Know Your Macronutrients: Carbohydrates

Introduction

Carbohydrates are a type of macronutrient (components of foods that are broken down and absorbed by the body to support the maintenance of life) that provide one of the main sources of dietary energy for infant growth and development.¹ Carbohydrates are either hydrolyzed (broken down) in the small intestine to provide energy to the body, or are, in the case of dietary fiber, resistant to hydrolysis.² In addition to being a primary energy source for the body, carbohydrates also help shape the gut microbiome and gastrointestinal tract.³ Carbohydrates are found in many foods like bread, pasta, fruit, vegetables, pastries, cookies, and dairy products. Infants also consume carbohydrates in human milk and infant formula – mature milk contains up to 7% (60-70 g/L) carbohydrates, primarily lactose,⁴ and formula can contain different sources of carbohydrates such as lactose, maltodextrin, or corn syrup solids. To learn more, see the ‘Fast Facts: Carbohydrates’ box for more in-depth information. 

FDA Requirements

The United States Food & Drug Administration (FDA) establishes strict standards for infant formula ingredients in terms of ingredient quantity, but infant formula companies have flexibility when it comes to ingredient type. This means that infant formula companies have choices about which carbohydrate sources to include in their formulas, including lactose, corn syrup solids, maltodextrin, sucrose, glucose, glucose syrup, maltose, pre-cooked starch, and gelatinized starch. Importantly, sucrose and/or glucose are only permitted under specific circumstances (e.g., in hydrolyzed infant formula). 

Carbohydrate sources may have different functional impacts on infant health. For example, potential impacts of corn syrup solids and maltodextrin include negative alterations of the gut microbiome,⁵ increased risk of obesity,^6,7 and possible implications for early learned preference for sweet taste due to higher glycemic indices.^6,7 Lactose, a slow-release disaccharide (a carbohydrate made up of two sugar units, glucose and galactose), is the predominant carbohydrate source in mature human milk.⁷ Not only is it associated with reduced stool transit time,⁸ but it acts as a prebiotic in the small intestine by promoting the growth of commensal bacteria.^7–9 

Lactose: A Shift in Paradigm

Lactose used as a carbohydrate source was previously believed to result in maldigestion and malabsorption in infants.^10–12 Advancements in human milk science, however, have established lactose as the predominant carbohydrate source in human milk, and nearly all infants are equipped with adequate lactase (the intestinal brush border enzyme that hydrolyzes lactose) to adequately digest lactose.¹⁰ Infant formulas containing lactose are therefore well-tolerated by most healthy term infants. While there are clinical classifications of lactose intolerance in infants (e.g., developmental lactase deficiency in infants <34 weeks gestation, secondary lactose intolerance, or congenital lactase deficiency), these conditions are rare11 and low-lactose and lactose-free formulas for healthy, term infants are very rarely clinically indicated. Lactose is also known to lessen reward potential (i.e., the motivation to consume additional, or excessive, palatable foods) with its low glycemic index of 46, and to be less cariogenic than glucose.⁷ Lactose is also utilized in synthesis of oligosaccharides, glycoproteins, and glycolipids,⁸ and to enhance absorption of calcium, sodium, and water.¹³ 

Nutrient Spotlight: Prebiotic Fibers

Prebiotics are mostly indigestible carbohydrates that undergo fermentation by commensal bacteria. Human milk oligosaccharides (HMO), fructo-oligosaccharides (FOS), and galacto-oligosaccharides (GOS) are three types of prebiotics that help establish the infant gut microbiome. They are very important during an infant’s first 1,000 days, as this time represents a critical window for gut microbiome development associated with maturation of the infant’s immune system.¹⁴ 

There are several hundred types of HMOs naturally found in human milk, and although they provide no direct energy to the infant, they are known to have structure-specific biological functions. HMOs have several key functions, including, but not limited to, selectively feeding the commensal probiotic Bifidobacteria in the infant gastrointestinal tract and influencing immune system development.¹⁵ Some of the smaller, more abundant HMOs have been manufactured at scale and added to some infant formulas to promote infant gut and immune health.¹⁶ However, structurally more complex HMOs are largely unavailable for commercial use.¹⁷ 

While neither GOS, nor FOS, are naturally found in human milk, they are known to mimic several functions of HMOs including the promotion of the probiotics Bifidobacteria and Lactobacilli, playing a role in gut barrier function, inhibition of pathogen adhesion, and promoting softer stools.^17–20 These well-researched oligosaccharides are synthesized from lactose and plants, respectively. GOS and FOS are common prebiotic ingredients in infant formulas, used to achieve gut health outcomes that are similar to those in infants fed human milk.