What are the different classifications of oral nutritional supplements (ONS) based on osmolarity and how are they chosen for patients with various medical conditions, such as diabetes, impaired renal function, or gastrointestinal disorders?

ONS Osmolarity Classification

Oral nutritional supplements are not formally classified by osmolarity in ESPEN guidelines; instead, they are categorized as nutritionally complete versus incomplete, with osmolarity considerations primarily relevant for enteral tube formulas and parenteral solutions. 1

Primary Classification System for ONS

The ESPEN guidelines establish a two-tier classification system for oral nutritional supplements that does not rely on osmolarity 1:

Nutritionally Complete ONS

• Can serve as the sole source of nutrition for prolonged periods with balanced macro- and micronutrients including essential amino acids, essential fatty acids, and micronutrients reflecting dietary recommendations for healthy individuals 1
• Most commonly used as supplements to regular diet when food intake is insufficient 1
• Appropriate for general malnutrition without specific metabolic requirements 1

Nutritionally Incomplete ONS

• Not suitable as sole nutrient source because they contain specific nutrients in higher amounts while other nutrients are lacking or insufficient 1
• Disease-specific formulations modified for conditions including diabetes, pressure ulcers, cirrhosis, cancer, renal failure, and pulmonary disease 1
• Can be either complete or incomplete depending on formulation 1

Osmolarity Considerations in Clinical Practice

While osmolarity is not the primary classification criterion for ONS, it becomes clinically relevant in specific contexts:

Evidence on Osmolarity Effects

Research demonstrates that osmolarity primarily affects gastrointestinal tolerance rather than nutritional efficacy 2:

• A randomized controlled trial of 118 patients showed that hypertonic diet (430 mmol/kg) resulted in significantly better nitrogen intake and balance compared to isotonic formulations (300 mmol/kg), with no difference in gastrointestinal side effects 2
• Diarrhea was significantly associated with concurrent antibiotic treatment rather than diet osmolality 2
• "Starter regimens" that gradually increased osmolality reduced nutrient intake without reducing symptoms 2

The "Goldilocks Zone" for Special Populations

For patients with ileostomies, osmolality between 100-290 mOsm/kg optimizes fluid and electrolyte absorption 3:

• Ileostomy output increased by 57 g/6 hours for every 100 mOsm/kg increase in supplement osmolality above 290 mOsm/kg 3
• Supplements with osmolality 290-600 mOsm/kg showed linear correlation with increased ileostomy output and sodium loss 3

Route-Specific Osmolarity Concerns

Hyperosmolar medications (>500 mOsm/kg) administered via nasogastric tube do not affect nutritional adequacy or gastrointestinal complications, but postpyloric administration increases diarrhea risk 4:

• Nasogastric administration of hyperosmolar medications showed no association with feeding intolerance, diarrhea, or other GI complications 4
• Nasoduodenal administration of hyperosmolar medications was associated with dramatically increased diarrhea risk (OR 138.7) 4

Disease-Specific Selection Algorithm

For Renal Failure Patients

Standard ONS can be used in hemodialysis patients, with formula phosphorus and potassium content requiring verification 1:

• Hemodialysis-specific formulas should be preferred for tube feeding but standard ONS are acceptable for oral supplementation 1
• Formulas designed for conservative chronic renal failure should NOT be used in dialysis patients due to inadequate protein content 1
• Intradialytic ONS delivery improves compliance and nutritional parameters 1

For Diabetes and Metabolic Conditions

Disease-specific incomplete ONS are modified to meet specific metabolic demands 1:

• These formulations adjust macronutrient composition for glycemic control 1
• Must be supplemented with other nutrient sources as they cannot serve as sole nutrition 1

For Gastrointestinal Disorders

Formulas containing peptides and medium-chain triglycerides facilitate absorption in malabsorption or short bowel syndrome 1:

• Standard complete ONS appropriate for most GI conditions with intact absorptive capacity 1
• Modified formulations reserved for severe malabsorption 1

Hierarchical Approach to Selection

ONS represent the mandatory first step when artificial nutrition is indicated, before considering tube feeding or parenteral nutrition 5:

• Initiate ONS when patients cannot meet energy needs from normal food alone 5
• Continue ONS as long as oral intake remains possible, even if inadequate 5
• Progress to tube feeding only when ONS proves insufficient to meet nutritional requirements 5

Common Pitfalls to Avoid

• Do not use "starter regimens" that gradually increase osmolality - they reduce nutrient intake without reducing symptoms 2
• Do not assume hyperosmolar formulas cause diarrhea - antibiotic use is the primary culprit in most cases 2
• Do not administer hyperosmolar supplements postpylorically unless absolutely necessary due to dramatically increased diarrhea risk 4
• Do not use renal-restricted formulas in dialysis patients - they contain inadequate protein for catabolic needs 1
• For ileostomy patients, avoid supplements with osmolality >290 mOsm/kg to minimize output and sodium losses 3