Both our bodies and those of our horses have a variety of mechanisms for compensating for the ups and downs of the diet. When excesses occur, they can be excreted in the urine, bile and into the digestive tract. The only exception to this is iron, which the body holds onto as if it were gold.
It’s also possible to slow down excretion of nutrients in poor supply, sometimes to absorb them in greater amounts than normal and also to save some of them (minerals and fat-soluble vitamins) away in the tissues for future use. These compensatory mechanisms work a lot better for people because we eat a much more varied diet than our horses. What’s missing in one of our meals may be in abundant supply in the next one, or one the next day, but horses usually eat exactly the same thing day in and day out for months on end.
The bodily adjustments also have their limits. They can’t create a mineral out of thin air if it’s not in adequate amounts and can only deal with so much excess before the mineral starts to accumulate with harmful effects.