It used to be easy. Children drank cow’s milk. How much? Whatever their parents told them to drink.

Now medical research at St. Michael’s Hospital in Toronto, Ontario, has fine-tuned the milk mantra: For children between the ages of 2 and 5, two cups a day is about right.

Less, and children might not get enough Vitamin D. More, and they might lose iron.

And before age 1, they shouldn’t have cow’s milk at all.

A Vitamin D deficiency in children has been associated with bone health issues.

Iron deficiency has been associated with anemia and delays in cognitive development.

This study involved more than 1,300 children and sound scientific reasoning.

But the answer — not too much, not too little — is a good formula for many questions in life, for example, how much the federal government should raise taxes or cut spending.

Not too much, not too little works in almost every case.

The challenge is figuring how much is too much and how much is too little when people whose doctorates are in economics can’t come close to agreeing.

And when politicians do what they do best — milk the issue.