Bioavailability and Drug–Nutrition Interface

Bioavailability and Drug–Nutrition Interface

Introduction

·       The term bioavailability refers to the fraction of an ingested nutrient or drug that reaches the systemic circulation and is available for physiological activity, storage, or utilization.

·       It is a critical concept in nutrition, pharmacology, and clinical practice because even if a food or supplement contains adequate amounts of nutrients, their physiological effectiveness depends on how much is actually absorbed and utilized by the body.

·       Similarly, the drug–nutrition interface describes the interaction between drugs and nutrients in the body, which can alter the absorption, metabolism, and efficacy of both.

·       Understanding bioavailability and the drug–nutrition interface is essential in planning therapeutic diets, preventing deficiencies, optimizing supplementation, and managing patients on long-term medications.

Bioavailability

Definition

·       Bioavailability is the proportion of an ingested nutrient (or drug) that is absorbed from the gastrointestinal tract and becomes available for use or storage in the body.

·       Mathematically (for drugs):

o   Bioavailability (F)=Amount of nutrient/drug reaching systemic circulationAmount ingested×100\text{Bioavailability (F)} = \frac{\text{Amount of nutrient/drug reaching systemic circulation}}{\text{Amount ingested}} \times 100

·       In nutrition, it refers to the efficiency with which nutrients are absorbed and utilized for metabolic functions.

Factors Affecting Bioavailability

·       Bioavailability is influenced by host-related factors, food-related factors, and nutrient–nutrient/drug–nutrient interactions.

A. Physiological/Host Factors

• Age – Infants and elderly may have reduced absorption due to immature or declining GI function.
• Genetics – Variations in enzymes (e.g., lactase persistence or deficiency) affect nutrient metabolism.
• Gastrointestinal health – Conditions like celiac disease, Crohn’s disease, or chronic diarrhea reduce absorption.
• Gastric acidity – Hypochlorhydria (low stomach acid) decreases absorption of vitamin B12, calcium, and iron.
• Pregnancy and lactation – Increased demand may reduce effective bioavailability.
• Nutritional status – Deficiency may enhance absorption (adaptive response), while overload may reduce absorption (homeostatic regulation).

B. Food/Nutrient Matrix Factors

• Chemical form of nutrient – Heme iron (from animal sources) is more bioavailable than non-heme iron (plant sources).
• Food matrix – Fiber, phytates, oxalates, tannins, and polyphenols reduce mineral absorption.
• Processing and cooking – Heat, fermentation, sprouting can enhance or reduce bioavailability.
• Fat content – Fat-soluble vitamins (A, D, E, K) require dietary fat for absorption.
• Meal composition – Vitamin C enhances non-heme iron absorption; calcium competes with iron and zinc.

C. Drug–Nutrient Interactions

• Antibiotics like tetracyclines bind to calcium, iron, and magnesium, reducing both nutrient and drug absorption.
• Proton pump inhibitors (PPIs) reduce stomach acid, decreasing absorption of B12 and calcium.
• Anticonvulsants (e.g., phenytoin) interfere with vitamin D metabolism.

Nutrient-Specific Bioavailability

• Iron –
• Heme iron: 15–35% absorbed (meat, poultry, fish).
• Non-heme iron: 2–20% absorbed (plants, cereals). Absorption enhanced by vitamin C, inhibited by phytates/tannins.
• Calcium –
• Dairy calcium highly bioavailable.
• Calcium from spinach poorly absorbed due to oxalates.
• Absorption requires vitamin D.
• Zinc –
• Better absorbed from animal products.
• Phytates and high calcium intake reduce absorption.
• Vitamin B12 –
• Requires intrinsic factor and gastric acid for absorption.
• Deficiency common in elderly and those on PPIs.
• Folate –
• Naturally occurring folate less bioavailable than synthetic folic acid (50% vs 85%).
• Fat-soluble vitamins (A, D, E, K) –
• Require dietary fat and bile salts for absorption.
• Deficiency risk in fat-malabsorption disorders.
• Carotenoids –
• Lycopene and β-carotene bioavailability enhanced by cooking and fat.

Improving Nutrient Bioavailability

A. Food Preparation and Processing

• Soaking, fermenting, sprouting, and germination reduce phytates and oxalates, enhancing mineral absorption.
• Cooking tomatoes increases bioavailability of lycopene; mild cooking enhances carotenoids.
• Fortification and supplementation improve intake and bioavailability of iron, folate, and vitamin D.

B. Dietary Combinations

• Vitamin C-rich foods with plant iron sources improve iron absorption.
• Fat-rich foods enhance fat-soluble vitamin absorption.
• Avoid simultaneous high intake of competing minerals (calcium vs iron vs zinc).

C. Clinical/Medical Approaches

• Enzyme replacement therapy (e.g., lactase for lactose intolerance).
• Chelated mineral supplements (e.g., ferrous bisglycinate) for better absorption.
• Controlled-release formulations in drugs.

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