Not enough Mn and Vitamin D   muscle cramps
Magnesium and Vitamin D  Vti D Council Newsletter   July 2009
Dr. Cannell: Is it important to take magnesium with vitamin D? Judith, New York
Yes, it is important to have adequate magnesium intake and most Americans do not. A number of people have written about muscle cramps after they start sunbathing or taking Vitamin D. This is likely caused from the neuromuscular hyperexcitability of magnesium deficiency that is somehow unmasked by higher Vitamin D levels. Abbott LG, Rude RK. Clinical manifestations of magnesium deficiency. Miner Electrolyte Metab. 1993;19(4–5):314–22.
The latest survey of magnesium (Mg) intakes of Americans (NHANES) indicates the majority of Americans have Mg intakes below the Recommended Daily Allowance (RDA) in all age and race groups tested. In fact, the daily intakes were: 70 mg/day less than recommended in Caucasian men; 130 mg/day less in African American men; 60 mg/day less in Caucasian women; and 120 mg/day less than recommended in African American women. (The RDA is 320 mg/day for women and 420 mg/day for men.) Also, one statistic—called the standard error of the mean—was quite low. For example, +/- 6 for Caucasian men, raising the possibility that the vast majority of Americans are Mg deficient.
Even more interesting were some of the top ten contributors for American Mg intake: coffee, 3.7% of intake; milk, 2.2%; beer, 1.8%; French fries, 1.1%. Not a word about Americans eating many seeds and nuts, the foods loaded with Mg. Dr. Earl Ford of the CDC, the lead author, concluded, "Because magnesium has many potential health benefits, increasing the dietary intake of magnesium in the U.S. population should be an important public health goal." Ford ES, Mokdad AH. Dietary magnesium intake in a national sample of US adults. J Nutr. 2003 Sep;133(9):2879–82.
Apparently, Mg is better absorbed from foods than from supplements and Mg absorption varies with the degree of Mg deficiency. Mg is at the heart of the chlorophyll molecule, which is why green vegetables are a good Mg source. Other good sources are nuts, seeds, whole grains, dried fruit, and some fish. The richest source by far on a per gram basis is dried seeds, like pumpkin, sunflower, and sesame seeds, containing between 340–535 mg per 100 gram serving. High Mg foods were probably staples of Paleolithic man. Magnesium. Office of Dietary Supplements, NIH Clinical Center, National Institutes of Health.

Magnesium (Mg) is the forgotten mineral, an "orphan," as Professor Robert Heaney of Creighton University says. It is the fourth most abundant mineral in the body, for it is involved in more than 300 biochemical reactions. All the enzymes that metabolize Vitamin D require Mg. It is also required in each of the steps concerned with replication, transcription, and translation of genetic information, and thus it is also needed for the genetic mechanism of action of Vitamin D. Zofková I, Kancheva RL. The relationship between magnesium and calciotropic hormones. Magnes Res. 1995 Mar;8(1):77–84. Carpenter TO. Disturbances of vitamin D metabolism and action during clinical and experimental magnesium deficiency. Magnes Res. 1988 Dec;1(3–4):131–9.

Besides these two reviews, any scientist interested in Vitamin D and the immune system should read Interactions between magnesium and vitamin D: possible implications in the immune system.

Two interesting cases of Mg dependent Vitamin D-resistant rickets appeared in the Lancet in 1974. Two children, one age two and the other age five, presented with classic rickets. 600,000 IU of Vitamin D daily for ten days did not result in any improvement in six weeks—in either x-rays or alkaline phosphatase—and the doctors diagnosed Vitamin D-resistant rickets. Almost by accident, serum Mg levels were then obtained, which were low in both children. After the treatment with Mg, the rickets rapidly resolved. Reddy V, Sivakumar B. Magnesium-dependent vitamin-D-resistant rickets. Lancet. 1974 May 18;1(7864):963–5.
What does that mean? How can one treat rickets with Mg? Remember, these children took a total of 6 million units, that's a total of 6,000,000 IU of vitamin D over ten days (it was given as injections so we know the children actually took it). Thus, they had plenty of vitamin D but, in their cases, the vitamin D needed Mg to work.
In 1976, Dr. Ramon Medalle and colleagues at the Washington University School of Medicine described five patients with Mg deficiency and low blood calcium whose calcium blood levels would not return to normal after Vitamin D treatment, a condition known as Vitamin D resistance. However, serum calcium promptly returned to normal in all five patients after treatment with Mg, raising the possibility that such Vitamin D resistance may be caused from simple, but severe, Mg deficiency. Medalle R, Waterhouse C, Hahn TJ. Vitamin D resistance in magnesium deficiency. Am J Clin Nutr. 1976 Aug;29(8):854–8.
What is not known is how mild to moderate Mg deficiencies (like most Americans apparently have) affect Vitamin D metabolism. The safe thing to do is to eat green leafy vegetables and a handful of sunflower seeds every day (Trader Joe's sells a variety of seeds). If you can't, won't, or don't end up doing that, then take a Vitamin D supplement with added Mg.

In fact, there are now supplements on the market that contain all the co-factors vitamin D needs to work properly (including magnesium): zinc (the base of the fingers of the Vitamin D Receptor each contains a zinc molecule), Vitamin K2 (Vitamin K helps direct Vitamin D to calcify the proper organs and prevents calcification of improper organs), boron (boron is involved in the rapid, non-genomic action of Vitamin D on the cell wall), a small amount of genestein (about one-half the amount the average Japanese consumes every day) which helps activated Vitamin D stay around longer at the receptor site, and a tiny amount of Vitamin A. Again, the wisest thing to do is to eat green leafy vegetables and a handful of seeds every day as that combination contains the co-factors Vitamin D needs, the co-factors many Americans are deficient in.