Table 1 Summary of human milk macronutrient analysis and clinical implications.

Carbohydrates

Lactose

Makes up 70–85% of the total carbohydrates in human milk. Digestible by infant for energy.

• IR carbohydrate values depend on whether instrument was calibrated to total carbohydrates (includes lactose and HMOs) or lactose.

• While IR is not strongly correlated with chemical methods for measuring carbohydrates, it is accurate within FDA requirement of +/−15%

• IR instruments calibrated to total carbohydrates will overestimate the available energy in human milk.

• Reliable methods for measuring lactose that are not influenced by HMOs include AOAC 984.22 and AOAC 2006.06.

HMOs

Makes up 15–25% of the total carbohydrates in human milk. Not digestible by infant. Serves multiple functions in the infant gut.

Monosaccharides

Makes up less than 1% of the total carbohydrates in human milk.

Total carbohydrates

Sum of monosaccharides, lactose, and HMOs.

Fat

Total fat

Primarily in the form of triglycerides; a major contributor to energy and the most variable macronutrient in human milk.

• Total fat values are profoundly influenced by sample collection and handling practices.

• Lack of proper sample handling may contribute to errors and bias.

• IR provides reliable values for total fat, provided proper sample handling techniques are used.

• Creamatocrit reliability decreases when fat concentrations are low, and when milk has undergone storage.

Protein

Crude Protein

Also referred to as Total Protein. Assumes all the nitrogen in milk is from protein; therefore, crude/total protein over-estimates protein in human milk.

• Protein is estimated from the amount of nitrogen in milk.

• Nitrogen in human milk can be measured with IR or the Kjeldahl assay.

• In human milk, 20–50% of nitrogen is from non-protein sources.

• Kjeldahl can be adapted to also measure non-protein nitrogen, whereas IR cannot distinguish non-protein nitrogen and uses a correction factor to estimate true protein value (e.g., Miris IR calculates true protein as [crude protein]*0.8)

True Protein

Accounts for the non-protein nitrogen (NPN) either through measurement of NPN or applying a constant discount. This is the most nutritionally relevant value.

Energy

Gross Energy

Energy provided when all organic molecules are burned in a bomb calorimeter.

• Metabolizable energy requires distinguishing protein from non-protein nitrogen, and lactose from HMOs.

• FAO’s recommended metabolizable energy conversion factors are 9, 4, 4, and 2 per gram of fat, true protein, digestible carbohydrate (lactose) and indigestible carbohydrate (HMOs).

• Energy estimates from IR are based on how the instrument is calibrated and configured. Miris IR instrument estimates gross energy using crude protein, total fat, and total carbohydrate measures.

Metabolizable Energy

The energy that is biologically available to an infant after digesting and metabolizing nutrients.