Which of the following vitamins has a relatively high RDA?

BTSC Staff Nurse 1 August-2025

Applied Nutrition & Dietetics

Medium

Appeared in: BTSC Staff Nurse 1 August-2025

Explanation

Vitamin C has a much higher Recommended Dietary Allowance (RDA), around 75-90 mg/day for adults, compared to the other options.
The primary reason is its water-solubility. The body does not store Vitamin C; any excess is readily excreted through urine, necessitating a consistent and larger daily intake.
Crucially, the RDA for Vitamin C is measured in milligrams (mg), whereas the RDAs for the fat-soluble vitamins A, D, and K are measured in micrograms (mcg). One milligram is 1,000 times larger than one microgram, highlighting the significant difference in required quantity.

Option B: Vitamin D is a fat-soluble vitamin. Its RDA is very low, around 15-20 micrograms (mcg) per day, which is thousands of times less than that of Vitamin C.
Option C: Vitamin A is a fat-soluble vitamin. While its RDA (700-900 mcg/day) is numerically higher than Vitamin D or K, it is still in micrograms and represents a much smaller mass than the milligram-based RDA of Vitamin C.
Option D: Vitamin K is a fat-soluble vitamin with a relatively low RDA of about 90-120 micrograms (mcg) per day for adults.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Comparison of Recommended Dietary Allowances (RDA) for different vitamins as background academic context rather than a clinical decision trigger.
Understanding RDAs is crucial for nurses when providing dietary education, especially for at-risk populations like pregnant women, the elderly, or patients with malabsorption disorders.
Knowing the difference between water-soluble and fat-soluble vitamins is vital for patient safety. Nurses can educate patients that while excess Vitamin C is usually excreted, excess fat-soluble vitamins (A, D, K) can accumulate and cause toxicity.
What if? If the patient is a smoker, the RDA for Vitamin C increases by about 35 mg/day. A nurse providing smoking cessation counseling should include this nutritional advice, as it makes the high requirement for Vitamin C even more significant.

How to Approach the Question

First, identify the core task of the question: to compare the Recommended Dietary Allowance (RDA) of the four listed vitamins and find the highest one.
Recall or deduce the basic properties of these vitamins. Remember that Vitamin C is water-soluble, while Vitamins A, D, and K are fat-soluble.
Apply the fundamental nutritional principle: water-soluble vitamins are not stored in the body and require higher daily intake, whereas fat-soluble vitamins are stored, leading to lower daily requirements.
Critically, pay attention to the units of measurement. The RDA for water-soluble vitamins is often in milligrams (mg), while for fat-soluble vitamins it is in micrograms (mcg).
Recognize that 1 mg is 1000 times larger than 1 mcg. This unit difference is the key to identifying the correct answer.
Conclude that Vitamin C, with its RDA measured in mg, has a substantially higher requirement than the others measured in mcg.

Concept Tested & Keywords

Concept Tested: Comparison of Recommended Dietary Allowances (RDA) for different vitamins.
Stem keywords: vitamins, RDA, Recommended Dietary Allowance
Lead-in keywords: Which, relatively high

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