Body bacteria weigh far less than 2 kg

A modern estimate for a 70-kg “reference” adult man is about 0.2 kg of bacteria in the body, roughly 0.3% of body mass. That is far below the older popular claims of 1–3 kg. The estimate varies by person and with changing gut contents, so 0.2 kg is not an exact value for everyone. There are still trillions of bacterial cells; low mass does not mean low biological importance.