2.2 Micronutrients & Water Balance

2.2 Micronutrients & Water Balance

Dietary Reference Intakes (DRIs) Framework

The Dietary Reference Intakes (DRIs) represent a comprehensive set of nutrient reference values established by the Food and Nutrition Board of the National Academies of Sciences, Engineering, and Medicine. DRIs serve as the quantitative standard for planning and assessing diet quality in healthy populations and clinical settings.

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Fat-Soluble vs. Water-Soluble Vitamins

Vitamins are organic compounds required in micro-gram or milli-gram quantities to support metabolic reactions. They are classified into fat-soluble and water-soluble groups based on solubility, absorption, transport, and tissue storage capacity.

Fat-Soluble Vitamins (A, D, E, K): Physiology and Deficiency

Fat-soluble vitamins require biliary micelle formation and pancreatic lipase activity for intestinal absorption. They enter systemic circulation via chylomicrons in the lymphatic system and are stored in hepatic and adipose tissue, making toxicities possible when exceeding the UL.

Water-Soluble Vitamins (B-Complex and Vitamin C)

Water-soluble vitamins function primarily as coenzymes in enzymatic energy metabolism and single-carbon transfers. With the exception of Vitamin B12, they are not stored in significant quantities and are excreted in urine when tissue thresholds are exceeded.

• Thiamine (Vitamin B1): Coenzyme thiamine pyrophosphate (TPP) for pyruvate dehydrogenase and transketolase. Deficiency causes Beriberi (Wet: high-output heart failure, edema; Dry: peripheral neuropathy) and Wernicke-Korsakoff Syndrome (ataxia, confusion, nystagmus, confabulation in chronic alcoholism).
• Riboflavin (Vitamin B2): Precursor to FAD and FMN coenzymes. Deficiency causes ariboflavinosis (cheilosis, angular stomatitis, glossitis, magenta tongue).
• Niacin (Vitamin B3): Precursor to NAD and NADP. Synthesized from tryptophan ($60\text{ mg tryptophan} = 1\text{ mg Niacin Equivalent}$). Deficiency causes Pellagra, characterized by the 4 Ds: Dermatitis (photosensitive Casal necklace), Diarrhea, Dementia, and Death. High-dose nicotinic acid causes cutaneous flushing.
• Pyridoxine (Vitamin B6): Coenzyme pyridoxal phosphate (PLP) for transamination, decarboxylation, and heme synthesis. Deficiency causes microcytic anemia, peripheral neuropathy, and seizures. Isoniazid therapy antagonizes B6.
• Folate (Vitamin B9): Coenzyme tetrahydrofolate (THF) for single-carbon transfer, DNA synthesis, and conversion of homocysteine to methionine. Deficiency causes macrocytic megaloblastic anemia and fetal neural tube defects (spina bifida, anencephaly). RDA = 400 mcg DFE; Pregnancy = 600 mcg DFE.
• Cobalamin (Vitamin B12): Contains cobalt; coenzyme for methionine synthase and methylmalonyl-CoA mutase. Requires intrinsic factor (IF) secreted by gastric parietal cells for ileal absorption. Deficiency causes megaloblastic anemia with neurological degeneration (subacute combined degeneration of spinal cord). Elevated methylmalonic acid (MMA) distinguishes B12 deficiency from folate deficiency.
• Vitamin C (Ascorbic Acid): Water-soluble antioxidant; cofactor for prolyl and lysyl hydroxylases in collagen synthesis, and enhances non-heme iron absorption. Deficiency causes Scurvy (petechiae, perifollicular hemorrhage, bleeding gums, impaired wound healing).

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Essential Macro and Trace Minerals

Iron Dynamics and Anemia Classification

Iron is an essential trace mineral required for hemoglobin, myoglobin, and cytochrome synthesis. Transported by transferrin and stored as ferritin or hemosiderin. Non-heme iron absorption (plant sources, ferric $Fe^{3+}$) is enhanced by Vitamin C and inhibited by phytates, oxalates, polyphenols, and calcium. Heme iron (animal tissue, ferrous $Fe^{2+}$) is absorbed via heme carrier protein 1 (HCP1).

Trace Minerals: Zinc, Iodine, Selenium, and Copper

• Zinc: Cofactor for over 300 metalloenzymes (RNA polymerase, superoxide dismutase). Deficiency leads to hypogeusia (loss of taste), hyposmia, impaired wound healing, alopecia, and acrodermatitis enteropathica. High zinc supplements inhibit copper absorption.
• Iodine: Essential component of thyroid hormones ($T_3$ triiodothyronine and $T_4$ thyroxine). Deficiency causes endemic goiter and congenital hypothyroidism (cretinism).
• Selenium: Key component of glutathione peroxidase (antioxidant defense). Deficiency causes Keshan disease (endemic cardiomyopathy).
• Copper: Component of ceruloplasmin and cytochrome c oxidase. Facilitates iron oxidation for transferrin binding. Deficiency causes microcytic anemia and neutropenia.

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Water Balance, Fluid Requirements, and Electrolyte Dynamics

Water accounts for 50% to 60% of total body weight in adults (60% in men, 50% in women, 75% in infants). Intracellular fluid (ICF) comprises two-thirds of total body water, while extracellular fluid (ECF) comprises one-third (interstitial fluid and intravascular plasma).

Fluid Requirement Estimation Formulas

1. Body Weight Method: $30\text{ to }35\text{ mL/kg}$ body weight for average adults; $35\text{ to }40\text{ mL/kg}$ for young active adults; $25\text{ to }30\text{ mL/kg}$ for elderly adults or renal/cardiac restriction.
2. Caloric Method: $1.0\text{ mL/kcal}$ of total energy intake for adults ($1.5\text{ mL/kcal}$ for infants).
3. Holliday-Segar Method (Pediatric):
   • First 10 kg: $100\text{ mL/kg}$
   • Next 10 kg (11–20 kg): $+ 50\text{ mL/kg}$
   • Each kg over 20 kg: $+ 20\text{ mL/kg}$

Major Electrolytes: Sodium, Potassium, and Chloride

• Sodium ($\text{Na}^+$): Major ECF cation (Normal range: 135–145 mEq/L). Regulates ECF volume and plasma osmolality ($280\text{--}295\text{ mOsm/kg}$). Hyponatremia (<135 mEq/L) causes confusion, muscle twitching, and cerebral edema.
• Potassium ($\text{K}^+$): Major ICF cation (Normal range: 3.5–5.0 mEq/L). Controls cardiac membrane resting potential and neuromuscular conduction. Hyperkalemia (>5.0 mEq/L) risks fatal cardiac arrhythmias (peaked T waves).

Clinical Scenario: Fluid & Electrolyte Assessment

Case: A 72-year-old female patient weighing 60 kg consumes a 1,800 kcal enteral formula daily.

• Fluid requirement by weight ($30\text{ mL/kg}$): $60\text{ kg} \times 30\text{ mL/kg} = 1,800\text{ mL/day}$.
• Fluid requirement by caloric intake ($1\text{ mL/kcal}$): $1,800\text{ kcal} \times 1\text{ mL/kcal} = 1,800\text{ mL/day}$.
• If enteral formula is 80% free water ($1,800\text{ mL} \times 0.80 = 1,440\text{ mL}$ free water), the NDTR recommends additional water flushes of $360\text{ mL/day}$ ($1,800 - 1,440 = 360\text{ mL}$) divided into four 90 mL flushes.