenesis step into the breach?
I was pleased to come across a paper by Volek et al describing research with 20 well matched elite athletes 10 of whom were regular high carb guys and 10 were low carbers (and had been for at least 9 months). Naturally, when you get your hands on such a group, you take muscle biopsies and put them on a treadmill for three hours, take more muscle biopsies and measure everything you can think of. Then you give them two hours to recover before taking another muscle biopsy!
The rate of fat oxidation was two to three times higher in the low carb athletes and it peaked at a higher level of effort. Glycogen stores, usage and replacement were very much the same in both groups.
Their conclusion …
Compared to highly trained ultra-endurance athletes consuming an HC (High Carb) diet, long-term keto-adaptation results in extraordinarily high rates of fat oxidation, whereas muscle glycogen utilization and repletion patterns during and after a 3 hour run are similar.
At the ultra marathon level the benefits of using fat as fuel are appealing to more and more competitors, you just don’t run out.
Back in my marathon days (30+ years ago) I remember reading a prediction that it wouldn’t be long before women were beating the men over the magic distance of 26.2 miles. The logic was very simple, fat is an excellent fuel, fit women athletes carry more fat than fit men athletes therefore women would be better over long distances than men once they matched the men in training.
So far it hasn’t happened, fastest man – Eliud Kipchoge of Kenya 2:01:39 fastest woman Brigid Kosgei also from Kenya 2:14:04. Getting on for 10% slower.
At about 9 calories per gram fat is an excellent fuel trouble is for endurance sports the body is very happy burning glucose and glycogen even though they pack only 4 calories per gram. Stores of glycogen exist in muscle and in the liver amounting to about 600 grams. In ball park figures that’s about 2400 calories for a marathon requiring about 2600. A runner can expect to absorb 50 to 60 grams of glucose from the gut per hour which easily makes up the shortfall.
So fat burning doesn’t really get into the equation during an elite marathon. If it did even the thinnest male athlete has enough fat to go the distance.
Glycogen is king. Glycogen replenishment happens faster after a carbohydrate rich meal. What are the implications for the athlete following a keto diet? Depends who you ask. According to Harvey, Holcomb & Kolwicz the keto athlete is operating at a distinct disadvantage although it is an excellent diet for weight loss.
Dr Caryn Zinn on the other hand is more optimistic …
while Professor Asker Jeukendrup sums up what is known but leaves the question open. That article is well worth reading.
Fat oxidation rates are on average 0.5 grams per min at the optimal exercise intensity. So in order to oxidise 1kg of fat mass, more than 33 hours of exercise is required! Walking or running exercise around 50-65% of VO2max seems to be an optimal intensity to oxidise fat. The duration of exercise, however, plays a crucial role, with an increasing importance of fat oxidation with longer exercise.
There is no doubt that the reforming couch potato can successfully lose weight and burn fat at moderate rates of exercise on a keto diet. Fat around the middle is as much a handicap as lead in the saddle bags. VO2max is the upper limit of your ability to burn fuel a good measure of your fitness. More precisely it’s milliliters of oxygen consumed in one minute, per kilogram of body weight (mL/kg/min) at sustained maximum effort. In other words get the kilograms down and the VO2max goes up without any extra training.