Dietary Antioxidants — MCQs
Most potent lipid phase antioxidant:
Glutathione does all of the following except?
Which of the following is not an antioxidant?
Which mineral is known for its antioxidant properties similar to those of vitamin E?
Maximum concentration of vitamin A is found in which organ?
Which of the following is not recommended for patients with coronary artery disease?
Dietary fibres have :
Which is not a dietary fiber ?
In G6PD deficiency, which enzyme's function is MOST directly impaired due to decreased NADPH availability, leading to reduced protection against oxidative stress?
Which of the following trace elements has vitamin E-like action?
Dietary Antioxidants Indian Medical PG Practice Questions and MCQs
Question 1: Most potent lipid phase antioxidant:
- A. Vitamin A
- B. Vitamin E (Correct Answer)
- C. Vitamin C
- D. Vitamin K
Explanation: ***Vitamin E*** - **Vitamin E** (primarily alpha-tocopherol) is a **fat-soluble antioxidant** that is highly effective at neutralizing **lipid peroxyl radicals** within cell membranes and lipoproteins, making it the most potent lipid-phase antioxidant. - It protects against **oxidative damage** to **polyunsaturated fatty acids** in lipid bilayers, which is crucial for maintaining cell membrane integrity. *Vitamin A* - **Vitamin A** (retinol and its derivatives) has antioxidant properties, particularly **beta-carotene**, which can scavenge **single oxygen radicals**. - However, its primary role is not as potent a lipid-phase chain-breaking antioxidant compared to vitamin E. *Vitamin C* - **Vitamin C** (ascorbic acid) is a **water-soluble antioxidant** that works primarily in aqueous environments, such as the cytosol and plasma. - It regenerates expended vitamin E by reducing the **tocopheroxyl radical**, but it doesn't directly act in the lipid phase. *Vitamin K* - **Vitamin K** is essential for **blood coagulation** and **bone metabolism**, serving as a cofactor for gamma-glutamyl carboxylase. - While it has some very limited antioxidant activity in specific contexts, it is not considered a significant or potent antioxidant, especially in the lipid phase.
Question 2: Glutathione does all of the following except?
- A. Scavenge peroxides
- B. Decreases the stability of erythrocyte membranes (Correct Answer)
- C. Form conjugates with some drugs to increase water solubility
- D. Participates in the transport of amino acids across some cell membranes
Explanation: ***Decreases the stability of erythrocyte membranes*** - Glutathione actually **increases** the stability of erythrocyte membranes by protecting them from oxidative damage, particularly through its role in reducing **hydrogen peroxide** and organic hydroperoxides. - A deficiency in glutathione or the enzymes that reduce it (like **G6PD**) leads to increased oxidative stress, which **destabilizes** erythrocyte membranes and can cause **hemolysis**. *Scavenge peroxides* - Glutathione, in its reduced form (GSH), serves as a crucial **antioxidant** by **scavenging peroxides** through the action of **glutathione peroxidase**. - This reaction converts harmful **hydrogen peroxide** into water, protecting cellular components from oxidative damage. *Form conjugates with some drugs to increase water solubility.* - Glutathione plays a vital role in phase II detoxification reactions, where it forms **conjugates** with various xenobiotics and drugs. - This **conjugation** typically increases the **water solubility** of these compounds, facilitating their excretion from the body. *Participates in the transport of amino acids across some cell membranes* - Glutathione is involved in the **gamma-glutamyl cycle**, which is a mechanism for the **transport of amino acids** into cells, particularly in the kidney. - In this cycle, glutathione donates its **gamma-glutamyl group** to an amino acid, forming a gamma-glutamyl amino acid that is then transported across the membrane.
Question 3: Which of the following is not an antioxidant?
- A. Enzymes like glutathione peroxidase, superoxide dismutase
- B. Vitamins like C and E
- C. Iron (Correct Answer)
- D. Hormones like insulin, cortisol
Explanation: ***Micronutrients like Iron*** - Iron is a **pro-oxidant**, not an antioxidant, as it catalyzes the formation of **free radicals** through Fenton reactions [1]. - While essential for many biological processes, it can lead to increased **oxidative stress** when present in excess. *Transport proteins like transferrin, Ceruloplasmin* - These proteins act as **antioxidants** by transporting metal ions and facilitating their redox reactions, helping to prevent oxidative damage. - Transferrin binds iron tightly, reducing its availability for free radical formation, thus exhibiting **protective effects**. *Enzymes like glutathione peroxidase, superoxide dismutase* - These enzymes are crucial **antioxidants** that neutralize reactive oxygen species (ROS) and protect cells from oxidative damage [1]. - They help maintain redox balance and prevent cellular damage caused by excessive **oxidative stress**. *Vitamins like C and E* - Vitamins C and E are well-known **antioxidants** that help neutralize free radicals in the body and protect cellular integrity. - Vitamin C acts as a **water-soluble** antioxidant, while Vitamin E is a **lipid-soluble** antioxidant, both essential for preventing oxidative damage. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Cellular Responses to Stress and Toxic Insults: Adaptation, Injury, and Death, p. 59.
Question 4: Which mineral is known for its antioxidant properties similar to those of vitamin E?
- A. Calcium
- B. Iron
- C. Selenium (Correct Answer)
- D. Magnesium
Explanation: ***Selenium*** - Selenium is a crucial component of **glutathione peroxidase**, an enzyme with significant **antioxidant properties** that protects cells from oxidative damage. - Its antioxidant function is often compared to that of **vitamin E**, as both work to neutralize **free radicals** in the body. *Calcium* - Calcium is primarily known for its role in **bone health**, muscle contraction, and **nerve transmission**, not for its antioxidant properties. - It does not directly participate in neutralizing free radicals like selenium or vitamin E. *Iron* - Iron is essential for **oxygen transport** in red blood cells and various metabolic processes. - While crucial, excessive free iron can actually promote the formation of **free radicals** (Fenton reaction), making it pro-oxidant rather than antioxidant. *Magnesium* - Magnesium is involved in over 300 enzymatic reactions, including **energy metabolism** and muscle and nerve function. - It does not share direct antioxidant properties similar to vitamin E or selenium.
Question 5: Maximum concentration of vitamin A is found in which organ?
- A. Liver (Correct Answer)
- B. Kidney
- C. Lung
- D. Heart
Explanation: ***Liver*** - The **liver** is the primary organ for **storage of vitamin A** (retinyl esters), accounting for 90% of the body's total vitamin A content. - **Hepatic stellate cells** within the liver are specialized for storing the majority of this fat-soluble vitamin. *Kidney* - The **kidney** plays a role in **vitamin D metabolism** and excretion, but not significant vitamin A storage. - While it helps regulate blood levels of various substances, it does not accumulate large quantities of vitamin A. *Lung* - The **lung** does not serve as a major storage site for **vitamin A**. - Its primary functions are related to **gas exchange**, not nutrient storage. *Heart* - The **heart** is responsible for **pumping blood** throughout the body and has minimal involvement in vitamin A storage. - It utilizes certain vitamins for its metabolic processes but does not act as a primary reservoir.
Question 6: Which of the following is not recommended for patients with coronary artery disease?
- A. Vitamin E (Correct Answer)
- B. Potassium
- C. Statins
- D. Daily exercise
Explanation: ***Vitamin E*** - **Vitamin E supplements** are generally not recommended for patients with coronary artery disease (CAD) based on current evidence. Some studies suggest a potential link between high doses of vitamin E and an increased risk of **heart failure** or even **overall mortality**. - There is no convincing evidence that vitamin E supplements provide cardiovascular benefits in patients with established CAD, and they may interfere with the efficacy of other beneficial medications like **statins**. *Daily exercise* - **Regular physical activity** is a cornerstone of CAD management, improving cardiovascular fitness, reducing blood pressure, and aiding in weight control [1]. - It helps in preventing disease progression and reducing the risk of future cardiovascular events when performed under appropriate medical guidance [1]. *Potassium* - Maintaining adequate **potassium levels** is crucial for patients with CAD, especially those on diuretics, as it helps regulate **blood pressure** and prevents **cardiac arrhythmias**. - Dietary sources of potassium (fruits, vegetables) are preferred, and supplementation may be necessary for those with deficiencies, but always under medical supervision. *Statins* - **Statins** are a class of medications widely recommended for patients with CAD due to their ability to significantly lower **LDL cholesterol** levels and reduce cardiovascular events [1]. - They stabilize **atherosclerotic plaques** and reduce inflammation, playing a critical role in secondary prevention of heart attacks and strokes [1].
Question 7: Dietary fibres have :
- A. catabolic effect
- B. sometimes anabolic and sometimes catabolic effect, depending on the type
- C. anabolic effect
- D. no metabolic effect (Correct Answer)
Explanation: ***no metabolic effect*** - Dietary fibers are **indigestible polysaccharides** that cannot be broken down by human digestive enzymes. - Humans lack enzymes like **cellulase** necessary to hydrolyze the β-glycosidic bonds in dietary fiber. - Dietary fibers pass through the gastrointestinal tract **without being metabolized** by human cells, meaning they do not participate in anabolic or catabolic pathways. - While gut bacteria can ferment some fibers producing short-chain fatty acids (SCFAs), this is **bacterial metabolism**, not human metabolism. - The physiological effects of fiber (improved bowel motility, reduced cholesterol absorption, glycemic control) are **mechanical and physicochemical**, not metabolic. *catabolic effect* - Catabolic processes involve **breakdown of molecules with energy release** (e.g., glycolysis, lipolysis). - Dietary fibers cannot undergo catabolism in humans because we lack the enzymes to break them down. - The fermentation by gut bacteria is not human catabolism. *anabolic effect* - Anabolic processes involve **synthesis of complex molecules** from simpler ones (e.g., protein synthesis, glycogenesis). - Dietary fibers are not absorbed or incorporated into human tissues, so they cannot participate in anabolic reactions. *sometimes anabolic and sometimes catabolic effect, depending on the type* - Regardless of fiber type (soluble or insoluble), **all dietary fibers remain non-metabolizable** by human enzymes. - Neither type undergoes anabolic or catabolic metabolism in human cells.
Question 8: Which is not a dietary fiber ?
- A. Lignin
- B. Pectin
- C. Cellulose
- D. Lactulose (Correct Answer)
Explanation: ***Lactulose*** - **Lactulose is NOT a dietary fiber** - it is a synthetic disaccharide used pharmaceutically as an osmotic laxative and for treating hepatic encephalopathy. - Unlike true dietary fibers, lactulose is a manufactured drug, not a naturally occurring food component. - While it is fermented by colonic bacteria (similar to fiber), it does not meet the definition of dietary fiber. *Lignin* - Lignin is a complex aromatic polymer that provides structural support to plant cell walls. - It is classified as a non-polysaccharide dietary fiber that is largely indigestible by human enzymes. - Contributes to fecal bulk and is considered an insoluble fiber. *Pectin* - Pectin is a soluble dietary fiber found naturally in fruits, particularly in apple peels and citrus fruits. - Forms a gel when mixed with water, slowing gastric emptying and aiding digestion. - Beneficial for gut health and blood glucose regulation. *Cellulose* - Cellulose is the most abundant dietary fiber and a major structural component of plant cell walls. - An insoluble fiber composed of β-1,4-linked glucose polymers that cannot be digested by human enzymes. - Contributes to stool bulk and promotes regular bowel movements.
Question 9: In G6PD deficiency, which enzyme's function is MOST directly impaired due to decreased NADPH availability, leading to reduced protection against oxidative stress?
- A. Catalase
- B. Pyruvate kinase
- C. Superoxide dismutase
- D. Glutathione reductase (Correct Answer)
Explanation: ***Glutathione reductase*** - **G6PD deficiency** impairs the production of **NADPH** through the pentose phosphate pathway - **Glutathione reductase** is NADPH-dependent and reduces oxidized glutathione (GSSG) back to reduced glutathione (GSH) - Without adequate NADPH, glutathione reductase cannot maintain sufficient **GSH levels**, which is the primary antioxidant protecting RBCs from oxidative damage - This explains why G6PD deficiency leads to **hemolysis** when exposed to oxidative stressors (antimalarials, sulfonamides, fava beans) *Catalase* - **Catalase** decomposes hydrogen peroxide to water and oxygen, protecting cells from oxidative damage - While important for antioxidant defense, catalase does **not require NADPH** for its function - Its activity is not directly impaired by decreased NADPH in G6PD deficiency *Pyruvate kinase* - **Pyruvate kinase** catalyzes the final step of **glycolysis**, producing ATP - Its function is **completely independent** of NADPH levels - Pyruvate kinase deficiency causes a separate hemolytic anemia unrelated to oxidative stress or G6PD deficiency *Superoxide dismutase* - **Superoxide dismutase (SOD)** converts superoxide radicals to hydrogen peroxide and oxygen - SOD functions **independently of NADPH** and uses metal cofactors (Cu/Zn or Mn) - While part of antioxidant defense, it is not directly affected by G6PD deficiency
Question 10: Which of the following trace elements has vitamin E-like action?
- A. Iron
- B. Selenium (Correct Answer)
- C. Copper
- D. Zinc
Explanation: ***Selenium*** - Selenium is an essential component of **glutathione peroxidase**, an enzyme that works alongside vitamin E to protect cells from **oxidative damage**. - Its antioxidant properties are similar to **vitamin E**, as both scavenge free radicals and prevent lipid peroxidation. *Iron* - Iron is vital for **oxygen transport** in hemoglobin and myoglobin, and for cellular respiration as a component of cytochromes. - While essential, iron does not have direct **antioxidant properties** akin to vitamin E; in excess, it can even promote oxidative stress. *Copper* - Copper is a cofactor for several enzymes, including **superoxide dismutase (SOD)**, an antioxidant enzyme, but its primary role is not directly analogous to vitamin E's lipid-soluble antioxidant function. - It also plays a role in **energy production**, iron metabolism, and neurotransmission. *Zinc* - Zinc is crucial for **immune function**, wound healing, and DNA synthesis, acting as a cofactor for over 300 enzymes. - Although it has indirect antioxidant effects by stabilizing cell membranes and reducing oxidative damage, its mechanism and direct action are not considered "vitamin E-like."
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