[chris (OP)]

Erik is wondering:

I was listening to your last podcast about the microbiome and I was wondering; If the bacteria that live on us weight nearly as much as our brain, how come after we have killed off a percentage of them after taking an antibiotic and after time they die off and we shed them how come we do not lose weight? Shouldn't killing off that many bacteria create a noticeable loss of weight?

What does everyone think?

[Bored chemist]

Dead bacteria weigh the same as live ones.
It's fairly common  for antibiotics to cause diarrhoea, and that does cause (temporary) weight loss.

[chris (OP)]

Good point, @Bored chemist

Also, bear in mind that not all of the bugs in the gut are sensitive to the antibiotics being administered. So the inevitable effect is to remove some bacterial groups (taxa), which de-represses others, which were formerly kept in check by the presence of their now-missing counterparts. In other words, there is a compensatory over-growth of other bacteria to make use of the vacant resources.

Hence the overall bacterial burden remains similar, but the relative population numbers change. This is actually what causes the diarrhoea that @Bored chemist mentioned.

[evan_au]

The microbiome contains many species of bacteria that are optimized for processing various types of food to fuel their own growth. They produce waste products which are consumed by other bacteria for their use. Some of these products may also be absorbed by the human gut.

Depending on the type of food you eat, killing off the bacteria may result in more or less energy being made available to the human.

If the food you eat is fairly human-digestible, you may find that the weight you lose in your microbiome is replaced by a similar weight of body fat?