A popular method of losing body fat is the strategy of performing cardio or other exercises in a fasted state. As in, before you’ve eaten a meal. This normally takes place first thing in the morning after someone has woken up and before they’ve had a chance to eat breakfast.
The belief here is that as there’s no food in the stomach
the body must turn to its stored energy, body fat, to fuel the activity. Doing
this repeatedly over time MUST mean that you’re losing body fat consistently
over time. Right?
Well, kind of but not necessarily.
It’s not incorrect that the body does turn to its fat stores
at time to fuel a workout, especially first thing in the morning before
breakfast, but this doesn’t always mean that you will lose body fat over time.
This will only happen if you are in a negative energy balance where you are
burning more calories than you consume over time. The rule of Calories In vs
Calories Out.
A fasted workout only leads to weight loss over time if you
DO NOT REPLACE those stored calories, that body fat, with more calories.
Let’s explain this in more detail.
To perform an activity, the body uses a molecule called ATP
to release energy into its cells. The body creates ATP by using several
different chemical processes and sources, some with or without oxygen, during
activity. You may be familiar with Aerobic and Anaerobic activity. During these
different forms of activity, the body creates ATP with or without the presence
of oxygen.
The body’s preferred sources of ATP come from Phosphocreatine,
Glycogen and Fats. Sometimes protein is used in the absence of all three, but
this would be as the result of severe depletion.
Phosphocreatine is used during the first 10 seconds of
activity anaerobic activity. The more creatine available the more explosive
energy you will have. Think of a weight-lifter lifting a heavy weight during a
bench press, or a sprinter at the beginning of a race. This is also why creatine is a popular supplement.
Glycogen is turned to during the next minute for about 45
to 90 minutes in both anaerobic and aerobic activity. At times fat comes in at the same time, but
glycogen is preferred. It’s stored in the muscles and liver and is extremely convenient
for ATP production. Glycogen is just the name for stored glucose which you get
from dietary carbohydrates. In the absence of dietary carbohydrates the body can
manufacture glycogen from other sources via gluconeogenesis/ketosis.
Finally, there is fat. Fat is the least efficient source of
ATP. It’s fairly challenging for the body to convert or switch over from
glycogen. If you’ve ever been for a long run and feel like you’ve ‘hit a wall’,
that’s the body’s reaction to switching from glycogen to stored fat.
What does this mean for fasted cardio and weight loss?
Well, this is just to point out that none of this matters
when it comes to weight loss. The body will convert whatever source is most
convenient to it at the time into ATP. And even if it’s converting fat to ATP,
if you’re still eating at maintenance or a surplus, those fat stores will just be replenished
for later use.
If your maintenance calories are 2000 and you burn 2000
calories a day through fasted activity you will maintain your weight if you continue
to eat 2000 calories a day. Any fat or glycogen that was used will still get
replenished for later use. You will not burn any additional fat. Your weight
will maintain.
All that training fasted does is get the body better suited
to training fasted or using fat for fuel.
Endurance athletes have been known to go on Low Carb/High
Fat diets in the run up to marathons for similar reasons.
Fasted cardio/fasted activity is not special. Just a different form of training.
If you want to lose body fat, just focus on CICO. If you like
training fasted, keep at it. But it is not a magic pill for fat loss.
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