Bipolar disorder and bone mineral density

There are decreases in bone mineral density in drug naive individuals with bipolar disorder compared to age- and gender-matched healthy controls. Individuals with bipolar I disorder have have high homocysteine levels. High homocysteine levels in individuals with bipolar disorder point to the transsulfuration pathway being dysregulated. Via the transsulfuration pathway L-cysteine is synthesized from homocysteine. L-taurine is synthesized from L-cysteine.

Taurine is required for calcium homeostasis. Taurine, also, is conjugated to various bile acids. Bile acids are are required for absorption of fat-soluble vitamins. Vitamin D and vitamin K are fat-soluble vitamins. Individuals with bipolar disorder are 4.7 times more likely to be vitamin D deficient than individuals amongst the general population of the Netherlands, however, deficient levels of vitamin D are not specific to bipolar disorder but are also present in individuals with schizophrenia. The taurine transporter is present in osteoblasts. Osteoblasts synthesize bone.

With taurine metabolism dysregulated calcium homeostasis is dysregulated and absorption of vitamin D and vitamin K is decreased. Decreases in bone mineral density in bipolar disorder could be due to dysregulation of the transsulfuration pathway which dysregulates calcium homeostasis and vitamin D and vitamin K absorption resulting in low bone mineral density.

Calcium homeostasis in osteoblasts and osteoclasts and bone formation and bone resorption

Osteoblasts are involved in bone formation. Osteoclasts break down bone. Dysregulation of calcium homeostasis in osteoblasts and osteoclasts could lead to bone abnormalities. In the forward mode of the Na/calium exchanger calcium is effluxed from cells while in reverse mode there is an influx of calcium into cells via the Na/calcium exchanger.

Taurine inhibits the reverse mode of the Na/calcium exchanger. The Na/calcium exchanger (NCX) is expressed in osteoblasts. The taurine transporter is expressed in osteoblasts. In osteoblasts inhibting the reverse mode of the Na/calcium exchanger would increase calcium net efflux from osteoblasts which would increase bone formation. Taurine inhibits osteoclastogenesis through the taurine transporter. In osteoclasts inhibiting the reveres mode of the Na/calicum exchanger would inhibit the influx of calcium into osteoclasts from bone which would inhibit bone resorption.

Extracellular calcium levels are very tightly controlled and are very, very frequently tested. However intracellular homeostasis of calcium in osteoblasts and osteoclasts could be very imporant as to whethere there is bone bone growth or bone resorption. Taurine is involved in calcium homeostasis in cells. Taurine both increases bone growth and inhibits bone resorption. The positive effects of taurine on bone fomation could be via the inhibition of the reverse mode of the Na/calcium exchanger in osteoblasts and osteoclasts.