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From the point of view of evolution and nutrition, breast milk is the most ideal food for babies. The World Health Organization recommends that babies should be exclusively breastfed for the first six months after birth, and then start to add complementary food after six months, and continue breastfeeding until they are 2 years old or longer. Lactose is considered as the most important macronutrient in human milk, the earliest milk component studied by people, and the main component that affects the osmotic pressure of human milk. Among all mammalian milk, human milk and monkey milk have the highest lactose content.

Lactose is a unique compound secreted by mammalian mammary glands and one of the main components of mammalian milk. It dissolves in milk. Lactose is a unique sugar in milk, which only exists in animal milk. It hardly exists in other tissues of animals, especially in the plant kingdom, and only exists in the anthers of forsythia suspensa. Among them, the carbohydrate in milk is about 4.7%, of which 99.8% is lactose; The carbohydrate content in human milk is about 7%, of which 90% is lactose and the rest is mainly oligosaccharides (HMOS1). In addition, it was found that the reducing sugar terminal of breast milk oligosaccharides (HMOs) all contained lactose groups 2 and 3 except a few oligosaccharides.

Lactose is considered to be the most important macronutrient in human milk, the earliest milk ingredient studied by people, and the main ingredient that affects the osmotic pressure of human milk. Lactose is composed of glucose and galactose linked by β- 1, 4- glycosidic bond, which is hydrolyzed by lactase in small intestine to produce glucose and galactose, which is absorbed and utilized as an important nutrient. Glucose is the main energy source of human cells, and galactose is involved in the development of brain cells and nervous system, and is a component of glycolipid. Lactose and its fermentation product lactic acid play an important role in the absorption and utilization of calcium and phosphorus.

As a natural carbohydrate, lactose is an early choice in the process of animal evolution. Compared with sucrose, lactose has the characteristics of mild sweetness (only 1/6- 1/5 of sucrose), low osmotic pressure and slow absorption speed. However, its most remarkable and often overlooked feature is that lactose contains a precursor substance, which is naturally produced and contributes to the physical growth and intellectual development of infants. Lactose is the main source of heat energy for infants during lactation.

Lactose is heated by dilute acid or hydrolyzed by β -galactosidase to produce glucose and galactose. The scientific name of β -galactosidase is lactase. It can catalyze the hydrolysis of β -galactoside bonds in β -galactoside compounds and has the function of transgalactoside. There are many microbial sources, and the optimum pH and temperature of lactase from different microbial sources are also different. Because of the different optimum pH, it can be divided into two categories: acidic lactase and neutral lactase. Acid lactase is derived from Aspergillus Niger, Aspergillus oryzae, etc. Neutral lactase is derived from yeast, Escherichia coli and so on.

Due to the transgalactoside action of lactase, when lactose is hydrolyzed by lactase, some oligosaccharides (such as galactooligosaccharides GOS) will actually be produced.

Lactose can also be used by lactic acid bacteria, which secrete lactase, which acts on lactose and breaks it down into glucose and galactose. Other enzymes produced by lactic acid bacteria react with glucose and galactose and are converted into lactic acid substances through complex intermediate reactions.

1. Provide energy for the body

Lactose is one of the first energy substances obtained from milk after a baby is born. The carbohydrate content in human milk is about 7%, of which 90% is lactose. In the first six months, infants need lactose10-14g per kilogram of body weight every day, and 8-9g for 6- 12 months. The lactose content in milk is about 4.7%. This is also a basic reason why lactose must be added to milk-based infant formula. If sucrose, glucose, fructose, maltose and other carbohydrates are added, it will cause infantile diarrhea and obviously increase the burden on liver and kidney. This is why lactose is the first choice of carbohydrates in the regulations and standards of infant formula milk powder, and the content of lactose must account for more than 90% of the total carbohydrates.

Regulating intestine

Lactose is not hydrolyzed in the stomach, only a small amount is absorbed in the upper part of the small intestine, and at the end of the small intestine, lactose is converted into lactic acid, forming an acidic environment. Promote the growth of lactic acid bacteria and bifidobacteria in colon drugs, and inhibit the reproduction of other harmful bacteria. Therefore, it plays a great role in maintaining intestinal microecological balance and preventing and treating infantile diarrhea.

The acidic environment caused by lactose prevents the growth of basophilic spoilage bacteria and acid-sensitive bacteria, and forms an environment that promotes the survival of acidophilic beneficial bacteria in the extremely low area of the intestine.

3. Promote intestinal absorption of minerals such as calcium, phosphorus and manganese.

Because lactose is more difficult to hydrolyze than other sugars and stays in the stomach for a long time, it is beneficial for intestinal microorganisms to metabolize lactose to produce organic acids (such as lactic acid), reduce the pH value in the intestine, improve the solubility of calcium salts, and promote the transportation of minerals such as calcium and phosphorus. Animal experiments show that lactose can significantly improve the absorption rate of phosphorus, manganese and other minerals, increase the solubility of calcium and phosphorus in blood, accelerate the entry of calcium into bones, and contribute to the improvement of bone quality.

The research shows that the absorption rate of pure active calcium in intestine is 62.59%, and the absorption rate of lactose plus calcium is 87.33%. Lactose can obviously promote calcium absorption.

4. Participate in the cell composition and activities of the body.

Lactose is a component of glycoprotein and glycolipid, and participates in many functions of cells. Oligosaccharides and glycoproteins in milk are anti-infective agents for newborns, which can effectively inhibit the growth of bacteria at the initial stage of infection. A large number of oligosaccharides and glycoproteins contained in milk can be used as receptor analogues of intestinal epithelial cells to combine with microorganisms to prevent them from adhering to intestinal wall. Therefore, breast-fed babies are less likely to be infected with gastrointestinal and respiratory diseases within one month.

5. Promote intellectual development 1

Galactose produced by lactose hydrolysis is a component of glycolipid in brain and nerve tissue, which is very important for infant's intellectual development and can promote the growth and development of brain stem. The production of mucopolysaccharide can promote the development of brain and nerve tissue. It has been scientifically proved that galactose is contained in brain tissues such as glycolipid of brain nerve cells and gray matter of brain. Lactose, as the precursor of galactose, once lacking, will lead to mental retardation or low intelligence level of larvae.

From the physiological function of lactose on infants, it can be seen that the nutritional function room of lactose is unique and irreplaceable by other carbohydrates and other nutrients.

After lactose enters the body, it is first decomposed into glucose and galactose by the total lactase in the small intestine, and then absorbed by the small intestine. However, if lactase is deficient in human body, most lactose cannot be decomposed into colon in small intestine, and is further metabolized by bacteria in colon. The metabolism of lactose in the intestine is influenced by many factors, including lactase activity of intestinal mucosa, the amount of lactose entering the intestine, gastric emptying rate, intestinal running time, the ability of intestinal bacteria to ferment lactose, and the compensation of large intestine to intestinal osmotic pressure. If there is a large amount of undiluted lactose in the small intestine, the osmotic pressure in the intestine will increase, and if the large intestine cannot restore the osmotic pressure in the intestine, it will cause discomfort and even diarrhea.

The metabolism of lactase plays a decisive role in human body. Lactase deficiency will lead to lactose metabolism disorder in human body. There are three common terms about lactose metabolic disorder.

Lactase deficiency

It refers to the insufficient quantity or low activity of lactase that can combine with villous cells of brush border of small intestine in human small intestine.

Lactose intolerance (plum)

It refers to the phenomenon of abdominal distension, abdominal pain, diarrhea, nausea and vomiting caused by the lack of lactase in small intestinal mucosa, which leads to the disorder of lactose metabolism.

Lactose malabsorption

It refers to the phenomenon that the lack of lactase in small intestinal mucosa only hinders the absorption of lactose by individuals without obvious clinical symptoms.

Lactase deficiency (LD) can be divided into congenital lactase deficiency, primary (adult) lactase deficiency and secondary lactase deficiency according to its etiology.

Congenital lactase deficiency, a rare autosomal recessive genetic disease, is the lack or low lactase activity at birth.

Primary lactase deficiency refers to the gradual decline or disappearance of lactase activity with age, which belongs to physiological lactase decline.

Secondary lactase deficiency refers to temporary low lactase activity caused by various reasons, such as infectious diarrhea, intestinal surgery, immunoglobulin deficiency, malnutrition, abdominal radiation and so on.

To sum up, lactose is one of the most important nutrients for lactating infants, and it has nutritional and physiological functions that other carbohydrates cannot replace. For infants with transient secondary lactase deficiency caused by various reasons, it is not recommended to replace their normal diet with lactose-free formula products. Instead, it is recommended to supplement lactase in the normal diet. How to choose lactase correctly (acidic lactase or neutral lactase)? Powder or drop? .....) products. You can refer to two popular science articles, Teaching You How to Choose Lactase for Infants and Discussion on Acidic Lactase and Neutral Lactase, which have detailed interpretations. 8.9

References:

1 Yin Shi 'an, etc. Composition-existing form, content, function and detection method of human milk.

Wu Junlin, Lin Weitie, et al. Application of oligosaccharides in infant food. Guangzhou Food Industry Science and Technology, 2003, 19:66-68.

Sun Jianhua and Xie Enping. Breastfeeding and neonatal immunity, Journal of Clinical Pediatrics, 20 12, 30: 204-7.

4 Cederlund A, Kai-Larsen Y, Printz G, et al. Lactose in breast milk is an inducer of innate immunity and has an effect on intestinal homeostasis. PloS one，20 13，8: e53876

5 Zhao Xinhuai, Zhang Yongzhong, et al. 2007 Dairy Chemistry. Beijing: Science Press.

6 He Ping, Zhang Liebing, etc. Handbook of Modern Dairy Industry 2005. Beijing: China Light Industry Press.

7 weeks, Li Na. Lactase deficiency. Clinical highlights. 20 12,6:548-550.

8 Aibei Nutrition. Teach you how to choose lactase for infants. Know almost

9 Aibei Nutrition. Discussion on acidic lactase and neutral lactase. Know almost