It makes sense. When we exercise we burn more energy. Our body needs more fuel, so we become more hungry and are driven to eat. Nothing groundbreaking there. Have you ever experienced though, on your days off from training, that you can't keep yourself away from the fridge? How come we can often become more hungry on our rest days?
At this point, we'll briefly cover some basic physiology and why we become hungry. There are other factors at play in determining how hungry you get. Not just the amount of exercise you do.
Our appetites are determined to a certain extent by the following factors:
- lean body tissue - i.e. your body composition and the amount of muscle mass you have
- Your basal, or resting metabolic rate
- How your stomach and the digestive system responds to your food intake
- Hormone levels - particularly insulin, ghrelin and leptin, among others
Here's some things to think about which might explain why we feel hungrier on rest days.
How exercise affects our hormone level
There is evidence that levels of our appetite hormone increase when we are in an energy deficit.
Put simply, the day after a big training day our body wants to replace the energy we've burned up.
It will increase the level of certain hormones that signal our brain to eat more. Our body is literally telling us it needs more fuel.
Like the fuel warning light on your car.
Not wanting to eat after exercise
You've probably experienced this at some point or another. You've trained particularly hard, at high intensity, and afterwards, you just don't feel hungry.
It might feel confusing, as you think you should eat, but your stomach isn't growling and you feel fine going without food.
When we train particularly hard, our muscles need lots of oxygen to keep performing. Our body will divert blood flow to these working muscles, so our red blood cells can deliver oxygen.
As a result, our digestive system may have its blood flow restricted. Our body prioritising the external activity over its internal processes. This is sometimes referred to as 'Fight or flight' versus 'Rest and digest.' Since our gastrointestinal activity is slowing down, our appetite signals diminish with it.
It appears that there is a bit of a delay in evening out our energy expenditure. There can be a lag of 1 or 2 days, or even longer, for the digestion to fully kick in and the body to start producing the hunger hormones at full capacity.
Regular exercise may affect, and improve our hunger and satiety signals
There is a theory, namely the glycogenostatis theory, that suggests it is glycogen availability that signals our appetite.
Our brain may be monitoring how fuelled up our muscles are and, as a result, provide signals to restore the energy balance.
When we exercise, our glycogen stores within the muscles deplete. Our body would naturally want to replenish these fuel stores, with the view of increasing carbohydrate levels in the body.
Glycogenostatis suggests our 'empty' muscles signal to the body to eat more, thus restocking carbohydrate. Exactly how this process may work, and the pathways involved, are still unknown at this point. It is an area of developing research.
Exercising regularly may affect our conscious food choices
This one I hear quite a lot from my clients and class participants. You may well have experienced it yourself. Once you are underway in a new exercise habit and have your routine set, you seem to want to crave healthier food.
It seems that regular physical activity may change the way our brains view macronutrient. This then affects the food choices we subsequently make.
I've said it before - our bodies are smarter than we are.
It makes sense that if we were to exercise strenuously, our body would naturally want lean protein and complex carbohydrates with which it can rebuild and refuel. There may indeed be a deep biological drive to seek out particular foods so our bodies can replenish blood sugar and/or glycogen levels.
It is known that if we become excessively hungry, we can start to crave sweet foods. Naturally sweet foods are those that are high in carbohydrates. Our body is crying out for more fuel.
Training interfering with meal times
For some people, it can simply be a case that they essentially miss a meal due to their workout schedule.
Without realising it, overall calorie intake may reduce. Instead of dinner, we might just have a 'quick snack' before we hit the gym. Then when we get home, we don't want a full meal before going to bed, so we just have a protein shake.
Hands up everyone who recognises that scenario.
We haven't necessarily skipped a meal, but we may experience an increase in appetite because our body is trying to recoup the lost fuel intake. Again, there can be a lag in the body's response, so our appetite may only increase gradually over a few days while we are still training.
Then when your rest day comes around... BAM!
The full effects of your appetite signalling take effect.