To better understand just what happens, imagine your body as a manufacturing plant with tens of thousands of production lines. Each line is responsible for making a certain chemical. It does this by executing a series of reactions, using nutrients as its raw materials. If a line doesn't have enough of a certain nutrient, the corresponding reaction cannot proceed. Production grinds to a halt until the nutrient is restocked.
Biochemists refer to the interrupted reaction as a rate-limiting step.
That's not so bad. If a nutrient remains out of stock for a long time, or if it repeatedly runs out, however, trillions of cells will be affected. That's very bad. A chronic nutrient deficiency undermines health and promotes degenerative disease, although the symptoms may not appear for many years. Running low on beta-carotene, for instance, opens the door to most types of cancer.