Bones have a natural turnover rate – osteoclasts break down damaged bone cells (bone resorption) and osteoblasts form fresh bone cells (bone formation). This process is necessary to adapt to exercise, recover from damage, remove old cells and maintain calcium homeostasis. Osteoporosis seems to be the result of too much bone resorption and/or too little bone formation.
Osteoporosis is thought to be due to a negative calcium balance, where the body compensates by taking calcium from the bones to maintain calcium levels. Under this paradigm dairy foods are essential, low intakes of calcium cause osteoporosis and calcium supplements are therapeutic.
Calcium supplementation only reduces the rate of bone loss by ~50% (it doesn’t stop it or reverse it) and in trials of high compliance calcium supplementation reduces fractures by ~20% (the low doses of vitamin D had very little effect, but did reduce the incidence of falls by 19%). Calcium supplementation has been found to have has other effects in the body: it increased heart attacks by about 24-31% and total cardiovascular disease by about 15-18%, but decreased cancer by 14%. These results suggest a low calcium intake can be part of the problem but not the problem we are facing. Something else is signalling the excessive bone resorption.
The dietary acid-base theory of osteoporosis suggests that an acidic diet (high in protein, meat, dairy, grains and soft drinks and low in fruit and vegetables) leeches calcium from the bones to balance acidity in the kidneys. But there’s no good evidence to support an alkaline diet for osteoporosis, our body regulates pH but doesn’t use calcium to do so, and in fact protein increases calcium absorption growth factors like IGF-1 that promote bone growth
Inflammation promotes bone resorption. Inflammatory markers (IL-6, CRP), sources of inflammation (LPS, homocysteine, oxidative stress) and inflammatory diseases (IBD, CVD) are strongly associated with osteoporosis. The beneficial effects that estrogen has on bone health seem to be largely mediated through anti-inflammatory and antioxidant activity.
Exercise should be effective as it increases bone mineral density, bone strength and prevents falls. Although the trials find only very moderate effects (~10% reduction in fracture)
Vitamin K2 activates osteocalcin (a mineral binding protein). People with osteoporosis have low K2 and inactivated osteocalcin is a strong risk factor for fractures (OR: 3.1-5.9). A meta-analysis of clinical trials found that vitamin K2 supplementation in mega-doses is very effective for osteoporosis, reducing fractures by 60%.
Finally, osteoporosis/osteopenia is strongly associated with arterial calcification and cardiovascular disease   . This suggests that there usually isn’t a calcium deficit, but rather that each disease can promote the other, there’s a source of chronic inflammation underlying both conditions, a problem with calcium handling (vitamin K2 deficiency) or another common mechanism such as vitamin D deficiency or hyperparathyroidism 
“In general, postmenopausal women are advised to take calcium supplements to prevent or treat osteoporosis, implying that bone loss is due to insufficient dietary calcium. Yet, in many patients with osteoporosis, loss of bone tissue from the skeleton occurs at the same time as formation of bone in the artery wall.” 
Strategies for Osteoporosis
This is for informational purposes only and is not meant to diagnose or treat any medical condition.
Reduce Chronic Inflammation
In the absence of a low vitamin K2 intake or calcium deficiency, it seems to me that inflammation may be the main cause of osteoporosis. See Causes of Inflammation.
Being overweight is thought to be protective due to putting the bones under greater stress, and it may be, but obesity seems to decrease bone mineral density    and cause fat infiltration in bone , which may be mediated by the pro-inflammatory environment usually present in obesity. Also, T1D and T2D are associated with an increased risk of osteoporosis 
I’ve mentioned calcium and vitamin K2, but there are other nutrients needed for bone health and deficiencies in them can result in osteoporosis  (this this topic seems to be poorly researched and low intakes of nutrients could simply reflect a poor diet).
Vitamin D is often mentioned but doesn’t seem to have much effect in clinical trials , which may be due to the low doses used in the trials, and may be probably more important for elderly (who have poor vitamin D synthesis) and people with vitamin D deficiency. Interestingly, vitamin D supplementation reduces the risk of falls  , which might be related to vitamin D deficiency causing myopathy and vitamin D supplementation having a slight effect on muscle recovery in healthy people 
Even though vitamin K2 is about 3x more effective than calcium I don’t consider it ‘better’ than calcium. Rather I think the reason why vitamin K2 is more effective is because most people get enough calcium due to consumption of dairy foods, whereas vitamin K2 intake is very low because we have been told to avoid animal fats (the only source besides fermented soy). Vitamin K1 (found mostly in plants) is not an adequate substitute for K2 . The cardiovascular risks of calcium supplementation may not be an issue if one is supplementing or has a good intake of vitamin K2 because vitamin K2 (but not K1) is associated with a reduced risk of coronary heart disease*  and can reverse arterial calcification in animal models . However, vitamins A, D and K2 work synergistically so if you take K2 you may need to increase vitamin A intake and get more sun to compensate
Exercise increases BMD and bone strength, but also would likely reduce the risk of falls (and improve the landing of falls). Resistance training and power/impact exercise are preferred (and obviously weight bearing exercises)  . Bone also desensitises to the anabolic effects of mechanical loading and responds better to intermittent loading and recovery periods .