Vitamin D is fat soluble and not found naturally in many foods. The best natural source is the flesh of fatty fish like salmon or fish liver oils. There is also some found naturally in select cheeses, beef liver, and egg yolks. This elusive vitamin is mostly synthesized naturally when the UV rays for the sun come in contact with the skin. Vitamin D promotes calcium absorption in the gut and is often added to milk. Insufficient Vitamin D can cause rickets which results in thin, brittle and misshapen bones. Some other roles it has in the body are reduction of inflammation, modulation of cell growth, neuromuscular and immune functions. Besides dietary inadequacy, Vitamin D deficiency can also be the result of impaired absorption and use, increased requirement, or increased excretion. Veganism and diets that lack milk due to allergy or intolerance are more likely to develop a vitamin D deficiency.
Vitamin D from food, sun, or supplements has to go through two hydroxylations in the body for activation. Serum concentrations of vitamin D are obtained by measuring 25(OH)D which has a serum half-life of 15 days.
Intake reference values for vitamin D and other nutrients are provided in the Dietary Reference Intakes (DRIs) from the Food and Nutrition Board at the Institute of Medicine of the National Academies. These values will vary based on age and gender, but include the recommended Dietary Allowance, which is the average daily level of intake sufficient to meet the nutrient requirements of nearly all (97-98%) healthy people. The Vitamin D RDAs are based on minimal sun exposure, with generally 400-600IU or 15mcg being adequate for both genders and a large age span.The Tolerable Upper Intake Level is the maximum daily intake unlikely to cause adverse health effects.The Tolerable Upper Intake Level is the maximum daily intake unlikely to cause adverse health effects. Based on theThe National Academies of Sciences, Engineering, and Medicine committee’s review of the evidence, the UL is 4,000 IU/day. Most reports suggest a toxicity threshold for vitamin D of 10,000 to 40,000 IU/day and serum 25(OH)D levels of 500–600 nmol/L (200–240 ng/mL).
The scientific community has not been unanimously agreed on what the RDA should be for vitamin D. A study published in 2015 by Veugelers and Ekwaru argues that “in its dietary reference intakes for calcium and vitamin D, the Institute of Medicine (IOM) had made a serious calculation error. Using the same data set as had the IOM panel, these investigators showed that the Recommended Dietary Allowance (RDA) for vitamin D had been underestimated by an order of magnitude. Veugelers and Ekwaru, using the IOM’s data, calculated an RDA of 8895 IU per day”.
Average intake from food ranged from 144-288 IU/day . In general serum levels for 25(OH)D were higher in younger individuals and higher in males than females. Over the past 20 years there has been a reported slight decline in levels among males. It is hypothesized this decline is due to increased body weight, reduced milk intake, and greater use of sun protection.
Vitamin D has been suggested to help in protection against diseases like cancer, Diabetes, MS, and hypertension. Deficiency has been linked to a number of other issues, including hair loss. Too much vitamin D can cause symptoms like weight loss, polyuria, heart arrhythmias, and even vascular and tissue calcification, including kidney stones. The federal government’s 2010 Dietary Guidelines for Americans says “nutrients should come primarily from foods”, suggesting that they do not recommend supplement intake of vitamin-D, unless prescribed a professional.
Written by Michelle Mader, PhD
March 15th, 2016