I think 4kg over 3years is a huge difference for many people. Not for morbidly obese maybe. Anyway, here it matters that difference exists.
There are many ways to make this difference in energy balance, by changing the kind of food eaten, while keeping the same intake volume, by changing the intake volume, or by adding an additional activity, like brooming.
Reducing intake by 100kcal by changing volume while maintaining composition is always going to be carbon wasteful. Do we agree on this?
There are many advisable ways to reduce the carbon effect. By changing the kind of food eaten, for sure. But also, but replacing manual brooming with less carbon-consuming process. One way does not cancel the other, does it?
By the way, we should be clear that instead of brooming one should not go for a run on something. Conversely, replacing some of the health-motivated physical activity with brooming is not a bad idea at all, that’s a large part of the reason I still do it. Still, both sport and manual brooming are somehow wasteful.