L3: Fat-Soluble Vitamins (A, D, E, K)

Comprehensive practice question-and-answer flashcards covering forms, metabolic functions, food sources, deficiencies, toxicities, absorption, and regulation of fat-soluble vitamins (A, D, E, K).

Where are fat-soluble vitamins (A, D, E, K) primarily stored in the body, and how are dietary fat-soluble vitamins transported from the intestine?

Fat-soluble vitamins are stored in adipose tissue and the liver. Dietary fat-soluble vitamins are packaged into chylomicrons (CM) with dietary fat for transport from intestinal cells.

Which fat-soluble vitamins can be synthesized endogenously by the human body or its intestinal microbiota?

Vitamin D (synthesized in the skin via sunlight exposure) and Vitamin K (synthesized by gut microbiota as menaquinone/$\text{K}_2$).

What are the three forms belonging to the preformed retinoid family of vitamin A, and what are their specific functions?

Retinol (used in supplements and stored in the diet as retinyl esters with fatty acids), Retinal (oxidized form of retinol, essential specifically for vision), and Retinoic acid (oxidized form of retinal, regulates gene transcription for cellular growth, acne, and wrinkle treatment).

Which carotenoid is the primary provitamin A in the human diet, how is it cleaved, and what is its biological activity relative to retinol?

$\beta\text{-carotene}$ is the most consumed carotenoid; it is cleaved in the small intestine into $2$ retinal molecules and exhibits $1/12$ the activity of retinol.

What specific form of vitamin A serves as the chromophore in rhodopsin, and what composes the visual pigment in photoreceptor cells?

$11\text{-cis}\text{ retinal}$ is the active component; it binds to the protein opsin to form rhodopsin, the primary photoreceptor in rod and cone cells.

How does retinoic acid regulate cellular differentiation and gene transcription in target tissues?

Retinol is oxidized to retinoic acid (RA), which enters the nucleus and binds to nuclear retinoic acid receptors (RAR); the RA–RAR complex then binds DNA to activate gene expression and inhibit keratin synthesis.

How is retinol transported from liver storage to peripheral target tissues?

Retinol is secreted from the liver in association with plasma retinol-binding protein (RBP) in a complex with transthyretin (TTR).

What are the primary clinical signs and symptoms associated with vitamin A deficiency?

Fatigue, increased susceptibility to infection, vision impairment, night blindness, xerophthalmia (severe dry eye resulting from excessive corneal keratin synthesis), infertility, and growth retardation.

What is the Tolerable Upper Intake Level (UL) for retinol, and why is high vitamin A intake particularly dangerous for pregnant women?

The UL is $3000\,\mu\text{g}$ of retinol (compared to an RDA of $900\,\mu\text{g}$ for men and $700\,\mu\text{g}$ for women); excessive vitamin A during pregnancy causes teratogenesis.

What are the preformed dietary forms of vitamin D, and what is its active hormonal form?

Preformed dietary forms are Ergocalciferol ($\text{D}_2$, from plants) and Cholecalciferol ($\text{D}_3$, from animals). The active form is $1,25\text{-dihydroxycholecalciferol}$ ($1,25\text{-diOH-D}_3$ or calcitriol).

What precursor in the skin is converted to Vitamin $\text{D}_3$ upon sunlight exposure?

$7\text{-dehydrocholesterol}$ in the skin is photochemically converted to cholecalciferol (Vitamin $\text{D}_3$) by ultraviolet (UV) light.

Describe the two sequential hydroxylation reactions required to produce active calcitriol ($1,25\text{-diOH-D}_3$).

1st step: Cholecalciferol is hydroxylated in the liver by 25-hydroxylase to form calcidiol ($25\text{-OH-D}_3$). 2nd step: Calcidiol is hydroxylated in the kidney by 25-hydroxy-cholecalciferol 1-hydroxylase to form calcitriol ($1,25\text{-diOH-D}_3$).

How is the kidney enzyme 25-hydroxy-cholecalciferol 1-hydroxylase regulated by plasma ions and hormones?

It is activated by low phosphate levels and parathyroid hormone (PTH), and inhibited by feedback from active calcitriol ($1,25\text{-diOH-D}_3$).

What are the main systemic actions of calcitriol in maintaining calcium homeostasis?

It upregulates intestinal calbindin to increase calcium absorption, promotes renal reabsorption (decreasing excretion) of calcium alongside PTH, and stimulates bone resorption to mobilize calcium when serum calcium is low.

Distinguish between Rickets and Osteomalacia in terms of target population and pathology.

Rickets occurs in children and involves incomplete mineralization of the bone collagen matrix, leading to soft bones and bowed legs. Osteomalacia occurs in adults/elderly and involves demineralization of existing mature bone, predisposing to fractures.

What is the Tolerable Upper Intake Level (UL) for Vitamin D, and what is the main clinical complication of hypervitaminosis D?

The UL is $100\,\mu\text{g}$ (RDA is $15\,\mu\text{g}$); toxicity from excessive supplement use causes hypercalcemia, leading to widespread calcium salt deposition in tissues.

Which specific vitamer of Vitamin E is most active in humans, and how does it protect cellular membranes?

$\alpha\text{-tocopherol}$ is the most active form; as a fat-soluble antioxidant, it scavenges reactive oxygen species (ROS) to prevent polyunsaturated fatty acid peroxidation in cell membranes and LDL oxidation.

How is Vitamin E distributed from the liver to peripheral tissues, and what are the main signs of Vitamin E deficiency?

Hepatic $\alpha\text{-tocopherol}$ transfer protein selectively transfers $\alpha\text{-tocopherol}$ into plasma lipoproteins for tissue distribution; deficiency causes retinopathy, neuropathy, and red blood cell fragility leading to hemolytic anemia.

What are the RDA and Tolerable Upper Intake Level (UL) for Vitamin E, and what symptoms result from toxicity?

The RDA is $15\,\text{mg}$ and the UL is $1000\,\text{mg}$; toxicity (occurring only with supplement use) causes bleeding, fatigue, and nausea.

Compare the three forms of Vitamin K (K1, K2, K3) regarding their structure and origin.

Phylloquinone ($\text{K}_1$) comes from plants; Menaquinone ($\text{K}_2$) is synthesized by intestinal bacteria and found in animal/fermented foods; Menadione ($\text{K}_3$) is a synthetic form that is converted to $\text{K}_2$ in the body.

What posttranslational protein modification does Vitamin K catalyze, and which blood clotting factors depend on it?

Vitamin K acts as a coenzyme for $\gamma\text{-glutamyl carboxylase}$ to convert glutamic acid residues into $\gamma\text{-carboxyglutamate}$ (Gla) residues on mature clotting factors II, VII, IX, and X in the liver.

How is active Vitamin K regenerated during protein carboxylation, and what drug targets this cycle?

Active hydroquinone vitamin K ($\text{KH}_2$) is oxidized to vitamin K epoxide ($\text{K}_e$) during carboxylation; Vitamin K epoxide reductase (VKOR) regenerates $\text{KH}_2$. Warfarin blocks VKOR to inhibit blood clotting factor maturation.

How does Vitamin K contribute to bone mineralization and matrix maintenance?

It acts as a coenzyme for the $\gamma\text{-carboxylation}$ of glutamic acid residues on osteocalcin (a protein secreted by osteoblasts), converting it into $\gamma\text{-carboxylated}$ osteocalcin required for bone tissue maintenance.

Why are newborn infants susceptible to Vitamin K deficiency, and what preventative measure is routinely taken?

Newborns have a sterile GI tract (lacking $\text{K}_2$ bacterial synthesis) and human milk contains low Vitamin K levels; a prophylactic dose of Vitamin K is administered at birth to prevent hemorrhagic disease.

Why is menadione (Vitamin K3) supplementation regulated, unlike natural forms K1 and K2?

Natural forms ($\text{K}_1$ and $\text{K}_2$) have no established Upper Limit or toxicity, whereas synthetic menadione ($\text{K}_3$) supplementation can cause hemolytic anemia and jaundice in infants.

Based on the skeletal diagram, what childhood metabolic bone disorder is characterized by these anatomical features, including pigeon chest, curved femur, enlarged epiphyses, and bowed legs?

Rickets, which is caused by vitamin D deficiency during childhood, leading to incomplete mineralization of the bone collagen matrix.

In the posttranslational modification cycle of clotting factors shown in the diagram, what enzyme carboxylates glutamyl residues, and what drug inhibits the regeneration of active vitamin K ($\text{K}_h$) by VKOR?

The enzyme is $\gamma\text{-glutamyl carboxylase}$, and the inhibitor drug is Warfarin.

According to the U.S. NHANES 2003–2006 prevalence data shown in the chart, which nutrient deficiency is most prevalent, and what is the deficiency rate for vitamin D ($\ge 1\text{ y}$)?

Vitamin B6 ($\ge 1\text{ y}$) is the most prevalent deficiency at $10.5\%$, while vitamin D deficiency ($\ge 1\text{ y}$) has a prevalence of $8.1\%$.