Thiamin is a water‑soluble vitamin whose nutrition is defined by its dietary sources and deficiency signs. The richest sources are pork, whole grains, legumes, and fortified cereals; dairy products and seafood do not provide large amounts, so they are considered poor sources. Leafy greens contribute only small amounts compared with the best sources. The skin/mucous membrane changes described as cheilosis and dermatitis are linked to riboflavin (B2) deficiency, not thiamin. A lack of thiamin leads to beriberi and related neurologic issues, not the mucocutaneous symptoms mentioned. Activation of prothrombin involves vitamin K, not thiamin. So, the statement that seafood and cheeses are poor sources correctly reflects thiamin nutrition.

Alcohol disrupts vitamin B6 status by causing the PLP coenzyme to dissociate from its enzyme. PLP is the active form of vitamin B6 that sits in the active site of many enzymes, forming a needed holoenzyme for reactions like transamination and decarboxylation. When alcohol is consumed, its metabolism alters cellular conditions (for example, increases in NADH and acetaldehyde) that can destabilize the PLP–enzyme complex. This makes PLP leak away from the enzyme, reducing the amount of active, bound coenzyme and thereby impairing PLP-dependent reactions. That direct release of PLP from enzymes explains why alcohol’s major effect is dislodging the coenzyme from its enzyme. The other ideas aren’t the primary mechanism. Alcohol doesn’t reduce acetaldehyde formation; it actually increases it. Fecal excretion isn’t established as the main route by which alcohol affects B6 status, and while liver function can influence PLP synthesis, the most immediate and impactful effect is the dislodgement of PLP from enzymes.

Low levels of vitamin C are commonly seen in people who chew tobacco because tobacco use increases oxidative stress and the body uses up more vitamin C to neutralize free radicals. Vitamin C acts as an antioxidant and is required for collagen synthesis and other important processes, but it isn’t stored in large amounts, so intake must be relatively regular. When tobacco is used, the higher oxidative burden depletes circulating vitamin C, leading to lower levels. This makes the statement about tobacco chewing the best fit: it aligns with how vitamin C status is influenced by external oxidative challenges. The other ideas aren’t correct: severe deficiency can be reversed with vitamin C supplementation, so it isn’t irreversible; extreme physical stress tends to raise the body's vitamin C needs rather than impair its mobilization; and deficiency signs don’t appear at a fixed 20% drop in blood levels.

Thiamin intake recommendations are set to ensure adequate energy metabolism and to prevent deficiency, not to match how actively thiamin-dependent enzymes are functioning. In other words, the dietary amount is determined by the amount needed to support carbohydrate metabolism and prevent deficiency, while enzyme activity is used as a functional check of status rather than the basis for setting the intake. Functional tests like transketolase activity can indicate whether thiamin status is adequate, but they don’t drive the recommended intake values. Severe thiamin deficiency can lead to edema (wet beriberi), and deficiency can also occur without edema (dry beriberi), so those characteristics are accurate.

Riboflavin is highly sensitive to light, so exposure to photons—especially from the ultraviolet range—drives photochemical reactions that break its chemical bonds and create inactive degradation products. This is why riboflavin-containing products are stored in opaque or amber packaging and kept out of light. While heat, acids, or bases can affect vitamins, riboflavin loses its activity much more readily to light exposure than to typical heating or pH changes, making ultraviolet light the condition that destroys it most easily.