I am new to this forum but have been vegan for 18 months. In 2006, as an omnivore at the age of 34, I had a baseline dexa scan done to check bone density since my ovaries were removed due to endometriosis the year before. I was shocked to learn I have osteoporosis. My T scores were -3.2 in spine and -1.8 in hip at that time. I have been lactose intolerant for years, always a low normal or underweight except for a short period of time in 2007, and have had hypothyroidism for 23 years. I found out my thyroid meds increase risk for loss of bone density over the long term, as do many other prescription and over the counter medications. I was able to improve those scores with supplementing calcium, vitamin D (at that time I took D3), weight bearing exercise, some weight gain, and increasing hormone replacement. It's hard to say how much of any of these factors made the most difference. My hip T score improved to -1.4 and spine to -3.0 (the spine is much harder to regain bone density). I have not had a dexa since going vegan which was 18 months ago (insurance covers it every two years). I refuse to take the osteoporosis drugs on the market. I still supplement with calcium but now use D2 (ergocalciferol) however I get headaches with D2 (no matter what brand I have tried or how much I take) so that has been a struggle, although my vitamin D levels have been on the high normal side. I run and cycle outside in the summer so I get my vitamin d from the sunshine at least part of the year, but I live in MInnesota so most of the year the sunshine isnt enough. I do drink fortified plant milks and at least 2 or more cups of leafy greens each day, and like others I use blackstrap molasses on hot cereals and also added to sauces for stir fries etc. I have to watch soy intake due to thyroid issues and in fact I had gotten lax with it over the last few months and my TSH shot up to 6.7 and I feel awful so I have had to get strict again and eliminate soy intake for a while.
I wouldnt supplement with the extra calcium if I wasnt on a medication that increases risk of bone density loss and didnt have osteoporosis. I think that diet is only one factor when looking at calcium and other minerals for bones. Other medical conditions and treatments for them are factors to look at also, as well as how active one is and how you use your muscles to stimulate bone growth. Hormones, or lack of, play a key role in bone density also. Those underweight and especially female athletes that do not menstruate need to consider this factor and how it may affect their bones also.
I worry about the long term calcium supplementation I have been doing but so far I have not had any problems. I only supplement 1000 mg to 1200 mg per day and get the rest from food. I have not had any fractures despite being hit by a car on my bike (though my spine was knocked out of alignment), falling down stairs, and other things. But I feel I have my age going for me yet. I have lost a little bit of height though and that worries me. I had an xray of my spine after I was hit by the car but nothing major showed up.