What is the approximate osmolality of an isotonic enteral formula?

• A. 0 mOsm/kg
• B. 300 mOsm/kg
• C. 550 mOsm/kg
• D. 700 mOsm/kg

Answer: (B)

Isotonic enteral formulas are designed to match the osmolality of body fluids to avoid shifting water across the gut wall. The target is about 300 mOsm/kg, which is close to the osmolality of plasma (roughly 285–295 mOsm/kg). This balance helps maintain fluid distribution in the body and supports better GI tolerance.

Values much higher than this, like 550 or 700 mOsm/kg, are hypertonic and can pull water into the gut lumen, leading to osmotic diarrhea and poor tolerance. A value near zero would be hypotonic and not feasible for an enteral formula, since it would disrupt fluid balance. Therefore, the approximate osmolality of an isotonic enteral formula is around 300 mOsm/kg.