This post was first published in the Run Experience in a slightly different version.
*Metabolic Efficiency is too long so let’s call it ME. Metabolic=metabolism and efficiency, well we know what it means. But the two together? How can your metabolism be efficient.
So here we go:
What is exactly Metabolic Efficiency?
It is the capability of burning fat instead of carbs for the same intensity. You can train your body’s metabolism to use more fat at higher intensity exercise so that it results in glycogen sparing and be more efficient. Glycogen is the sugar reserve that you have in your body and is in limited quantity. Does it make more sense now?
How do you achieve that?
Training: The more trained you are, the most able you are to burn fat. In fact there was a recent study that showed just that. They took 9 pro athlete and 9 recreational athlete and compared for the same RPE their performance. They actually consumed the same amount of carbohydrates, the same amount of lactate was produced but guess what the pro athletes were much faster because they burned 3 times as much fat as their counterpart, so more energy generated from fats for same RPE. So they were faster for same RPE.
Gender: Women are usually better at burning fat. Yes, for once we have an advantage. Maybe it has something to do with us carrying babies and having to rely more on our reserve to grow life when food is restricted.
Exercise mode: You burn more fat running than cycling because running is much more a whole body exercise, you recruit more muscle but at a lower intensity for each one. While cycling put a high demand on your legs muscle primarily. That is why you can get away with eating less during running compared to bicycling even when you burn much more energy.
Fasting: When you are fasting your body burns more fat because of lower insulin in your body. Insulin is known to shut down fat metabolism towards carb metabolism.
Age: As we get older we are better at burning fat. Yes really. Maybe that is why older athletes often shine in ultra endurance events.
Day to day nutrition: And that is the big one. Do you boat load carbohydrates on a daily basis or do you fuel mostly on fat? If you fuel mostly on fat your body will adapt and burn more fat. Even if on race day you decide to carb load because you want to go long and fast (and carbohydrates are necessary to do that), this effect won’t just reverse, you will still burn more fat and be able to spare glycogen. This has tremendous consequences on health and performance.
How can you measure your metabolic efficiency?
You can go to lab and get hooked on a metabolic cart. It is a machine that measures the oxygen that you breathe and the carbon dioxide that you exhale. Using an algorithm, it can tell you how much fat and carbohydrate you are burning while on a treadmill or a cycle ergometer.
From there you can derive your caloric needs during training and racing at different paces. As you increase intensity, you will start burning more and more carbohydrates.
Here are two examples on the treadmill with two different athletes. Important to note that both of them report similar RPE for similar speed:
This athlete has a crossover point between 9 and 8.5 min/mile, meaning that they start burning more carbohydrates passed the 8.75 min/mile mark.
In this second example, this athlete has no crossover and is still burning more fat at 7.5 min/mile. Interestingly, this athlete burn mostly fat at rest. Do you think that she needs to eat carbohydrate when working at her desk?
Two totally different pictures. Let’s do a little math and take the 8min/mile mark.
72% from carbs
500 kcal/hour or 125 g/hour of carbs
Approximately 30% comes from plasma glucose (from a classic paper by Luc van Loon et al. 2001)
87 g/carbs per hour
Assuming 300 glycogen reserve, that is 3.5 hours of fuel.
34% from carbs
208 kcal/hour or 52 g/hour of carbs
Approximately 30% comes from plasma glucose
36 g/carbs per hour
Assuming 300 glycogen reserve, that is 8.3 hours of fuel.
OK, I know, this might be an oversimplification. One caveat of this calculation is that as duration increases your body will naturally start burning more fat. Also we have reserves of fat stored in our muscles, the more fat burning you have the more you store. These reserve also can get depleted which leaves you with your bodily reserves which, as you know are pretty unlimited even for the leanest athlete. How easily are those reserves accessed though? It can be more challenging.
Overall this is just to show you the advantage for endurance events. There are other advantages such as health, minimizing bonking, minimizing GI distress and also less acidosis because less lactate produced.
Does it work for everyone?
Most people report feeling great when they train their body to burn more fat: less sugar craving, less bonking, less eating during training and less GI distress. Does it work for everyone? Everyone is different and you might reach a point where lowering carbohydrates too much is too low (unless ketosis is what you are looking for). Listen to your body! And remember that you still need some carbohydrates for long endurance event or even short and fast races, so timing is key! Consult with a good sports dietitian or nutritionist to know what works for you.
PS INprove.ME : the IN stands for Isabelle Nadeau and the ME for Metabolic Efficiency. You can always inprove.me. Get it? Get it?