Drs. Chris and Zand van Tulleken

The kidneys are a big part of this.

This is a complex system.

When do we think this evolved?

There's calcium in the sea.

So we're going back now 300, 400 million years.

So when we were primitive fish, basically.

So this is before the first dinosaurs, isn't it?

This is pre-mesozoic.

So you mentioned these fish.

The fish are swimming around in a sea which has loads of calcium dissolved in it.

So they don't have to worry about this.

When I think it was Tiktaalik that the first fish crawls out onto land to become, eventually become us.

Did the parathyroids we think, are they in the gills?

Osteoporosis is such a huge issue.

I think we're going to do a separate episode on it.

But can you talk us through the links between calcium and osteoporosis and what osteoporosis is?

And how much of it is due to calcium levels?

And once you have osteoporosis, how much can you do to maintain or build bone density?

If someone is in their 50s listening to this, if they start doing the impact exercise and they get enough vitamin D and calcium, can they turn things back a bit?

What medications work?