54.3 Nutritional Deficiencies: Vitamin D, B12, Iron & Malnutrition

Vitamin D Deficiency & Metabolic Bone Pathophysiology

Vitamin D is a seco-steroid prohormone essential for calcium and phosphorus homeostasis, skeletal mineralization, and neuromuscular function. It exists in two primary forms: vitamin D3 (cholecalciferol), synthesized endogenously in the epidermis upon exposure to solar ultraviolet B (UVB, 290–315 nm) radiation from 7-dehydrocholesterol or ingested from animal dietary sources (oily fish, egg yolks), and vitamin D2 (ergocalciferol), derived from fungal and plant irradiation of ergosterol.

Metabolism & Circulating Biomarkers

1. Hepatic 25-Hydroxylation: Both cholecalciferol and ergocalciferol are transported to the liver bound to vitamin D-binding protein (DBP), where the microsomal enzyme 25-hydroxylase (CYP2R1) converts them into 25-hydroxyvitamin D [25(OH)D, calcidiol].
   • The Clinical Gold Standard: Serum 25(OH)D is the only reliable biomarker reflecting total body vitamin D nutritional status. It has a circulating biological half-life of 2 to 3 weeks and is not tightly regulated by parathyroid hormone (PTH).
2. Renal 1-Alpha-Hydroxylation: In the proximal convoluted tubules of the kidney, the mitochondrial enzyme 1-alpha-hydroxylase (CYP27B1) converts 25(OH)D into the biologically active hormonal form, 1,25-dihydroxyvitamin D [1,25(OH)2D, calcitriol].
   • Calcitriol has a short half-life of 4 to 6 hours and circulates at concentrations 1,000-fold lower than calcidiol.
   • Renal CYP27B1 is tightly stimulated by PTH and hypophosphatemia, and inhibited by fibroblast growth factor 23 (FGF23) and hypercalcemia.

                  THE VITAMIN D METABOLIC CASCADE

   Solar UVB Radiation (290-315 nm)                  Dietary Intake
                 │                                         │
                 ▼                                         ▼
   Epidermal 7-Dehydrocholesterol ──► Vitamin D3 / D2 (Cholecalciferol / Ergocalciferol)
                                              │
                                              ▼ [Liver: 25-Hydroxylase (CYP2R1)]
                                   25-Hydroxyvitamin D [25(OH)D]
                                   • BEST BIOMARKER OF TOTAL BODY STORES
                                   • Half-life: 2–3 weeks
                                              │
                                              ▼ [Kidney: 1α-Hydroxylase (CYP27B1)]
                                              │   ▲ Stimulated by High PTH, Low Phos
                                              │   ▼ Inhibited by FGF23, High Ca
                                              ▼
                               1,25-Dihydroxyvitamin D [1,25(OH)₂D, Calcitriol]
                               • Biologically active hormone
                               • Promotes intestinal Ca²⁺ and PO₄³⁻ absorption

[!WARNING] THE COMMON LABORATORY PITFALL: ORDERING 1,25(OH)2D INSTEAD OF 25(OH)D Ordering 1,25-dihydroxyvitamin D to assess vitamin D status is a serious clinical mistake. In patients with severe nutritional vitamin D deficiency, low calcium absorption triggers compensatory secondary hyperparathyroidism. High circulating PTH strongly stimulates renal 1-alpha-hydroxylase, maintaining 1,25(OH)2D levels within the normal reference range or driving them paradoxically high, even when the patient's skeletal stores are completely depleted. Always order serum 25-hydroxyvitamin D [25(OH)D].

Laboratory Classification of Vitamin D Status (Endocrine Society)

• Deficiency: Serum 25(OH)D <20 ng/mL (<50 nmol/L)
• Insufficiency: Serum 25(OH)D 20 to 29 ng/mL (50 to 74 nmol/L)
• Sufficiency: Serum 25(OH)D ≥30 ng/mL (≥75 nmol/L) (optimal range: 30 to 50 ng/mL)
• Toxicity: Serum 25(OH)D >100 to 150 ng/mL (>250 to 375 nmol/L) (manifests with hypercalcemia, hypercalciuria, acute kidney injury, and soft tissue calcification)

Clinical Consequences: Osteomalacia & Rickets

When serum 25(OH)D drops below 20 ng/mL, intestinal calcium absorption falls from ~30-40% down to 10-15%. The resulting hypocalcemia stimulates parathyroid chief cells to secrete PTH (secondary hyperparathyroidism). PTH induces renal calcium retention, promotes renal phosphate wasting (hypophosphatemia), and increases osteoclastic bone resorption:

• Osteomalacia (Adults):
  • Characterized by defective and delayed mineralization of newly synthesized organic bone matrix (osteoid).
  • Clinical features: Insidious, diffuse, dull aching bone pain (especially in the pelvis, spine, ribs, and lower extremities); proximal muscle weakness (causing a waddling, antalgic gait and difficulty rising from a chair or climbing stairs); bone tenderness upon firm palpation of the anterior tibia and sternum.
  • Laboratory profile: Low or low-normal serum calcium, low serum phosphate, elevated serum intact PTH, elevated bone-specific alkaline phosphatase (BSAP), and markedly low 25(OH)D.
  • Radiographic features: Generalized osteopenia, cortical thinning, and pathognomonic Looser zones (pseudofractures / Milkman lines)—narrow, transverse, radiolucent bands perpendicular to the cortex, typically bilateral and symmetric, located along the medial femoral neck, pubic rami, ribs, and lateral axillary border of the scapulae.
• Rickets (Children):
  • Occurs prior to the fusion of epiphyseal growth plates. Defective chondrocyte apoptosis and impaired mineralization of cartilage matrix lead to growth plate widening and deformities: craniotabes (soft, thinning skull bones), rachitic rosary (enlargement of costochondral junctions), Harrison groove (diaphragmatic pull indentation of lower ribs), and progressive bowing of the weight-bearing long bones (genu varum or genu valgum).

Pharmacologic Treatment of Severe Deficiency

• High-Dose Loading Protocol (for 25(OH)D <20 ng/mL):
  • Ergocalciferol (Vitamin D2) 50,000 IU orally once weekly for 8 weeks (or Cholecalciferol [Vitamin D3] 50,000 IU weekly x 8 weeks).
  • Alternatively: 6,000 IU of daily vitamin D3 for 8 weeks.
• Maintenance Therapy:
  • Following the 8-week loading regimen, transition to maintenance therapy with Cholecalciferol (Vitamin D3) 1,000 to 2,000 IU orally daily (or 50,000 IU every 2 to 4 weeks).
  • Repeat serum 25(OH)D measurement at 3 months to confirm achievement of sufficiency (target ≥30 ng/mL).
• High-Risk Populations Requiring Higher Maintenance Doses (3,000–6,000 IU/day):
  • Patients with obesity (BMI ≥30 kg/m²: vitamin D is fat-soluble and sequestered within excess subcutaneous adipose tissue, reducing circulating bioavailability).
  • Malabsorptive disorders (celiac disease, Crohn's disease, post-bariatric surgery [RYGB]).
  • Patients taking hepatic cytochrome P450-inducing anticonvulsants (phenytoin, carbamazepine, phenobarbital) or rifampin, which dramatically accelerate the metabolic catabolism of 25(OH)D and 1,25(OH)2D into inactive calcitroic acid.

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Vitamin B12 (Cobalamin) Deficiency: Hematologic & Neurologic Manifestations

Vitamin B12 is an essential water-soluble organometallic compound synthesized exclusively by microorganisms. Dietary sources are limited strictly to animal products (meat, poultry, fish, shellfish, eggs, and dairy products) and fortified foods.

Physiology of Absorption & Etiological Drivers

1. Gastric Phase: Ingested B12 is bound to dietary proteins. In the stomach, gastric acid and pepsin cleave B12 from proteins. B12 immediately binds to salivary haptocorrin (R-factor), which protects it from acid degradation.
2. Pancreatic & Duodenal Phase: In the duodenum, pancreatic proteases degrade haptocorrin, releasing free cobalamin. Free cobalamin then binds to intrinsic factor (IF), a 45-kDa glycoprotein secreted by gastric parietal cells.
3. Ileal Phase: The stable B12-IF complex transits through the jejunum into the terminal ileum, where it binds to specific cubilin-amnionless receptors on enterocyte brush borders and is absorbed via calcium-dependent endocytosis.

                  ETIOLOGICAL SPECTRUM OF VITAMIN B12 DEFICIENCY

   Etiological Category       Specific Mechanism                                     Clinical Context
   ═════════════════════════════════════════════════════════════════════════════════════════════════════════════════
   Pernicious Anemia          Autoimmune destruction of gastric parietal cells ──►   Middle-aged/older adults; coexists
                              Complete loss of Intrinsic Factor and HCl             with Hashimoto's, vitiligo, T1D

   Strict Vegan Diet          Zero intake of animal products; total body stores      Onset delayed 3-5 years due to
                              (2-5 mg stored in liver) gradually deplete             efficient enterohepatic recycling

   Surgical Anatomical        • Roux-en-Y Gastric Bypass (loss of IF & acid)         Mandates lifelong prophylactic
   Modifications              • Terminal Ileal Resection (>20-30 cm resected)        oral or parenteral cobalamin

   Gastrointestinal Disease   • Ileal Crohn's disease (mucosal inflammation)         Malabsorption of intact
                              • Celiac sprue; Small Intestinal Bacterial Overgrowth   B12-IF complex; bacterial consumption

   Drug-Induced               • Metformin: Interferes with Ca²⁺-dependent ileal      Occurs in 10-30% of chronic users;
   Cobalamin Malabsorption      binding of B12-IF complex                             monitor levels every 1-2 years
                              • PPIs / H2-Blockers: Hypochlorhydria impairs          Cleavage of food-bound cobalamin
                                peptic cleavage from dietary protein                  impaired (free B12 absorbed normally)
   ═════════════════════════════════════════════════════════════════════════════════════════════════════════════════

Clinical Manifestations

• Hematologic Features (Megaloblastic Anemia):
  • Macrocytic anemia with an MCV typically >100 fL (frequently >110–120 fL).
  • Peripheral blood smear: Hypersegmented neutrophils (defined as the presence of ≥1 six-lobed neutrophil or ≥5% five-lobed neutrophils), anisocytosis, poikilocytosis, and oval macrocytes.
  • Severe deficiency results in ineffective erythropoiesis and intramedullary hemolysis: severe pancytopenia (leukopenia, thrombocytopenia), elevated indirect bilirubin, elevated lactate dehydrogenase (LDH, often marked), and low reticulocyte production index.
• Neurologic Manifestations: Subacute Combined Degeneration (SCD):
  • Cobalamin is an essential cofactor for methionine synthase, which converts homocysteine to methionine (vital for S-adenosylmethionine [SAM] synthesis, the methyl donor for myelin sheath maintenance), and methylmalonyl-CoA mutase, which converts methylmalonyl-CoA to succinyl-CoA. Impaired mutase activity results in accumulation of methylmalonic acid and abnormal odd-chain fatty acids that incorporate into and destabilize neural membranes.
  • Dorsal (Posterior) Columns Demyelination: Produces loss of vibration sensation and proprioception in lower extremities, sensory ataxia (wide-based, unsteady, slapping gait), positive Romberg sign, and Lhermitte-like electrical sensations.
  • Lateral Corticospinal Tracts Demyelination: Produces upper motor neuron signs: lower extremity spasticity, hyperreflexia, clonus, and positive extensor plantar responses (Babinski sign).
  • Peripheral Sensorimotor Neuropathy: Symmetric distal sensory paresthesias ("stocking-glove" numbness, tingling, burning).
  • Neuropsychiatric Features ("Megaloblastic Madness"): Progressive memory loss, irritability, depression, paranoia, hallucinations, and reversible cognitive decline / dementia.

[!CAUTION] THE FOLIC ACID TRAP (NEUROLOGICAL DISASTER) Folic acid (vitamin B9) supplementation bypasses the B12-dependent methionine synthase block in the thymidylate synthesis pathway, completely restoring DNA replication in hematopoietic precursors and curing the megaloblastic anemia. However, folic acid does NOT correct the impaired myelin sheath maintenance or methylmalonyl-CoA mutase dysfunction. Consequently, administering high-dose folic acid to a patient with unrecognized B12 deficiency masks the anemia while allowing irreversible spinal cord demyelination (Subacute Combined Degeneration) and permanent paraplegia to progress unimpeded. Always measure serum B12 prior to or concurrently with folate supplementation!

Diagnostic Evaluation & Reflex Testing

• Serum Vitamin B12 Level:
  • <200 pg/mL (<148 pmol/L): Definitively diagnostic of deficiency.
  • 200 to 350 pg/mL: Borderline / equivocal zone. Up to 30% of patients with tissue-level B12 deficiency have serum levels in this intermediate range.
  • >350 pg/mL: Deficiency is highly unlikely.
• Metabolic Biomarkers: Methylmalonic Acid (MMA) & Total Homocysteine:
  • Indicated whenever serum B12 is borderline (200–350 pg/mL) or when high clinical suspicion exists despite normal serum levels.
  • Elevated Serum MMA (>0.40 umol/L) AND Elevated Homocysteine (>15 umol/L): Definitively confirms Vitamin B12 Deficiency (sensitivity >98%).
  • Normal MMA AND Elevated Homocysteine: Diagnostic of Folate (Vitamin B9) Deficiency.
  • Note: Serum MMA can be falsely elevated in chronic kidney disease due to reduced renal excretion.
• Pernicious Anemia Autoantibodies:
  • Anti-Intrinsic Factor (anti-IF) Antibodies: Extraordinarily high specificity (>95%), moderate sensitivity (~50–70%). A positive test establishes pernicious anemia.
  • Anti-Parietal Cell Antibodies: High sensitivity (~90%), but low specificity (~50%; found in atrophic gastritis and autoimmune thyroiditis).

                  DIAGNOSTIC ALGORITHM: B12 VS. FOLATE DEFICIENCY

                            Borderline Serum B12 (200-350 pg/mL)
                                              │
                                              ▼
                              Order Serum MMA and Homocysteine
                                              │
                    ┌─────────────────────────┴─────────────────────────┐
                    ▼                                                   ▼
         BOTH MMA & Homocysteine                             Homocysteine ELEVATED
                 ELEVATED                                        MMA NORMAL
                    │                                                   │
                    ▼                                                   ▼
       VITAMIN B12 DEFICIENCY                                FOLATE DEFICIENCY
                    │                                                   │
         Order Anti-Intrinsic Factor                             Prescribe Oral
              Antibodies                                        Folic Acid 1 mg Daily

Pharmacotherapy & Route of Administration

• Intramuscular Cyanocobalamin: Traditional regimen for severe hematologic manifestations, neurologic symptoms, or malabsorptive states:
  • 1,000 mcg IM daily for 7 days, followed by
  • 1,000 mcg IM once weekly for 4 weeks, then
  • 1,000 mcg IM once monthly for life (or transition to high-dose oral).
• High-Dose Oral Cyanocobalamin (1,000 to 2,000 mcg PO daily):
  • Multiple randomized controlled trials demonstrate that high-dose oral cobalamin is equally effective to intramuscular injections for normalizing hematologic indices and resolving neurologic deficits, even in patients with pernicious anemia or bariatric surgery.
  • Physiological Basis: Approximately 1% to 2% of an oral dose of free crystalline cobalamin is absorbed via passive, non-carrier-mediated diffusion across the entire mucosal surface of the gastrointestinal tract, completely independent of intrinsic factor or terminal ileal receptors. A daily oral dose of 1,000 to 2,000 mcg delivers 10 to 20 mcg of absorbed B12, far exceeding the daily physiological requirement (2.4 mcg/day).

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Iron Deficiency Anemia (IDA): Biomarkers, Endoscopy & Dosing Protocols

Iron deficiency is the single most common nutritional deficiency worldwide, affecting over 1.5 billion people. In primary care, iron deficiency anemia (IDA) is rarely a primary disease itself; rather, it is a clinical manifestation of an underlying pathological bleeding site or malabsorptive disorder that demands systematic investigation.

Iron Kinetics & Biomarker Interpretation

                  LABORATORY PROFILES: IRON DEFICIENCY VS. ANEMIA OF CHRONIC DISEASE

   Biomarker                     Normal Range           Iron Deficiency Anemia       Anemia of Chronic Disease (ACD)
   ═════════════════════════════════════════════════════════════════════════════════════════════════════════════════
   Serum Ferritin                30 – 300 ng/mL         < 30 ng/mL                   Normal to Elevated (>100 ng/mL)
                                                        (Most sensitive & specific)   (Acute-phase reactant; hepcidin-driven)

   Transferrin Saturation (TSAT) 20% – 50%              < 15% – 20%                  Normal or Low (10% – 20%)
   (Serum Iron / TIBC x 100)

   Serum Iron                    60 – 170 ug/dL         Low (< 40 ug/dL)             Low (< 50 ug/dL)

   Total Iron-Binding Capacity   240 – 450 ug/dL        Elevated (> 450 ug/dL)       Low or Normal (< 250 ug/dL)
   (TIBC)                                               (Liver upregulates transferrin)

   Soluble Transferrin Receptor  1.2 – 3.0 mg/L         ELEVATED                     NORMAL
   (sTfR)
   ═════════════════════════════════════════════════════════════════════════════════════════════════════════════════

[!IMPORTANT] SERUM FERRITIN IN THE PRESENCE OF SYSTEMIC INFLAMMATION Serum ferritin is an intracellular iron storage protein and a potent acute-phase reactant upregulated by circulating inflammatory cytokines (IL-6, IL-1, TNF-α). In healthy individuals, a ferritin <30 to 45 ng/mL is virtually 100% specific for iron deficiency. However, in patients with chronic inflammatory states (e.g., chronic kidney disease, rheumatoid arthritis, active inflammatory bowel disease, congestive heart failure, malignancy, or acute infections), ferritin can be falsely elevated into the normal range (50 to 100+ ng/mL). In such patients, an iron deficiency state is diagnosed if serum ferritin is <100 ng/mL OR if transferrin saturation (TSAT) is <20%.

Clinical Etiologies & Mandatory Endoscopic Workup

• Premenopausal Females: Menorrhagia and pregnancy are the leading causes. However, gastrointestinal pathology must still be considered if anemia is severe or refractory.
• Adult Men & Postmenopausal Females:
  • Dietary iron deficiency in an adult consuming a Western diet is practically non-existent.
  • The Golden Rule: Iron deficiency anemia in an adult male or postmenopausal female is presumed to be caused by occult gastrointestinal bleeding until proven otherwise.
  • Mandatory Diagnostic Standard: Bidirectional endoscopy—comprising both Colonoscopy (to rule out right-sided colon cancer, polyps, vascular ectasias) AND Esophagogastroduodenoscopy (EGD) with duodenal biopsies (to rule out gastric adenocarcinoma, peptic ulcer disease, Cameron lesions in hiatal hernias, and celiac disease)—is indicated, regardless of whether fecal occult blood tests are positive or negative.

Oral Iron Therapy: The Paradigm Shift to Alternate-Day Dosing

• The Biology of Hepcidin: Ingestion of an oral iron bolus stimulates hepatic synthesis and secretion of hepcidin, the master systemic regulator of iron metabolism. Hepcidin binds to and internalizes ferroportin, the sole cellular iron exporter on duodenal enterocytes and reticuloendothelial macrophages. Following an oral iron dose, hepcidin levels spike for 24 to 48 hours, blocking subsequent iron absorption from successive doses and trapping unabsorbed iron in the intestinal lumen, causing severe gastrointestinal toxicity (nausea, constipation, epigastric cramps, dark tarry stools).
• Alternate-Day Dosing Protocol:
  • Prescribe ferrous sulfate 325 mg (containing 65 mg of elemental iron) once every other day (e.g., Monday, Wednesday, Friday, Sunday mornings).
  • Landmark randomized trials have demonstrated that alternate-day dosing yields equal or superior total fractional iron absorption compared to daily or three-times-daily dosing, with a significantly lower incidence of gastrointestinal adverse effects and markedly superior patient adherence.
  • Administration Instructions: Take on an empty stomach with a source of ascorbic acid (vitamin C 250 to 500 mg or orange juice), which maintains iron in its soluble ferrous ($\text{Fe}^{2+}$) state. Avoid co-ingestion with calcium supplements, antacids, dairy products, coffee, or tea (tannins and polyphenols bind iron, forming insoluble non-absorbable chelates).

Indications for Intravenous Iron Pharmacotherapy

Modern intravenous iron formulations (e.g., ferric carboxymaltose, ferric derisomaltose, ferumoxytol, iron sucrose) have an outstanding safety profile with negligible risk of anaphylaxis (unlike historical high-molecular-weight iron dextran):

1. Intolerance or failure of oral iron (severe gastrointestinal side effects despite alternate-day dosing or failure to raise hemoglobin by ≥1.0 g/dL after 4 weeks).
2. Severe malabsorptive states (post-Roux-en-Y gastric bypass, active celiac sprue, short bowel syndrome).
3. Active Inflammatory Bowel Disease (Crohn's disease, Ulcerative Colitis): Unabsorbed luminal oral iron exacerbates mucosal inflammation, ulceration, and dysbiosis; IV iron bypasses the gut.
4. Chronic Kidney Disease (Stages 3–5D): Especially patients receiving erythropoiesis-stimulating agents (ESAs).
5. Heart Failure with Reduced Ejection Fraction (HFrEF, NYHA Class II–III): The FAIR-HF and CONFIRM-HF trials proved that IV iron improves functional capacity, exercise performance, and reduces HF hospitalizations in patients with ferritin <100 ng/mL (or ferritin 100–299 ng/mL with TSAT <20%), even in the absence of anemia.
6. Second or Third Trimester Pregnancy with severe anemia (Hb <10.0 g/dL) or oral iron intolerance, where rapid repletion is essential prior to delivery.

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Pediatric & Geriatric Malnutrition Syndromes

Pediatric Malnutrition: Failure to Thrive (FTT)

Failure to thrive is a state of undernutrition characterized by inadequate weight gain or physical growth over time in infants and young children:

• Diagnostic Anthropometric Criteria:
  • Weight-for-age falling below the 3rd or 5th percentile on standardized WHO (age <2 years) or CDC (age ≥2 years) growth charts.
  • Weight-for-length falling below the 3rd or 5th percentile.
  • Deceleration of growth velocity across two or more major percentile lines (e.g., falling from the 50th to below the 10th percentile) over a 3- to 6-month period.
• Etiological Classification:
  • Inadequate Caloric Intake (Most Common, >80% of cases): Incorrect infant formula preparation (excessive water dilution due to financial poverty or misunderstanding), feeding aversion, severe oral motor dysfunction, neglect, breastfeeding failure, parent-child interaction difficulties.
  • Inadequate Nutrient Absorption: Celiac disease, cystic fibrosis (exocrine pancreatic insufficiency), cow's milk protein enteropathy, short bowel syndrome.
  • Increased Energy Expenditure: Congenital heart disease, chronic pulmonary disease (bronchopulmonary dysplasia, cystic fibrosis), chronic infections (HIV, recurrent UTIs), hyperthyroidism.
• Primary Care Management: Comprehensive dietary recall (3-day food diary), observing a feeding encounter, high-calorie nutritional supplementation (fortifying formula to 24–30 kcal/oz), and interdisciplinary collaboration (social work, lactation consulting, speech pathology for swallow mechanics).

Geriatric Malnutrition & The Mini Nutritional Assessment (MNA)

Protein-energy malnutrition in older adults is strongly associated with sarcopenia, immune dysfunction, impaired wound healing, functional decline, and heightened mortality.

• Diagnostic Warning Flags:
  • Unintentional weight loss of ≥5% of body weight in 1 month, or ≥10% in 6 months.
  • Serum albumin <3.0 g/dL or prealbumin <10 mg/dL (reflects disease acuity and inflammation rather than pure nutritional status).
• The Mini Nutritional Assessment (MNA):
  • A validated 18-item clinical tool designed specifically for geriatric populations. The short-form MNA (MNA-SF) scores food intake decline, weight loss, mobility, acute disease/psychological stress, neuropsychological problems (dementia/depression), and BMI (or calf circumference):
    • 12 to 14 points: Normal nutritional status.
    • 8 to 11 points: At risk of malnutrition.
    • 0 to 7 points: Malnourished.
• Primary Care Interventions:
  • Identification and treatment of reversible drivers: depression, poorly fitting dentures, xerostomia, polypharmacy causing anorexia (e.g., digoxin, anticholinergics, SSRIs), dysphagia, food insecurity, and isolation.
  • Nutritional prescription: High-protein, nutrient-dense oral nutritional supplements (ONS) between meals; liberalization of overly restrictive chronic disease diets (e.g., loosening strict low-salt or diabetic diets that impair palatability).