Dietary Iron

Overview

Iron is an essential trace mineral required for oxygen transport, electron transfer, and numerous enzymatic reactions. The body contains 3-5 grams of iron, with approximately 65% in hemoglobin, 10% in myoglobin, and the remainder in storage (ferritin) and transport (transferrin) forms. Iron deficiency is the most common nutritional deficiency worldwide, while iron overload causes significant organ damage.

Biological Functions

• Oxygen transport: Central component of hemoglobin (carries O2 from lungs to tissues) and myoglobin (oxygen storage in muscle)
• Electron transport: Essential component of cytochromes in mitochondrial ATP production
• DNA synthesis: Required for ribonucleotide reductase activity
• Enzymatic cofactor: Necessary for catalase, peroxidases, and cytochrome P450 enzymes
• Immune function: Required for lymphocyte proliferation and antimicrobial activity
• Neurotransmitter synthesis: Cofactor for tyrosine hydroxylase (dopamine synthesis) and tryptophan hydroxylase (serotonin)

Health Significance

Iron balance is tightly regulated because humans lack an active excretion mechanism. Absorption is adjusted based on body stores and erythropoietic demand. Iron deficiency progresses through stages: depleted stores, iron-deficient erythropoiesis, and finally iron deficiency anemia. Conversely, iron overload (hereditary hemochromatosis or transfusion-related) causes oxidative damage to liver, heart, and endocrine organs.

Clinical Interpretation Guidelines

When reviewing iron intake data:

• Distinguish iron forms: Heme iron (animal sources) has 15-35% absorption; non-heme iron (plants, supplements) has 2-20% absorption
• Assess menstrual status: Premenopausal women lose significant iron monthly; requirements nearly double
• Review dietary patterns: Vegetarians/vegans need 1.8x RDA due to exclusive non-heme intake
• Consider meal composition: Vitamin C dramatically enhances non-heme iron absorption
• Evaluate timing: Iron supplements are best absorbed on empty stomach but cause more GI side effects
• Check for chronic blood loss: GI bleeding, heavy menses, frequent blood donation increase needs

Deficiency

Iron deficiency stages:

1. Depleted stores: Low ferritin, normal hemoglobin
2. Iron-deficient erythropoiesis: Low transferrin saturation, rising TIBC
3. Iron deficiency anemia: Low hemoglobin, microcytic hypochromic RBCs

Symptoms:

• Fatigue and weakness (reduced oxygen delivery)
• Pallor (decreased hemoglobin)
• Dyspnea on exertion
• Cognitive impairment and poor concentration
• Pica (craving non-food items like ice, dirt)
• Restless legs syndrome
• Brittle nails (koilonychia/spoon nails in severe cases)
• Glossitis and angular cheilitis
• Impaired thermoregulation
• Reduced immune function

High-risk groups:

• Menstruating women (especially heavy flow)
• Pregnant women
• Infants and young children
• Adolescents during growth spurts
• Vegetarians/vegans
• Frequent blood donors
• Chronic disease (CKD, heart failure, IBD)

Toxicity/Excess

Acute iron poisoning (typically accidental pediatric ingestion):

• GI hemorrhage, vomiting, diarrhea
• Metabolic acidosis
• Liver failure
• Cardiovascular collapse

Chronic iron overload:

• Liver: Cirrhosis, hepatocellular carcinoma
• Heart: Cardiomyopathy, arrhythmias, heart failure
• Endocrine: Diabetes (pancreatic damage), hypogonadism, hypothyroidism
• Joints: Arthropathy
• Skin: Bronze discoloration

Populations at risk for overload:

• Hereditary hemochromatosis (HFE mutations)
• Thalassemia and sickle cell disease (transfusion-dependent)
• Chronic liver disease
• Those taking unnecessary supplements

Food Sources

Heme iron sources (animal-derived, 15-35% absorbed):

• Organ meats: Liver, kidney (highest concentration)
• Red meat: Beef, lamb
• Poultry: Dark meat chicken, turkey
• Seafood: Oysters, clams, mussels, sardines

Non-heme iron sources (plant-derived, 2-20% absorbed):

• Legumes: Lentils, chickpeas, kidney beans, soybeans
• Fortified cereals and bread
• Dark leafy greens: Spinach, Swiss chard
• Tofu and tempeh
• Pumpkin seeds, quinoa
• Dried fruits: Raisins, apricots

Absorption Factors

Enhancers of non-heme iron absorption:

• Vitamin C (ascorbic acid): Most potent enhancer; 75 mg can triple absorption
• MFP factor: Meat, fish, poultry enhance absorption of co-consumed non-heme iron
• Organic acids: Citric acid, lactic acid (fermented foods)
• Low iron stores: Body upregulates absorption when depleted

Inhibitors:

• Phytates: Whole grains, legumes, nuts (soaking/sprouting reduces)
• Polyphenols: Tea, coffee, wine, chocolate (tannins bind iron)
• Calcium: Inhibits both heme and non-heme absorption; separate by 2 hours
• Oxalates: Spinach, rhubarb
• Soy protein: Contains phytate and specific inhibitory factors
• Antacids and PPIs: Reduced stomach acid impairs iron solubility
• Excess zinc or copper: Compete for absorption

Special Populations

• Pregnant women: Plasma volume expansion and fetal demands require 27 mg/day; routine supplementation often recommended
• Menstruating women: Average monthly loss of 30-40 mg iron; heavy flow increases substantially
• Vegetarians/vegans: Should aim for 1.8x RDA; focus on vitamin C pairing and avoiding absorption inhibitors at iron-rich meals
• Endurance athletes: "Sports anemia" may reflect plasma expansion; true deficiency from foot-strike hemolysis, GI bleeding, sweat losses
• Infants: Iron stores from birth deplete by 4-6 months; fortified formula or iron supplementation needed
• Chronic kidney disease: Functional iron deficiency common despite adequate stores; IV iron often required
• Inflammatory conditions: Hepcidin elevation blocks iron absorption and release; may need IV iron
• Bariatric surgery patients: Bypass of duodenum (primary absorption site) necessitates supplementation

Drug Interactions

• Thyroid hormones (levothyroxine): Iron reduces absorption; separate by 4 hours
• Bisphosphonates: Iron impairs absorption; separate by 2 hours
• Tetracyclines/fluoroquinolones: Form non-absorbable complexes; separate by 2-4 hours
• Levodopa/carbidopa: Iron may reduce absorption
• ACE inhibitors: May increase risk of iron-related adverse effects
• Proton pump inhibitors/H2 blockers: Reduce iron absorption (need gastric acid)
• Calcium supplements: Inhibit iron absorption; take at different meals
• Cholestyramine: May bind iron in gut

Caveats & Limitations

• HealthKit data represents intake, not absorption or iron status
• Heme vs. non-heme distinction not captured, though absorption differs 5-10x
• Meal composition affecting absorption not tracked
• Food database accuracy varies; fortification levels differ by brand/region
• Supplement iron forms (ferrous sulfate, gluconate, citrate) have different bioavailability
• Serum ferritin is the best single marker of iron stores; intake data alone is insufficient for status assessment
• Ferritin is an acute phase reactant; inflammation falsely elevates levels
• Cooking in cast iron cookware can increase food iron content (not captured)