Nutrition and Metabolism — MCQs

Q: The limiting amino acid in maize is

Lysine. ***Lysine*** - **Lysine** is the **primary limiting amino acid** in maize (corn). - This means that maize contains a relatively low amount of lysine compared to the body's requirements, which limits the overall protein quality and biological value when maize is the sole protein source. - **Quality Protein Maize (QPM)**, developed through breeding programs, has significantly increased levels of **lysine** and **tryptophan** to improve nutritional quality. - Lysine deficiency in maize-based diets can lead to **pellagra** when combined with low niacin intake, as tryptophan (also deficient) is a precursor for niacin synthesis. *Tryptophan* - **Tryptophan** is the **second limiting amino acid** in maize, but not the primary one. - While maize is relatively low in tryptophan, the lysine deficiency is more pronounced and nutritionally significant. - Tryptophan is important as a precursor for **niacin (vitamin B3)**, **serotonin**, and **melatonin**. *Phenylalanine* - **Phenylalanine** is an **essential amino acid** but is generally present in sufficient quantities in maize and is **not considered a limiting factor**. - It serves as a precursor for **tyrosine**, **dopamine**, **epinephrine**, and **norepinephrine**. *Cysteine* - **Cysteine** is a **non-essential (conditionally essential) amino acid** in humans, as the body can synthesize it from methionine. - Therefore, it is **not considered a limiting amino acid** for dietary protein quality assessment.

Q: What is the biological value of egg protein?

100. ***Correct Answer: 100*** - The **biological value (BV)** of a protein measures how efficiently the body utilizes the protein consumed for growth and maintenance, with 100 representing the highest possible value. - **Egg protein** is often used as a reference point for protein quality because it contains all **essential amino acids** in proportions that closely match human requirements, making its BV approximately **100**. - Egg protein serves as the **gold standard** for biological value assessment. *Option: 80* - A biological value of **80** indicates a good quality protein, but it suggests that the protein is not as efficiently utilized as egg protein. - Proteins with a BV of 80 might be deficient in one or more essential amino acids compared to a perfect profile. - Examples include beef and some legumes which have BV around 80. *Option: 90* - A biological value of **90** signifies excellent protein quality, indicating that it is highly utilized by the body. - While very high, it is still slightly lower than the perfect biological value assigned to egg protein. - Some fish proteins approach this value but do not reach 100. *Option: 120* - A biological value of **120** is theoretically **not possible** for food proteins because BV is measured against how much nitrogen (from protein) is retained by the body compared to how much is absorbed. - The maximum BV is **100**, meaning all absorbed protein is retained; values above 100 would imply the body synthesizes protein from non-protein sources, which is not what BV measures. - This tests understanding that BV has an upper limit of 100.

Q: In a clinical scenario, what is the approximate total body iodine content necessary for optimal thyroid function?

15-20 mg. ***Correct: 15-20 mg*** - The approximate total body iodine content in a healthy adult is **15-20 mg**, which is necessary for optimal thyroid function - Approximately **70-80% of this (10-15 mg)** is concentrated in the **thyroid gland** itself - This total body content is critical for the synthesis of **thyroid hormones (T3 and T4)**, which regulate metabolism, growth, and development - This represents the physiological requirement for maintaining normal thyroid hormone production *Incorrect: 10 mg* - While **10 mg** represents approximately the amount of iodine stored specifically in the **thyroid gland** (not total body content), the question asks for **total body iodine content** - Total body iodine includes iodine in the thyroid gland plus iodine distributed in other tissues, blood, and organs - Therefore, 10 mg underestimates the total body requirement *Incorrect: 30 mg* - **30 mg** of total body iodine content is higher than the typical physiological range (15-20 mg) found in healthy individuals - Such a high concentration is not generally required for normal synthesis of **thyroid hormones** - While not necessarily toxic, this exceeds the optimal range *Incorrect: 50 mg* - **50 mg** is significantly higher than the physiological total body iodine content required for optimal thyroid function - Excessive iodine accumulation can lead to **thyroid dysfunction**, including both the **Jod-Basedow phenomenon** (iodine-induced hyperthyroidism) and the **Wolff-Chaikoff effect** (iodine-induced hypothyroidism) - This level far exceeds normal physiological requirements

Practice by Chapter

Macronutrients and Energy Requirements

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Protein Quality and Nitrogen Balance

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Essential Amino Acids and Proteins

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Essential Fatty Acids and Lipids

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Dietary Fiber and Complex Carbohydrates

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Glycemic Index and Glycemic Load

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Micronutrients: Vitamins and Minerals

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Trace Elements and Metabolism

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Malnutrition: Biochemical Consequences

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Dietary Antioxidants

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Functional Foods and Nutraceuticals

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Dietary Guidelines and Recommendations

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