What are Dietary Reference Intakes (DRIs)?
Before diving into specific numbers, it is helpful to understand the terminology. Dietary Reference Intakes (DRIs) are a set of reference values for nutrient intake developed by the Health and Medicine Division of the National Academies of Sciences, Engineering, and Medicine. These values are used for planning and assessing nutrient intake for healthy people and include:
- Recommended Dietary Allowance (RDA): The average daily dietary intake level sufficient to meet the nutrient requirements of nearly all (97–98%) healthy individuals in a particular life stage and sex group.
- Adequate Intake (AI): A value based on observed or experimentally determined approximations of nutrient intake by a group of healthy people. It is used when an RDA cannot be scientifically determined.
- Tolerable Upper Intake Level (UL): The maximum daily intake unlikely to cause adverse health effects.
What is the current DRI for vitamin D?
For most of the North American population, the DRI for vitamin D was last updated in 2010 by the Institute of Medicine (now the National Academies of Sciences, Engineering, and Medicine). These recommendations are based on achieving a serum 25-hydroxyvitamin D (25(OH)D) level of at least 50 nmol/L (20 ng/mL) for bone health.
Vitamin D Recommendations by Life Stage
| Life Stage | RDA (International Units - IU) | RDA (micrograms - mcg) |
|---|---|---|
| Infants (0-12 months) | 400 IU (AI) | 10 mcg (AI) |
| Children (1-18 years) | 600 IU | 15 mcg |
| Adults (19-70 years) | 600 IU | 15 mcg |
| Adults (>70 years) | 800 IU | 20 mcg |
| Pregnant and Lactating Women | 600 IU | 15 mcg |
Factors influencing individual vitamin D needs
While the DRIs provide a general guideline, several factors can influence a person's actual vitamin D requirements. These variables can impact how much vitamin D your body produces from sunlight or how efficiently it uses what is consumed from food and supplements.
- Sun Exposure: The body produces vitamin D when sunlight's ultraviolet B (UVB) rays strike the skin. Factors that limit sun exposure can reduce production, including living in northern latitudes, wearing sun-protective clothing, and spending less time outdoors.
- Skin Pigmentation: Melanin, the pigment that gives skin its color, acts as a natural sunscreen. Individuals with darker skin require more sun exposure to produce the same amount of vitamin D as those with lighter skin.
- Age: As people age, their skin becomes less efficient at producing vitamin D from sunlight. Older adults may also spend less time outdoors, further increasing their risk for deficiency.
- Body Weight: A higher body mass index (BMI) is associated with lower vitamin D levels. Excess body fat can sequester vitamin D, preventing it from circulating in the bloodstream. This may mean that individuals with obesity require higher doses to maintain adequate levels.
- Medical Conditions: Certain conditions can impair vitamin D absorption or metabolism. Examples include celiac disease, Crohn's disease, cystic fibrosis, and kidney or liver disease.
- Medications: Some medications, such as steroids and certain cholesterol-lowering drugs, can interfere with vitamin D metabolism.
Sources of vitamin D
To meet the DRI, individuals must obtain vitamin D from a combination of sources. The three main sources are sunlight, diet, and supplements.
Dietary Sources Few foods naturally contain significant amounts of vitamin D. The best sources are:
- Fatty fish (e.g., salmon, mackerel, trout, sardines).
- Cod liver oil.
- Beef liver and egg yolks contain smaller amounts.
- Mushrooms exposed to UV light.
Fortified Foods Many common foods are fortified with vitamin D to help people meet their needs. These include:
- Milk (cow's milk and plant-based alternatives like soy and almond milk).
- Fortified breakfast cereals.
- Some brands of orange juice, yogurt, and margarine.
Supplements For those who do not get enough vitamin D from sun exposure or fortified foods, supplements are an effective way to meet the DRI. Vitamin D is available in two forms: D2 (ergocalciferol) and D3 (cholecalciferol). Research suggests that vitamin D3 is generally more effective at raising and maintaining blood vitamin D levels.
The risk of vitamin D deficiency and toxicity
Vitamin D deficiency is a common problem globally and can lead to serious health issues, particularly related to bone health. Symptoms can include fatigue, bone and muscle pain, and an increased risk of fractures. Severe deficiency can cause rickets in children and osteomalacia in adults.
However, it is also possible to have too much vitamin D, which typically occurs from over-supplementation rather than excessive sun exposure. Vitamin D toxicity (hypervitaminosis D) can cause hypercalcemia (high calcium levels), nausea, weakness, and, in severe cases, damage to the kidneys and heart. The Tolerable Upper Intake Level (UL) is 4,000 IU for adults.
Conclusion
The Dietary Reference Intake for vitamin D is a vital guideline, with the RDA for most adults set at 600 IU per day, increasing to 800 IU for those over 70. However, an individual's specific needs can differ based on various factors, including sun exposure, skin tone, and health status. Obtaining sufficient vitamin D from a combination of sun, diet, and supplements is crucial for maintaining bone health and preventing deficiency-related problems. Always consult a healthcare provider to determine your optimal intake level and before starting any high-dose supplementation. The NIH Office of Dietary Supplements provides reliable information on vitamin D and other nutrients(https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/).
