When we typically look at intermittent fasting (skipping breakfast) and exercise performance, we see a decline in overall work capacity.
However, these are when the exercise sessions are done in a fasted state.
The question is; how does skipping breakfast impact performance when training later on in the day [5pm]?
In a 2015 study by Clayton et al, they examined how skipping breakfast impacts work capacity, desire to eat and overall calories consumed within the day.
This test was done in 10 habitual breakfast eaters, so when they skipped breakfast, levels of hunger increased and levels of fullness decreased in the morning – which makes sense.
This caused them to overeat slightly at lunch time when they were given a buffet style meal and told to eat until ‘comfortably full and satisfied.’
However, they ended up eating slightly less at dinner – meaning that overall calorie intake for the day was less, and therefore can be a nice way to decrease overall calorie intake and portion sizes without having the need to count calories (for some).
When these gentlemen jumped on an exercise bike and underwent a performance test, they ended up performing 4.5% less work that those who ate breakfast.
When we take a closer look at the data, those who ate breakfast consumed ~258g of carbs before their 5pm cycling test. Where the breakfast skippers only consumed ~148g of carbs before their test.
This makes sense – we know that high intensity exercise uses carbohydrates for fuel at a high rate. Therefore having more carbohydrate prior to the exercise bout is more advantageous for performance…
An interesting take home from this study was that afternoon subjective measures of hunger, fullness and overall desire to eat were the same as between groups. So, if you can manage the increased appetite in the morning, skipping breakfast will be beneficial for fat loss.
But then, you’ll have the compromise of worse performance.
I would imagine that if both groups were programmed to have exactly the same amount of carbs before their performance test, the performance gap would be far smaller…
Some food for thought.
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