It is often said vegetarianism is a healthy choice when you want to live a long life. But what about bone mineral density?
Apparently, scientists wondered the same thing, so there are several studies in which a vegetarian lifestyle was examined in comparison with a traditional non-vegetarian lifestyle. A higher dietary acid load (caused by greater consumption of acid-forming foods, such as cereals and meat, than of alkali-forming foods, such as fruit and vegetables) is believed to result in bone mineral dissolution and greater bone resorption, which results in the release of carbonate, citrate, calcium, sodium, and potassium.

A diet rich in fruit and vegetables may result in a more alkaline environment, which has been shown to reduce urinary calcium excretion. Fruit and vegetables also are rich sources of antioxidant vitamins such as vitamin C and carotene. These could act by combating oxidative stress, which has been shown to be negatively associated with BMD in adults. Vitamin C also has an essential regulatory role in bone build-up and collagen formation and therefore positively influences bone health.
Vegetables are the principal source of vitamin K1 in the diet, and evidence is growing that vitamin K1 has a role in bone mineralization by acting as a cofactor in the bone-building protein osteocalcin.
This seems enough proof that in order to maintain healthy bone structure, it is important to get in enough vitamin C and K from fruit and vegetables. But how about meat intake? Does it contribute positively or negatively to bone density?

From another study in which three diets were compared, one with high meat, low meat and a third one with low meat but additional mineral supplements such as potassium, phosphate, iron, magnesium and zinc to increase the intake up to the same level as in the high meat diet. Only one mineral showed to be positively influenced by a higher meat intake: zinc. For other minerals weren't significant as long as total protein intake was adequate at 0.8g protein/kg body weight.
So, what about a low-carb diet? Unfortunately, a ketogenic very-low carb diet which too severely restricts carbohydrates, particularly potassium-rich fruits and vegetables, can have deleterious effects on bone structure.

It is generally accepted how a diet with adequate dietary calcium intake and a good vitamin D status are important factors in maintaining bone health. As a calcium-rich food, dairy is seen as the perfect choice to deliver adequate calcium.
Much to the surprise of Swedish researchers, plain milk didn't contribute positively at all. It is thought that the sugar galactose is the culprit. Milk sugar lactose consists of two sugars: galactose and glucose. (table sugar consists of fructose and glucose) . It is hypothesized galactose boosts free radical activity and the aging process.
Fortunately, in fermeneted milk products the galactose is no longer present, so cheese, buttermilk and yoghurt are associated with lower rates of fracture and mortality.

In conclusion, a moderate protein intake with fermented dairy such as cheese and yoghurt, combined with adequate intake of potassium-rich vegetables and fruits are best for strong bones. A too high carb diet that is low in protein, will not result in adequate mineral intake, while a high-protein diet with a low carb intake is too acidic and low in potassium and vitamins.
When you want to know more about what factors contribute to strong bones, take a gander at Ergo-Log.

Addendum upon the alarming news regarding unfermented dairy, several stricter experiments were done in which it was impossible to reproduce the same results. It appears there is no real correlation between dairy consumption and hip fractures after all. Still, there are enough other reasons why limiting yourself to fermented dairy.  

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