Understanding Dietary Reference Intakes (DRIs)
Dietary Reference Intakes (DRIs) are a set of reference values used for planning and assessing nutrient intakes of healthy people. They are developed by the Food and Nutrition Board of the National Academies of Sciences, Engineering, and Medicine. The DRIs include several categories, such as the Recommended Dietary Allowance (RDA) and the Tolerable Upper Intake Level (UL). The RDA is the average daily level of intake sufficient to meet the nutrient requirements of nearly all healthy individuals in a particular life stage and gender group. The UL, on the other hand, is the maximum daily intake unlikely to cause adverse health effects.
Recommended Dietary Allowance (RDA) for Selenium
The RDA for selenium is based on the amount needed to maximize the synthesis of the selenoprotein glutathione peroxidase, a crucial antioxidant. The specific recommended amounts vary based on age, gender, and life stage. According to the National Institutes of Health, the RDAs for selenium are as follows:
- Infants 0–6 months: 15 mcg
- Infants 7–12 months: 20 mcg
- Children 1–3 years: 20 mcg
- Children 4–8 years: 30 mcg
- Children 9–13 years: 40 mcg
- Teens 14–18 years: 55 mcg
- Adults (19+ years): 55 mcg
- Pregnant Women: 60 mcg
- Breastfeeding Women: 70 mcg
Tolerable Upper Intake Level (UL) for Selenium
While selenium is essential, excessive intake can lead to serious health problems. The Tolerable Upper Intake Level (UL) is the maximum amount of daily intake that is unlikely to cause adverse health effects. For adults, the UL is set at 400 micrograms per day. Symptoms of toxicity, known as selenosis, can occur with intakes above this level and may include hair loss, brittle nails, and gastrointestinal issues.
Key Food Sources of Selenium
The selenium content of food can vary significantly depending on the concentration of selenium in the soil where plants are grown or where animals are raised. For this reason, average food values can be unreliable, especially for plant-based foods.
The Influence of Soil Selenium Levels
Soils with high selenium content lead to higher levels of the mineral in local crops. Conversely, low-selenium soil results in crops with lower selenium content. Foods widely distributed through national markets, however, provide more consistent selenium levels. Animal products tend to have more reliable levels as animals maintain predictable tissue concentrations and often receive supplemented feed.
Top Selenium-Rich Foods
- Brazil Nuts: A single ounce (6–8 nuts) can contain over 500 mcg of selenium, exceeding the daily UL.
- Seafood: Yellowfin tuna, sardines, and shrimp are excellent sources.
- Meats and Poultry: Pork chops, beef steak, turkey, and chicken are good sources.
- Eggs: One hard-boiled egg can provide a significant portion of the daily RDA.
- Grains: Spaghetti and oatmeal are common sources of selenium.
Health Implications of Selenium Intake
Symptoms of Selenium Deficiency
Though rare in the United States and Canada due to adequate dietary intake, selenium deficiency can manifest with several symptoms, particularly in populations with extremely low soil selenium levels. Symptoms include:
- Infertility in both men and women
- Muscle weakness and fatigue
- Thin or dull hair, hair loss, and white spots on nails
- Weakened immune system
- Impaired thyroid function, potentially leading to hypothyroidism
In severe cases in specific regions, deficiency has been linked to Keshan disease (a form of heart disease) and Kashin-Beck disease (a disorder affecting bones and joints).
Signs of Selenium Toxicity (Selenosis)
Consuming very high amounts of selenium, most commonly through supplements, can cause toxicity. Symptoms include:
- A characteristic garlic-like odor on the breath
- Hair loss and brittle, discolored nails
- Gastrointestinal upset, nausea, and diarrhea
- Fatigue and irritability
- Nervous system abnormalities
- In extreme cases, severe toxicity can lead to heart failure, kidney failure, and death.
Comparison of Selenium Intake Recommendations (US vs. Europe)
| Feature | US Guidelines (NIH) | European Guidelines (EFSA) |
|---|---|---|
| RDA for Adults | 55 mcg/day (men & women) | 70 mcg/day (men) & 60 mcg/day (women) |
| RDA for Pregnancy | 60 mcg/day | 65 mcg/day |
| RDA for Lactation | 70 mcg/day | 75 mcg/day |
| Tolerable Upper Intake Level (UL) for Adults | 400 mcg/day | 255 mcg/day |
| Note | RDA is based on maximizing plasma glutathione peroxidase activity. | UL is based on preventing alopecia as the critical adverse effect. |
The Role of Selenoproteins
The biological function of selenium is primarily carried out through selenoproteins. The human body uses selenium to create these proteins, many of which act as powerful antioxidants. They are involved in protecting cells from oxidative stress, regulating thyroid hormone metabolism, supporting the immune system, and aiding in reproduction. There are more than two dozen known selenoproteins, each with a specialized function. The activity of these vital proteins depends directly on the availability of selenium in the diet. For instance, in times of selenium deficiency, the body prioritizes the synthesis of certain selenoproteins over others, potentially impairing some bodily functions.
Conclusion
The dietary reference intake for selenium is a vital guideline for ensuring adequate consumption of this essential trace mineral without reaching toxic levels. The RDA of 55 mcg for adults is generally met through a balanced diet rich in seafood, lean meats, and grains. However, it is important to be mindful of the wide variability in plant-based selenium content due to soil differences. While deficiency is rare in developed countries, toxicity is a significant risk with over-supplementation, especially when consuming very high-selenium foods like Brazil nuts regularly. Both deficient and excessive intake can have adverse health effects, emphasizing the importance of staying within the established safe intake range for optimal health. For more details on this and other nutrients, please visit the NIH Office of Dietary Supplements.
