Oxidative stress impacts every aspect of the body, including the brain. It refers to the degree to which our cells are in balance with the activity of the mitochondria inside nearly every cell. (Red blood cells are a notable exception, and it wouldn’t really do to have mitochondria in the shuttle for oxygen.)
Mitochondria allow plants and animals to make a lot more ATP. Oxygen, phosphoric acid and carbohydrates go in and ATP, H2O and CO2 come out– if the system is balanced.
There are a few ways the system can get out of balance. Making more ATP than the cells can use drops the available phosphoric acid, changing intracellular pH. Fructose facilitates this tendency by skipping ATP feedback controls.
If the mitochondria can’t get H from phosphoric acid to make H2O, it will instead make OO- “superoxide”. The cell can generate an enzyme called superoxide dismutase, which turns superoxide into hydrogen peroxide (which is less damaging but still problematic). Superoxide then stiffens the cell membrane in a effort to limit insulin mediated carb uptake. But excess salt will bypass this defense.
And then there’s fat, especially solid (saturated) fats, which the cell membrane is largely composed of. Saturated fats make stiffer membranes, promoting insulin resistance.
Sugar, salt, and solid fats were all used as traditional preservatives, helping people prepare to survive winter. Insulin resistance was an important feature for extratropical populations that lived in 4 seasons. But modern times of plentiful food and no need for physical exertion have made this defense mechanism problematic.
Altitude, sunlight and temperature control all interact with these intracellular balances.