Hi jacksa. It's not just that you are eating a lower amount of calories but it's because there is a minimum amount of calories that the body needs to keep itself alive. The energy that is in fat cells takes a while to turn back into useable energy. So if you don't have the minimum requirement of 'ready' energy this is when you will start to feel ill and your body will hold on to weight as it thinks it's being starved.
Not only that, but the packs are nutricianally balanced so in order to get 100% of all the nutrients a human needs we need to be eating the amount of packs recommended by our councillors.
I hope helps a bit more the science side of things
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