Vitamin D and Muscle Wasting

Dr. Robert Mak, a pediatric nephrologist, gave a talk at the 2014 CRF family conference about the effects of Vitamin D on muscle wasting in cystinosis.  His talk generated a lot of excitement and interest, so I thought it would be worthwhile to break it down here.

Muscle wasting is one of the major complications of cystinosis, and it is typically seen later in life, in the second and third decade.  Cystine accumulation damages muscle cells.  The muscles predominantly affected are swallowing and limb muscles.  Many patients with cystinosis have trouble swallowing, especially as they get older, and the deterioration of these muscles can lead to aspiration of food and saliva into the lungs, which can cause serious complications, even death.  Wasting of the limb muscles can cause weakness in grip strength and affect dexterity, as well as exercise endurance.

Currently, there are no great therapies for muscle wasting.  Cysteamine depletes the cystine in the muscle tissue, but even patients taking cysteamine eventually develop muscle wasting.  Many cystinosis patients take carnitine supplements since this compound is wasted in the urine.  Carnitine is required for muscles to break down fat into energy, and carnitine deficiency leads to accumulation of fat in muscle tissue.  Giving patients carnitine supplements will normalize blood and muscle levels of carnitine, and it reduces accumulation of fat in muscle tissue, but no studies have been done to assess whether carnitine replacement results in higher muscle mass or better growth in the long term.   Dr. Doris Trauner, a neurologist, mentioned in her remarks at the CRF conference that other proposed treatments for muscle wasting include coenzyme Q and the different B vitamins. 

Dr. Mak has been studying the effect of vitamin D on muscle wasting using a mouse model for cystinosis.  In the mouse model, the gene that codes for the cystinosin protein has been "knocked out," so the mouse no longer makes the protein.  This effectively creates a mouse with cystinosis, and it is a good surrogate for testing different therapies, like vitamin D. 

Normally we make vitamin D in our skin, with exposure to sunlight.  We can also get vitamin D from fortified foods, or from supplements.  If the supplement is from a plant, it's called ergocalciferol, or D2.  If it's from animals or our skin, it is called cholecalciferol, or D3.  Whether we make it in our skin or eat a supplement, the vitamin D is modified by the liver to make 25-Vitamin D.  This form of vitamin D is then modified again by the kidneys, to 1,25-Vitamin D, which is the active form.  This form is also called calcitriol, and it is required to maintain calcium and phosphate levels in the blood and promote bone growth and remodeling.  1,25-Vitamin D increases absorption of calcium and phosphate in the intestines, both of which are needed for bone mineralization.  Patients with renal failure often have to take calcitriol because their kidneys can't make the active form anymore.

Dr. Mak pointed out three reasons cystinosis patients have low vitamin D: they lose it in their urine, they spend less time outside in the sun because of photophobia, and they develop chronic kidney disease. Doctors have always known that it is important to treat vitamin D deficiency in cystinosis patients, since vitamin D deficiency leads to rickets (in addition to the phosphorous wasting seen in Fanconi syndrome).  Low vitamin D levels may also be bad for the muscles.

Dr. Mak has shown that cystinosis mice with vitamin D deficiency have smaller muscle fibers, weaker grip strength, poor balance, and energy wasting.  He has also shown that vitamin D deficiency leads to genes being turned on that break down protein, which leads to muscle wasting.  He treated cystinosis mice with 25-Vitamin D (the kind your liver makes) and 1,25-Vitamin D (the active form that your kidneys make).  He found that in the mice treated with 25-Vitamin D, there was restoration of muscle mass, muscle fiber size, grip strength and balance, more than in the mice treated with 1,25-Vitamin D.

Remember that 25-Vitamin D is just the regular over-the-counter supplement after it is modified by the liver.  This is the form that the muscle uses, because the muscle has its own enzyme to activate it.  So while 1,25 Vitamin D (calcitriol) is crucial for bone health, it does not appear to be as important for muscle health. 

So what is the take home message?  Vitamin D deficiency is bad for muscles, so taking a vitamin D supplement is a good idea if you have low levels, and it may even help prevent muscle wasting.  This doesn't mean you should just start taking big doses of vitamin D, however, because you can theoretically get vitamin D toxicity.  Dr. Mak and Dr. Grimm said that if your doctor hasn't done it already, get your vitamin D levels checked, and if you are deficient, then take a supplement.

We built Sam and Lars a rock climbing wall in their bedroom to help them with limb muscle strengthening, especially arms and hands.  We have absolutely no evidence that it will make a difference in the long run, but it seems like a good idea, and they have a lot of fun with it!