When I was in school, I had to take an advanced nutrition course that examined the biochemistry of fats, carbs, and protein. In that class, my professor argued that by starving yourself or fasting for long periods of time, you may actually end up preparing your body to store more fat.
Turns out, this was only half the story. I’ll try to explain…
What Happens If You Don't Eat For A Long Time?
If you don’t eat for long stretches of time, the body is forced to find something for fuel (energy). The theory has been that when we don’t eat for a long period of time, the body says to itself, “Uh, oh! We haven’t had anything to eat for a while. I better hang on to my best source of long-term energy!”
Do you know what our bodies’ best source of long-term energy happens to be?
You guessed it: fat.
So according to this idea, when we don’t eat for a long stretch, our bodies say, “I don’t know when we’re going to get our next meal, so I better hang on to fat because I may need it for energy!”
Does this happen in reality?
Short answer: not necessarily.
This is because there are a number of factors that determine whether the body burns fat for fuel. Fasting isn’t the only factor. A person’s age, gender, current level of fitness, muscle mass, and typical diet are important to consider as well.
Marathon Runners and Burning Fat
Let me give you an example: researchers love to study marathon runners. As you can imagine, they're great test subjects because they are a very unique group of individuals – they are like real-life superheroes! Think about it: they can run continuously for long distances for hours on end without stopping. But if you were to look at their body type, many (not all) have a very thin physique. Their body fat percentages are extremely low.
That made researchers think: “It doesn’t make sense! Where do they get their energy from to run for that long??” You would think they would store a lot of fat so they could use that for energy when they’re running. Well, here’s what they learned…
Marathon runners often have a low body fat percentage because they have trained their bodies to burn fat for fuel ALL THE TIME. Because they have such a high fitness level, their bodies are so used to burning fat for fuel, they have trained their bodies to make burning fat a priority.
Why Fitness Level is Important to Consider
Here’s what happens in a research lab when they’re studying marathon runners, triathletes, or really anyone with a high fitness level: they hook these folks up to all these machines so they can see how their bodies respond to different scenarios. Researchers will say, “Ok, Mr. or Ms. Marathon Runner… now that we’ve hooked you up to all of these machines, we’re going to begin testing you. Go ahead and step on this treadmill and we’ll begin.”
The runner steps up on the treadmill and the machines immediately start to go haywire. The researchers look at the machines going haywire and think, “Something must be wrong. We haven’t even started the testing but the data is saying our marathon runner is already burning fat for fuel!”
The machines aren’t wrong. We’ve learned that those with a high level of fitness, like marathon runners, burn fat for fuel pretty much all the time. They merely take a few steps and their bodies say, “Well, we’re moving again… must mean we’re going to be moving for another 26.2 miles, so better start burning fat for fuel.” Even though the marathon runner is, let’s say, just walking from the parking lot to the grocery store, they have trained their bodies so well that anytime they move, their bodies think it’s time to burn fat for energy. Remarkable, right? This is why fitness level is so important to consider.
If you go without eating anything for an extended period of time, will your body then be more likely to store fat?
I will repeat my answer: not necessarily.
Now hopefully you know why.
Calorie Deficits: Conclusion
The bottom line is this: in the beginning, if your body isn’t used to something, it will resist you.
If your fitness level is average to below-average and your body is used to getting some food (or fuel) every 3-4 hours, but then you up and decide to not eat for a long period of time, it will probably hang on to its fat stores. But if you consistently reduce your calorie intake for weeks and months and create a sustained calorie deficit, the body will adapt and will eventually turn to burning fat for energy. You basically leave it no choice.
So-called yo-yo dieting, where folks start and stop diets, can delay the body from turning to fat for fuel. But long-term calorie deficits will force the body to use fat for fuel. When will that happen? Again, it depends on your age, gender, and of course, your fitness level.