Lactose and Human Breast Milk: How Sweet it Is!

The medical definition of “lac” is milk. It is therefore no surprise that the most abundant sugar (carbohydrate) that comprises human milk is named lactose.

Lactose is a disaccharide, meaning it is comprised of two monosaccharides or simple sugars. The two simple sugars bonded together (glycosidic linkage) to form lactose are glucose and galactose. Glucose is the primary metabolite used by cells to generate energy through various metabolic cycles. Galactose is converted to glucose in the blood so that essentially, you get two simple sugars for the price of one lactose.

The Best Sugar for Nutrition

It would be reasonable to ask why lactose, and not the sugar utilized in metabolism (glucose), is the major carbohydrate in human breast milk. The answer is to keep the concentration (dilution in water) of the sugar consistent and reproducible.

Consider if, physiologically speaking, you required one molecule of water for every molecule of sugar. That would mean one molecule of water per molecule of lactose. Lactose will produce 2 molecules of sugar after it is enzymatically cleaved by lactase (more on this later). If you were to have glucose instead of lactose and wanted the same energy equivalent, you would need 2 glucose molecules. Those 2 glucose molecules would require 2 water molecules and thus per volume, deliver half the energy or caloric equivalent. Glucose rather than lactose would have more water molecules and thus dilute other compounds such as proteins. Lactose appears to produce the proper osmolarity of human milk as demonstrated across other mammalian species and not just human milk.

Making 2 Sugars from One

Sugars, like proteins and other compounds that enter the GI tract, are broken down by enzymes to products that can be absorbed by the intestinal tract. An enzyme known as lactase hydrolyzes lactose into glucose and galactose, which are then readily absorbed by the intestinal lumen and enter the blood stream.

Galactose is converted by cellular enzymes to glucose, which is metabolized through a series of steps and cycles to generate energy currency used by cells. Or in a separate step, are combined to form polymers (glycogen) that is stored in the liver for later use.

Bugs like it too!

The problem with lactose, in terms of the GI tract, is reduced metabolism to glucose and galactose. Lactose is not absorbed by the intestinal lumen. It must be hydrolyzed, or broken into sugars with water, via the enzyme lactase. If lactase is insufficient in quantity or has reduced enzyme activity, the lactose will pass further into the intestine where bacteria (aka bugs) will break it down for their own source of nutrition and hence increase metabolism and their growth (and not yours). The byproducts of sugar metabolism in bacteria are gases that may distend the intestine and thus cause pain and distress. These metabolites may also be toxic or cause inflammation in the GI tract.

Lactase: Not just an enzyme in your GI Tract

To give this article further context about the importance of lactase, it is important to note that lactase is the active ingredient in some OTC preparations. The most familiar product is Lactaid® sold at grocery stores and pharmacies in a variety of forms. One product containing the enzyme, lactase, is taken orally (it is available in a variety of formulations) shortly before, during, or after a meal. This enzyme is activated in the stomach and passes into the intestine where is hydrolyzes lactose to glucose and galactose. Many adults cannot break down lactose efficiently due to its deficiency or reduce rate of enzyme activity. As described previously, the undigested lactose feeds microbiota of the GI tract, producing gas.

Oral lactase supplements are endogenous (existing) lactase to provide more enzyme activity and digestion of lactose. For example, after eating milk products rich in lactose such as whole milk, ice cream, and cheese, the amount of lactose consumed may be too high to be metabolized by lactase before the bacteria consume it. Therefore, supplementing lactase helps the enzyme “keep-up” with lactose digestion. Note that some food products are pretreated with lactase to break down the disaccharides rather than take a pill. For example, a milk product that is enriched with lactase is sold as Lactaid® milk. Donor human breast milk, like whole cow’s milk, may also be pretreated with lactase. One such product is provided by Medolac Laboratories as Lac Free®.

A Different Perspective of our GI Tract

It helps to consider the following analogy when thinking about our GI tract: It is a tube with different compartments that go from an entrance such as the oral cavity to the exit, our rectum. The different compartments would be the mouth, stomach, small and large intestines, and rectum, with efficient valves on both ends that control input and output.

But like all pipes, it is exposed to the environment on both ends. It is not only an entrance for food, water and nutrients, but also bacteria, viruses, and toxins, and a route of administration of certain pharmaceuticals. You can measure input and output to estimate how much is transferred across these pipes and compartments into blood to be delivered to tissues and organ systems.

Like any pipe that exists in nature, our GI tract is exposed to the environment and living organisms. Our GI tract is quite complex, with various compartments and segments. What is most fascinating is its length and surface area. The small intestine is not only long in length but has a large surface area made possible by numerous folds and protrusions from the villi on its surface. This surface area provides maximal opportunity to digest and absorb nutrients. Food products that are not broken down and absorbed into our blood and that are not digested by microorganisms are eliminated as fecal matter. Lastly, our GI tract is quite an underestimated organ system relative to the attention the heart and brain gets in medicine and science until recently. There are quite a few jokes about who is boss: The brain, the heart, or the stomach. All joking aside, a healthy GI tract is key to good health.

The enzymatic activity of lactase varies by age and genetics. We know by experience that as we age, many of us do not digest milk or milk products with the same ease we did as children or young adults. The very low birthweight infant (preterm) is not fully mature and it is thought that the lactase enzymatic action of converting lactose to glucose and galactose is reduced. This lactase inefficiency causes lactose to not be fully broken down, and since it is not absorbed in the small intestine, it is available as food for microorganisms. Their byproduct of metabolism is gas either as CO2 or small gaseous molecules that distend, inflame, or cause toxicity.

Some NICUs supplement lactase in their human breast milk products in the first week of life to assist in the digestion of lactose. At Medolac we developed a product that pretreated milk with lactase so it has the two products, glucose, and galactose rather than lactose. This products contains the nutrients required for absorption by the GI tract and because it is not in the form of lactose, the amount of bacteria feeding on the undigested milk sugar is reduced and as such the metabolic byproducts of their digestion, excess gas and distention.

It is also thought that lactose is required to encourage development of lactase activity. Therefore treatment with a lactase supplement should be reduced over time. Neonatologists may also consider its use or a different sugar (i.e fructose) that doesn’t require lactose for treatment in the early stages of NEC. The reason is to reduce gas, inflammation, and toxicity caused by organisms feeding on lactose. Further research in these areas is ongoing.

Observation regarding NEC

NEC, or necrotizing enterocolitis, is a disease of the GI tract for very preterm infants where the gut doesn’t grow and develop. Instead it begins to break down (necrotizes) and dies in certain segments. Surgery is required to remove part of the necrotic intestines in serious cases. Although some infants do not survive, the shortening of the intestinal tract creates problems with short- and long-term consequences in normal digestion. As described above, a healthy GI tract is a long and large effective surface to absorb nutrients required for nutrients that supply energy and growth complete with symbiotic microorganisms that aid in digestion. Anything that harms the GI tract by reducing its efficiency, whether that is NEC or other acute problems such as diarrhea, creates an unhealthy condition.

In Summary

Understanding the function and activity of the GI tract is important in nutrition. Human breast milk and its various components are designed for a “normal” GI tract in newborns. With preterm infants, the challenge is modifying practices to suit the needs of that infant. These practices change with time and age and require further investigation along with the exciting field of the microbiome. At the end of the day, growth and optimal health requires absorption of nutrients from the food we eat and that requires a healthy GI tract.

About the Author

Donald H. Chace, PhD, MSFS, FACB is the Chief Scientific Officer for Medolac, a public benefit corporation. He is one of the primary developers of newborn metabolic screening using tandem Mass Spectrometry. Developed 25 years ago with the first screening publication in Clinical Chemistry that describes the MS-based newborn screening of PKU, the method is now used to screen millions of infants per year, worldwide. Dr. Chace is an expert in metabolism and clinical chemistry using mass spectrometry as well as microsample analysis, e.g. the dried blood spot. Learn more about Dr. Chace by visiting his LinkedIn profile.