Vitamin D

Vitamin D is free.
You get it from the sun.
You can't buy it off anybody.
No company can get money for it from you. Unless they sell you eels or oily fish, which contain Vitamin D.
It can cure all manner of cancer and diseases.
Its the "happiness vitamin" because sunshine is known to lower depression and lift mood. No wonder europeans take so many anti depressants.
UV-B rays, which are what cause Vit D production in your body are most prevalent between the hours of 10am and 2pm.
Vitamin D deficiency can cause rickets and bone diseases, especially in young people whose bones are just developing.
Young people who stay indoors all summer won't have enough Vit D to tide them over winter.
Darker skinned people need more sunshine than fair skinned people to be able to produce the same amount of Vit D.

If I were a conspiracy theorist, I would be starting to see a problem here.