If you're not keeping your paths of elimination clean enough, exercise will stir up toxins that have no place to go and so they keep circulating, causing brain fog. Sustained exercise releases toxins from muscle and fat, which cause toxic symptoms.
Exercise creates toxins like lactic acid, which is a by-product of muscle energy, which the body must then metabolize and exit.
Lack of oxygen causes more toxic symptoms and more lactic acid.
Increased circulation - always a good thing - will help the liver do its job, but if the liver is already overworked, tired and congested, too many things will be left in the bloodstream.
Exercise will allow the body to sweat, and many toxins can *only* be released via the pores of the skin. Sweat is essential to having all your systems balanced. Too much and your balance is thrown off, but too little and your kidneys are overworked.
It's possible to eat foods in such a way that you do not add to your toxic overload, and it's possible to make food choices so that the toxic by-products are reduced.