Q: I ate a whole pizza on Wednesday after expending 4000kcal that day (6km run, 90 mins touch football and living). One week later, weight is still up by 3kg. Is this water retention or has the pizza caused 3kg fat gain?
The most likely cause would be water retention.
Pizza contains a lot of carbohydrates which are stored in the muscles and liver as glycogen.
Each gram of glycogen brings 3-4mls of water along with it.
So if you had depleted your glycogen - which is likely with that level of activity - there is now the capacity to store 350-500g of glycogen.
Let’s run the numbers on that using 425g of glycogen and 3.5 ml of water per gram of glycogen, using the middle of the range numbers.
425g + (3.5ml/g*425g) =
425g + 1487.5g = 1912.5g
Add in some dehydration from the physical activity (1-2% body weight) maybe a bit of body fat, and that could very well explain the 3kg.
The timing of the measurement would be important.
If the measurement was in the evening before eating the pizza, when glycogen and hydration were low, this would be the clear option.
Even if both measurements were made in the morning - which helps control hydration status - the 3kg weight gain is still likely from the water.
Water fluctuations can interfere with measuring body fat progression, which I will expand on tomorrow.