(... and the rest of the post follows!)This metastudy
points out that osteoporosis is a multifactorial disorder where nutrition only forms part of the problem. The studies it looked at included calcium in both supplement form and dairy foods, and all reported positive effect on bone health (though the author points out that there were methodological problems with those studies).
According to this paper
it doesn't appear that osteoporosis is a problem with absorbing calcium from dairy products, it's just that most people don't get enough calcium in the first place.
You could probably absorb calcium better from kale or broccoli but you'd have to eat an awful lot of them on a daily basis to meet your recommended daily intake! As a vegan who eats a hell of a lot of dark green leafy vegetables I'd be lucky to break even most days if I excluded fortified rice milk, and I take a calcium supplement just to be sure. Even when adding dairy products to the mix most people don't get enough calcium, so in my opinion at least it's more a question of quantity of calcium rather than quality of the source that's a problem in western diets.
There's also the issue of vitamin D which is needed for calcium absorption. I've heard that vitamin D deficiency is greater in the community than previously thought, so that may explain some of the problem with osteoporosis. I walk for between 40 minutes and an hour per day and have done so since I was a teenager so I'd get more sun than many other office workers, yet I was diagnosed as vitamin D deficient earlier this year which was a bit of a wake up call!
The book 'Eat, Drink and Be Healthy' sounds interesting. I might see if my local library has it. Thanks for starting this conversation, Steve! It's been really interesting researching this. In fact I'm going to look at changing my supplements to calcium citrate (costs permitting) because I didn't realise calcium carbonate created intestinal gas.