Glossary Detail

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name of the term: Lactose

Short introduction

  • Lactose is the major carbohydrate in mammalian milk. The content is variable in human breast milk. Research shows ranges from 4.0–8.0 g per 100 ml.
  • Lactose is a disaccharide composed of the monosaccharides glucose and galactose.
  • The intestinal villi (in lining of the small intestine) secrete the enzyme lactase. Lactase breaks down lactose (hydrolysis, digestion) into its small units glucose and galactose. Villi are small, finger-like projections that enable the small intestine to absorb nutrients.
  • Glucose and galactose are absorbed into the bloodstream and travel to the liver. Galactose is converted to glucose. Glucose is transported, by the bloodstream, to various tissues and organs and used as energy.
  • Young infants do not digest all lactose. Incomplete lactose digestion is normal and natural. Undigested lactose moves into the large intestine and is available for fermentation by colonic microbiota – stimulating growth of the ‘good bacteria’ Bifidobacteria and Lactobacilli (acts as ‘prebiotic’).
  • Recent research: role of lactose in immunity by increasing the response of specific antimicrobial compounds. This leads to protection of the neonatal gut against pathogens and, regulation of the microbiota.
  • Formulas for infants (stage 1, 0–6 months) contain a comparable amount of lactose as human milk.
  • Follow-on formulas (stage 2, from 6 months onwards) and growing-up formulas (from 12 months onwards) contain lactose as the main carbohydrate.
  • Formulas from 6 months and above contain a bit less lactose for prevention of complaints caused by a possible decline in lactase activity.
  • Lactose is not as sweet as other disaccharides e.g. sucrose, glucose or fructose.

Main natural sources

  • Milk and dairy products.

Main function

  • Source of energy: 1 g of carbohydrates (e.g. lactose) provides 4 kcal (17 kJ).
  • Possible role in immunity by increasing the response of specific antimicrobial compounds leading to protection of the neonatal gut against pathogens and, regulation of microbiota.

Deficiency disease

Note: this describes a deficiency of the enzyme lactase and not of the nutrient lactose.

  • Some people cannot properly digest lactose – due to a reduction of the enzyme lactase found in the lining of the small intestine (lactase deficiency).
  • Lactase deficiency results in lactose malabsorption. Lactose is not or incompletely hydrolysed (digested) and poorly absorbed into the bloodstream. It moves into the large intestine (colon). Bacteria in the colon react to the lactose, triggering symptoms.
  • Primary lactase deficiency is genetically inherited and occurs soon after weaning – as the diet includes less milk, lactase production drops. It may be a few years before lactose intolerance symptoms develop. Lowest rates are found in Northern Europe and highest in some parts of Africa, East Asia and among Native Americans.
  • Secondary or transient lactase deficiency is usually temporarily in infancy. It results from injury to the intestinal villi (production place of lactase) within the small intestine, causing loss of lactase activity, e.g. due to acute gastroenteritis.
  • Congenital lactase deficiency is a rare inherited condition. Lactase activity is decreased or absent at birth and persists throughout life.
  • Lactose intolerance describes the symptoms that occur as a result of the lactose malabsorption e.g. abdominal distension (belly swelling), flatulence, excess gas production, abdominal cramping and diarrhoea.
Synonyms: Lactose, Milk sugar

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