- Consuming less food requires the body to use fat-reserves. Some of these might have been storing heavy metals. With a quick enough change in diet, these might pose a significant risk.

- Consuming less food might alter your gut microbiology. Disheveling this balance might create the risk of infection or even worse.

- Consuming less food (to be precise, less lipids), might alter, or even disable certain parts of the metabolism, often leading to deficiencies in supplies for the immune system, thus making us more vulnerable to 'poisons'.

I don't know which of the above hypotheses are correct, but I am sure they are just as (un)likely as yours is.