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Lactose Intolerance

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Lactase Persistence

Lactase Persistence describes the presence of lactose in humans, the quantity of which is dependent on age, ethnicity and other factors.

 

Congenital Lactase Deficiency

CLD is a gastrointestinal disorder in the newborn. This disorder is seen in babies throughout the world but a high concentration in Finland suffers from congenital lactase deficiency.

 

Milk Allergy

Milk Allergy is a food allergy, an adverse immune reaction to one or more of the proteins in cow's milk and/or the milk of other animals — proteins that are normally harmless to the non-allergic individual. Milk allergy can cause anaphylaxis, a potentially life-threatening condition. Milk allergy is a separate and distinct condition from lactose intolerance.

 

Hydrogen Breath Test

In a hydrogen breath test, after an overnight fast, 50 grams of lactose (in a solution with water) is swallowed. If the lactose cannot be digested, enteric bacteria metabolize it and produce hydrogen. This, along with methane, can be detected in the patient's breath by a clinical gas chromatograph or a compact solid state detector. The test takes about 2 to 3 hours. A medical condition with similar symptoms is fructose malabsorption.

 

• Celiac Disease
• Bamboo Spine
• Silk Road Disease
• Lactose Intolerance
• Stevens-Johnson Syndrome
• Nasopharyngeal Carcinoma
• Esophageal Cancer

Nomenclature

According to Heyman (2006), approximately 70% of the global population cannot tolerate lactose in adulthood. Thus, some argue that the terminology should be reversed — lactose intolerance should be seen as the norm, and the minority groups should be labeled as having lactase persistence. A counter-argument to this is that the cultures that don't generally consume unmodified milk products have little need to discuss their intolerance to it, leaving the cultures for which lactose intolerance is a significant dietary issue to define its terminology.

 

 

Living Well with Lactose Intolerance by Jaime Aranda-michel, et al.

 

The Milk Sugar Dilemma: Living with Lactose Intolerance by Sherlyn Martens

 

Go Dairy Free: The Guide and Cookbook for Milk Allergies, Lactose Intolerance, and Casein-Free Living by Alisa Marie Fleming

 

 

 

 

 

Excerpts from Wikipedia.org

Lactose intolerance (hypolactasia, lactase non-persistence; 乳糖不耐症) is the inability to metabolize lactose, a sugar found in milk and other dairy products, because the required enzyme lactase is absent in the intestinal system or its availability is lowered. It is estimated that 75% of adults worldwide show some decrease in lactase activity during adulthood. The frequency of decreased lactase activity ranges from as little as 5% in northern Europe, up to 71% for Southern Europe, to more than 90% in some African and Asian countries.

Overview: Disaccharides cannot be absorbed through the wall of the small intestine into the bloodstream, so in the absence of lactase, lactose present in ingested dairy products remains uncleaved and passes intact into the colon. The operons of enteric bacteria quickly switch over to lactose metabolism, and the resulting in-vivo fermentation produces copious amounts of gas (a mixture of hydrogen, carbon dioxide, and methane). This, in turn, may cause a range of abdominal symptoms, including stomach cramps, nausea, bloating, acid reflux and flatulence.

Lactase Biology: The normal mammalian condition is for the young of a species to experience reduced lactase production at the end of the weaning period (a species-specific length of time). In humans, in non-dairy consuming societies, lactase production usually drops about 90% during the first four years of life, although the exact drop over time varies widely.

However, certain human populations have a mutation on chromosome 2 which eliminates the shutdown in lactase production, making it possible for members of these populations to continue consumption of fresh milk and other dairy products throughout their lives without difficulty. This appears to be an evolutionarily recent adaptation to dairy consumption, and has occurred independently in both northern Europe and east Africa in populations with a historically pastoral lifestyle. Lactase persistence, allowing lactose digestion to continue into adulthood, is a dominant allele, making lactose intolerance a recessive genetic trait. A noncoding variation in the MCM6 gene has been strongly associated with adult type hypolactasia (lactose intolerance).

Some cultures, such as that of Japan, where dairy consumption has been on the increase compared with traditional dietary patterns, demonstrate a lower prevalence of lactose intolerance in spite of a genetic predisposition.

Pathological lactose intolerance can be caused by coeliac disease, which damages the villi in the small intestine that produce lactase. This lactose intolerance is temporary. Lactose intolerance associated with coeliac disease ceases after the patient has been on a gluten-free diet long enough for the villi to recover.

Lactase Supplementation: When lactose avoidance is not possible, or on occasions when a person chooses to consume such items, then enzymatic lactase supplements may be used.

Lactase enzymes similar to those produced in the small intestines of humans are produced industrially by fungi of the genus Aspergillus. The enzyme, β-galactosidase, is available in tablet form in a variety of doses, in many countries without a prescription. It functions well only in high-acid environments, such as that found in the human gut due to the addition of gastric juices from the stomach. Unfortunately, too much acid can denature it, and it therefore should not be taken on an empty stomach. Also, the enzyme is ineffective if it does not reach the small intestine by the time the problematic food does. Lactose-sensitive individuals can experiment with both timing and dosage to fit their particular needs.

While essentially the same process as normal intestinal lactose digestion, direct treatment of milk employs a different variety of industrially produced lactase. This enzyme, produced by yeast from the genus Kluyveromyces, takes much longer to act, must be thoroughly mixed throughout the product, and is destroyed by even mildly acidic environments. Its main use is in producing the lactose-free or lactose-reduced dairy products sold in supermarkets.

 

 

 

 

History of Genetic Prevalence: The ability to digest lactose into adulthood (lactase persistence) would have only been useful to humans after the invention of animal husbandry and the domestication of animal species that could provide a consistent source of milk. Hunter-gatherer populations before the Neolithic revolution were overwhelmingly lactose intolerant, as are modern hunter-gatherers. Genetic studies suggest that the oldest mutations associated with lactase persistence only reached appreciable levels in human populations in the last ten thousand years. Therefore lactase persistence is often cited as an example of both recent human evolution and, as lactase persistence is a genetic trait but animal husbandry a cultural trait, gene-culture coevolution.

Several genetic markers for lactase persistence have been identified, and these show that the allele has multiple origins in different parts of the world (i.e. it is an example of convergent evolution). The version of the allele most common amongst Europeans is estimated to have risen to significant frequencies about 7,500 years ago in the central Balkans and Central Europe, a place and time approximately corresponding to the archaeological Linearbandkeramik and Starčevo cultures. From there, it most probably spread eastwards as far as India. Likewise, one of the four alleles associated with lactase persistence in African population, is also probably of European origin. Since North Africans also possess this version of the allele it is probable that it actually originated earlier, in the Near East, but that the earliest farmers did not have high levels of lactase persistence and, subsequently, did not consume significant amounts of unprocessed milk. Lactase persistence in Sub-Saharan Africa almost certainly had a separate origin, probably more than one, and it is also likely that there was a separate origin associated with the domestication of the Arabian camel. None of the mutations so far identified have been shown to be causal for the lactase persistence allele, and it is thought that there are several more yet to be discovered.

The evolutionary processes driving the rapid spread of lactase persistence in some populations are not known. In some East African ethnic groups lactase persistence has gone from negligible to near-ubiquitous frequencies in just three thousand years, suggesting a very strong selective pressure. But some models for the spread of lactase persistence in Europe attribute it primarily to a form of genetic drift. Competing theories on why the ability to digest lactose might be selected for include nutritional benefits, milk as a water source in times of drought, and increased calcium absorption helping to prevent rickets and osteomalacia in low-light regions.

Roman authors recorded that the people of northern Europe, particularly Britain and Germany, drank unprocessed milk. This corresponds very closely with modern European distributions of lactose intolerance, where the people of Britain, Germany, and Scandinavia have a high tolerance, and those of southern Europe, especially Italy, have a lower tolerance.

In east Asia, historical sources also attest that the Chinese did not consume milk, whereas the nomads that lived on the borders did. Again, this reflects modern distributions of intolerance. China is particularly notable as a place of poor tolerance, whereas in Mongolia and the Asian steppes horse milk is drunk regularly. This tolerance is thought to be advantageous, as the nomads do not settle down long enough to process mature cheese. Given that their prime source of income is generated through horses, to ignore their milk as a source of nourishment would be foolish. The nomads also make an alcoholic beverage, called Kumis, from horse milk, although the fermentation process reduces the amount of lactose present.

 

 

* A Worldwide Correlation of Lactase Persistence Phenotype and Genotypes by Yuval Itan, et al.

Interpolated Map of Old World LP Phenotype Frequencies

 

* Lactose Intolerance Seems Linked to Ancestral Struggles With Harsh Climate and Cattle Diseases, Cornell Study Finds by Susan S. Lang

Excerpt: Got milk? Many people couldn't care less because they can't digest it. A new Cornell University study finds that it is primarily people whose ancestors came from places where dairy herds could be raised safely and economically, such as in Europe, who have developed the ability to digest milk.

On the other hand, most adults whose ancestors lived in very hot or very cold climates that couldn't support dairy herding or in places where deadly diseases of cattle were present before 1900, such as in Africa and many parts of Asia, do not have the ability to digest milk after infancy."The implication is that harsh climates and dangerous diseases negatively impact dairy herding and geographically restrict the availability of milk, and that humans have physiologically adapted to that," said evolutionary biologist Paul Sherman, a professor of neurobiology and behavior at Cornell. "This is a spectacular case of how cultural evolution -- in this case, the domestication of cattle -- has guided our biological evolution."

 

* High Frequency of Lactose Intolerance in a Prehistoric Hunter-gatherer Population in Northern Europe by Helena Malmstrom, et al.

Abstract

Background: Genes and culture are believed to interact, but it has been difficult to find direct evidence for the process. One candidate example that has been put forward is lactase persistence in adulthood, i.e. the ability to continue digesting the milk sugar lactose after childhood, facilitating the consumption of raw milk. This genetic trait is believed to have evolved within a short time period and to be related with the emergence of sedentary agriculture.

Results: Here we investigate the frequency of an allele (-13910*T) associated with lactase persistence in a Neolithic Scandinavian population. From the 14 individuals originally examined, 10 yielded reliable results. We find that the T allele frequency was very low (5%) in this Middle Neolithic hunter-gatherer population, and that the frequency is dramatically different from the extant Swedish population (74%).

Conclusions: We conclude that this difference in frequency could not have arisen by genetic drift and is either due to selection or, more likely, replacement of hunter-gatherer populations by sedentary agriculturalists.

 

* Did Lactose Tolerance First Evolve in Central, Rather Than Northern Europe? Tolerance for cow's milk may have arisen in the Neolithic period among the Linearbandkeramik culture of central Europe, not with the Lutefisk-lovers of Scandinavia by Lynne Peeples

 

* The Origins of Lactase Persistence in Europe by Yuval Itan, et al.

Approximate Posterior Density of Region of Origin for LP/Dairying Co-evolution

 

* Genetics of Lactase Persistence and Lactose Intolerance by Dallas M. Swallow

 

* Comparison of Lactase Persistence Polymorphism in Ancient and Present-day Hungarian Populations by Dóra Nagy, et al.

Abstract: The prevalence of adult-type hypolactasia varies ethnically and geographically among populations. A C/T–13910 single nucleotide polymorphism (SNP) upstream of the lactase gene is known to be associated with lactase non-persistence in Europeans. The aim of this study was to determine the prevalence of lactase persistent and non-persistent genotypes in current Hungarian-speaking populations and in ancient bone samples of classical conquerors and commoners from the 10th–11th centuries from the Carpathian basin; 181 present-day Hungarian, 65 present-day Sekler, and 23 ancient samples were successfully genotyped for the C/T-13910 SNP by the dCAPS PCR-RFLP method. Additional mitochondrial DNA testing was also carried out. In ancient Hungarians, the T-13910 allele was present only in 11% of the population, and exclusively in commoners of European mitochondrial haplogroups who may have been of pre-Hungarian indigenous ancestry. This is despite animal domestication and dairy products having been introduced into the Carpathian basin early in the Neolithic Age. This anomaly may be explained by the Hungarian use of fermented milk products, their greater consumption of ruminant meat than milk, cultural differences, or by their having other lactase-regulating genetic polymorphisms than C/T-13910. The low prevalence of lactase persistence provides additional information on the Asian origin of Hungarians. Present-day Hungarians have been assimilated with the surrounding European populations, since they do not differ significantly from the neighboring populations in their possession of mtDNA and C/T-13910 variants.

 

* European Lactase Persistence Genotype Shows Evidence of Association With Increase in Body Mass Index by Johannes Kettunen, et al.

Abstract: The global prevalence of obesity has increased significantly in recent decades, mainly due to excess calorie intake and increasingly sedentary lifestyle. Here, we test the association between obesity measured by body mass index (BMI) and one of the best-known genetic variants showing strong selective pressure: the functional variant in the cis-regulatory element of the lactase gene. We tested this variant since it is presumed to provide nutritional advantage in specific physical and cultural environments. We genetically defined lactase persistence (LP) in 31 720 individuals from eight European population-based studies and one family study by genotyping or imputing the European LP variant (rs4988235). We performed a meta-analysis by pooling the β-coefficient estimates of the relationship between rs4988235 and BMI from the nine studies and found that the carriers of the allele responsible for LP among Europeans showed higher BMI (P = 7.9 × 10−5). Since this locus has been shown to be prone to population stratification, we paid special attention to reveal any population substructure which might be responsible for the association signal. The best evidence of exclusion of stratification came from the Dutch family sample which is robust for stratification. In this study, we highlight issues in model selection in the genome-wide association studies and problems in imputation of these special genomic regions.

 

* Evolution of Lactase Persistence: An example of human niche construction by Pascale Gerbault, et al.

Abstract: Niche construction is the process by which organisms construct important components of their local environment in ways that introduce novel selection pressures. Lactase persistence is one of the clearest examples of niche construction in humans. Lactase is the enzyme responsible for the digestion of the milk sugar lactose and its production decreases after the weaning phase in most mammals, including most humans. Some humans, however, continue to produce lactase throughout adulthood, a trait known as lactase persistence. In European populations, a single mutation (−13910*T) explains the distribution of the phenotype, whereas several mutations are associated with it in Africa and the Middle East. Current estimates for the age of lactase persistence-associated alleles bracket those for the origins of animal domestication and the culturally transmitted practice of dairying. We report new data on the distribution of −13910*T and summarize genetic studies on the diversity of lactase persistence worldwide. We review relevant archaeological data and describe three simulation studies that have shed light on the evolution of this trait in Europe. These studies illustrate how genetic and archaeological information can be integrated to bring new insights to the origins and spread of lactase persistence. Finally, we discuss possible improvements to these models.

 

* Genetic Evidence in Support of a Shared Eurasian-North African Dairying Origin by Sean Myles et al.

Abstract : The process by which pastoralism and agriculture spread from the Fertile Crescent over the past 10,000 years has been the subject of intense investigation by geneticists, linguists and archaeologists. However, no consensus has been reached as to whether this Neolithic transition is best characterized by a demic diffusion (with a significant genetic input from migrating farmers) or a cultural diffusion (without substantial migration of farmers). Milk consumption and thus lactose tolerance are assumed to have spread with pastoralism and we propose that by looking at the relevant mutations in and around the lactase gene in human populations, we can gain insight into the origin(s) and spread of dairying. We genotyped the putatively causal allele for lactose tolerance (–13910T) and constructed haplotypes from several polymorphisms in and around the lactase gene (LCT) in three North African Berber populations and compared our results with previously published data. We found that the frequency of the –13910T allele predicts the frequency of lactose tolerance in several Eurasian and North African Berber populations but not in most sub-Saharan African populations. Our analyses suggest that contemporary Berber populations possess the genetic signature of a past migration of pastoralists from the Middle East and that they share a dairying origin with Europeans and Asians, but not with sub-Saharan Africans.

 

* Independent Introduction of Two Lactase-Persistence Alleles into Human Populations Reflects Different History of Adaptation to Milk Culture by Nabil Sabri Enattah, et al.

Abstract: The T(-13910) variant located in the enhancer element of the lactase (LCT) gene correlates perfectly with lactase persistence (LP) in Eurasian populations whereas the variant is almost nonexistent among Sub-Saharan African populations, showing high prevalence of LP. Here, we report identification of two new mutations among Saudis, also known for the high prevalence of LP. We confirmed the absence of the European T(-13910) and established two new mutations found as a compound allele: T/G(-13915) within the -13910 enhancer region and a synonymous SNP in the exon 17 of the MCM6 gene T/C(-3712), -3712 bp from the LCT gene. The compound allele is driven to a high prevalence among Middle East population(s). Our functional analyses in vitro showed that both SNPs of the compound allele, located 10 kb apart, are required for the enhancer effect, most probably mediated through the binding of the hepatic nuclear factor 1 alpha (HNF1 alpha). High selection coefficient (s) approximately 0.04 for LP phenotype was found for both T(-13910) and the compound allele. The European T(-13910) and the earlier identified East African G(-13907) LP allele share the same ancestral background and most likely the same history, probably related to the same cattle domestication event. In contrast, the compound Arab allele shows a different, highly divergent ancestral haplotype, suggesting that these two major global LP alleles have arisen independently, the latter perhaps in response to camel milk consumption. These results support the convergent evolution of the LP in diverse populations, most probably reflecting different histories of adaptation to milk culture.

 

* Prevalence of Primary Adult Lactose Malabsorption in Three Populations of Northern China by Wang YG, et al.

Abstract: Lactose absorption capacity was examined in 641 apparently healthy adolescents and adults (447 males and 194 females with an average age of 22.9 years and an age range of 16-46 years) using a field version of the lactose tolerance test with breath hydrogen determination. In the total sample, 89 lactose absorbers and 552 lactose malabsorbers were identified. Lactose malabsorption was most frequent in a subgroup of Han (Chinese) from northeastern China (229 of 248 subjects, 92.3%). Among 198 Mongols from Inner Mongolia, there were 174 lactose malabsorbers (87.9%). The frequency of lactose malabsorption was lowest in a group of Kazakhs, traditional herders from the northwestern region of Xinjiang (149 of 195 subjects, 76.4%). Reported symptoms of lactose intolerance were significantly more frequent in lactose malabsorbers. The findings in northern Han are similar to the reported lactose malabsorption frequency in southern (mainly overseas) Chinese, and correspond with the absence of animal milk from traditional Chinese diets. The relatively low prevalence of lactose malabsorption among the Kazakhs suggests that lactose persistence may be frequent in herding pastoralist populations of southwest Asia.

 

* The Lactase Gene -13910T Allele Can Not Predict the Lactase-persistence Phenotype in North China by Hai-ming, et al.

Abstract: The frequency of lactase persistence varies widely in human populations. Study showed that the T allele of a C/T transition 13910bp upstream from exon 1 of lactase gene (LCT) was completely associated with lactase persistence in a Finnish population. To evaluate if the frequency of -13910T allele was in concordance with the lactase persistence in northern Chinese populations, in this study, we used Polymerase Chain Reaction-Restriction Fragment Length Polymorphism (PCR-RFLP) to detect the lactase -13910T allelic frequency in 5 northern Chinese populations for the first time. Results showed that the T allele frequency was low in these populations and that it did not match the lactase persistence phenotype in these populations. Therefore the -13910T allelic frequency can not serve as a predictor of the lactase persistence in these populations and this suggests the existence of other possible mechanisms of lactose tolerance in Chinese populations.

 

* The -22018A Allele Natches the Lactase Persistence Phenotype in Northern Chinese Populations by Xu L, et al.

Abstract

Objective: It has been reported that some single-nucleotide polymorphisms (-13910C/T, -22018G/A, -13907C/G, -13915T/G, and -14010G/C) within the lactase gene are associated with lactase persistence. In our previous study, we found that -13910C/T is not a good predictor of lactase persistence in Chinese populations. To obtain a better understanding of the mechanism of lactase persistence, we examined the frequencies in Northern China of the four other alleles that are associated with lactase persistence.

Material and Methods: We evaluated the allele frequencies of -22018G/A, -13907C/G, -13915T/G, and -14010G/C in six northern Chinese populations (Manchu, Mongol, Hezhen, Oroqen, Kazak, and northern Han) using the methods of polymerase chain reaction-restriction fragment length polymorphism and resequencing.

Results: By genotyping 1092 chromosomes, we found that the frequency of the -22018A allele was highest in the Kazak population and extremely low in the northern Han population. Although there are little available data about the frequency of lactase persistence in northern Chinese populations, we compared the allele frequencies with the phenotype frequencies that have been published previously. We found that the frequency of the -22018A allele was basically consistent with the reported frequencies of lactase persistence in Northern China. With respect to the -13907C/G, -13915T/G, and -14010G/C polymorphisms, we found no individuals with the derived allele.

Conclusions: The frequency of the -22018A allele differed significantly among the six populations and the frequency reflected the frequency of lactase persistence. Taking into consideration the results of previous studies, we believe that the origins of lactase persistence-associated alleles are different in different pastoral populations.

 

* Environmental Influences on Lactose Tolerance by Ronald C. Johnson, et al.

Abstract: This report is of three separate sets of data assessing possible environmental influences on lactose tolerance. The first study involves a comparison of the tolerance of persons of Chinese ancestry residing in Taiwan and Hawaii. Childhood use of dairy products had little influence on tolerance; however, living in a dairy product-using culture (Hawaii) decreased the risk for intolerance of persons at risk for intolerance (one or both parents intolerant). The second report concerns the reported lactose tolerance of Indochinese when residing in Indochina and Hawaii. In contrast to reports in the published literature, these Indochinese drank a great deal of milk in Indochina (usually evaporated or condensed milk mixed with water). Shifting to whole milk in the United States led to frequent but usually very brief episodes of intolerance, even though condensed or evaporated milk has levels of lactose as high as whole milk. The third study reported herein is of sibling pairs varying in lactose tolerance. About two-thirds of the intolerant siblings had a discernible environmental event that preceded their intolerance, and the severity of intolerance varied across types of event. Genetic influences on intolerance may vary in importance depending on the environmental experiences of individuals.

 

* Studies on the Etiology of Milk Intolerance in Japanese Adults by Yutaka Yoshida, et al.

Abstract

1.	The incidence of milk intolerance is approximately 19% in Japanese adults when 200 ml of milk is given. However, a much greater incidence was assumed when considered under the criteria of Western standard.
2.	The lactase activity was significantly greater in milk drinkers than non-drinkers. And, internation ally, the activity is higher in those nationalities whose milk consumption is greater.
3.	Lactase is an adaptive enzyme and rather easily induced by lactose load feeding in animals. From the data of our own and of the literature, it was further confirmed that environmental factors play a more important role than genetic factors in the etio-pathogenesis of milk intolerance.

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乳糖不耐症，又稱乳糖消化不良或乳糖吸收不良，是指人體內不產生分解乳糖的乳糖酶的狀態。主要癥狀為攝入大量乳糖後產生腹瀉、腹脹癥狀。該癥狀與否是基因決定的，不具傳染性。有些人的癥狀會隨時間減輕或加重。

鮮乳是幼兒斷奶以前的主要食物。這期間的乳糖不耐症應該及時諮詢醫生，以避免出現營養不良。斷奶以後出現的乳糖不耐症，則在白色人種以外的人中很常見。

生物原理: 哺乳動物的幼體在斷乳後，開始逐漸的減少乳糖酶的合成。人類的幼兒在4歲的時候通常會失去90%的乳糖消化能力，但各人之間的差異很大。一些人種的第2號染色體上發生基因突變，表現能終止乳糖酶的減少性狀，所以這些人種終生能消化乳糖。

關於該變異出現的時間和地點科學上尚無一致的結論。有人認為是前4000年左右在瑞典和阿拉伯半島分別獨立出現並逐漸在擴展中融合，有人認為是前4500年左右在中東地區出現後，擴展到別處。另外有人認為該變異在更晚的時候又獨立出現在東非的圖西族人中。目前的狀況是：多數祖籍西歐的人基因中存在該變異（意味著他們夠終生消化乳糖），而東亞人、撒哈拉沙漠以南的非洲人和美洲、大洋洲的原住民族則多數沒有該變異（意味著他們成年後會出現乳糖不耐症）。全球平均有75%的人成年後會出現癥狀，所以與其將之稱作疾病，還不如說是自然狀況更合適。

有乳糖不耐症的人不是一旦攝入微量乳糖立即出現腹瀉等癥狀，而是當攝入超過一定量之後才會出現。所以大多數有乳糖不耐症的人仍然是可以喝牛奶的，只是不能過量。比如雖然日本人九成以上有乳糖不耐，但大多數人可以每天喝200毫升的牛奶而沒有任何不適。

在不同人群中的比例: 上述耐受乳糖的變異基因為顯性基因，即在兩個等位基因中，只要有一個顯性基因，則表現顯性性狀，即耐受乳糖。只有當兩個等位基因都為隱形的不耐乳糖基因時，才會表現為乳糖不耐。

人群	取樣人數	不耐比例	等位基因頻率
瑞典人	N/A	2%	0.14
澳洲白人	160	4%	0.20
瑞士人	N/A	10%	0.316
美國白人	245	12%	0.346
芬蘭人	134	18%	0.424
非洲圖西人	N/A	20%	0.447
非洲富拉（Fula）人	N/A	23%	0.48
非裔美國人	20	75%	0.87
澳大利亞原住民	44	85%	0.922
非洲班圖（Bantu）人	59	89%	0.943
華人	71	93%	0.964
泰國人	134	98%	0.99
印第安人	24	100%	1.00

數字來源："Lactose and Lactase", Scientific American, October, 1972, by Norman Kretchmer。 因為各人群取樣人數不同，統計學上的顯著性差異也不同。

有意思的是家貓也有類似的趨勢：全球不少的家貓特別是亞洲品種成年後不耐攝入乳糖，喝牛奶會拉肚子。而很多歐洲品種則因為類似的基因變異不存在這個問題。

致病機理: 在缺乏乳糖酶的情況下，人攝入的乳糖不能被消化吸收進血液，而是滯留在腸道。腸道細菌發酵分解乳糖的過程中會產生大量氣體。造成腹脹、放屁。過量的乳糖還會升高腸道內部的滲透壓，阻止對水分的吸收而導致腹瀉。

診斷

• 隔夜禁食後，口服一定量的乳糖水溶液。乳糖不耐症可由下列幾種手段之一診斷：
  • 出現腹脹或腹部不適
  • 呼氣中監測到腸道細菌乳糖代謝產生的氫氣
  • 血糖上升值遠小於分別攝入同量的葡萄糖和半乳糖（乳糖酶解的自然產物）結果

• 最嚴格的診斷方法是進行腸道活檢來確定乳糖酶的活躍程度。

• 觀察各種乳製品攝入與腹脹、腹瀉癥狀的相關關係，也可作為非醫學的粗略診斷方法。例如第一天飲兩杯牛奶，第二天吃相當量的硬乾酪，分別觀察有無癥狀。只是第一天有癥狀的話大概是乳糖不耐症，兩天都有癥狀的話，則有可能是對乳製品過敏。

防治: 因為是基因原因導致，目前沒有根治的方法。基本的對策包括避免攝入乳糖，或者用人造的乳糖酶藥物來幫助消化。

避免攝入乳糖: 因為各人消化乳糖的能力不同，攝入多少量以下的乳糖不引起癥狀主要靠實踐中摸索來決定。

最根本的方法是限量食用奶製品，以及含有奶製品的食物。豆漿、豆奶、杏仁奶、豆腐等產品可以部分替代乳製品的營養（尤其是蛋白質和鈣）和烹飪效果，在價格上也比服用乳糖酶便宜很多。

酸奶因為乳酸菌分泌乳糖酶而且已經分解了一部分乳糖，比鮮奶容易消化。另外，瑞士乾酪、Cheddar等硬幹酪，由於其製作工藝和後來的細菌轉化，含乳糖也比同量的牛乳低得多。

避免乳制食品的同時，也要注意到其他很多食品等含的乳製成分，例如麵包里加入的乳清、高蛋白人造食品中的乳清蛋白、藥品中用來增加體積的乳糖。

此外，近年來食品工業界還發明了兩種降低鮮乳乳糖含量的技術：通過固定有乳糖酶的介質過濾，或者加入乳酸桿菌（L. acidophilus）。

乳糖酶藥物: 一般為口服片狀，有效時間為30-45分鐘。也有液體的乳糖酶，可以用來滴進鮮乳中提前消化乳糖。 長期使用乳糖酶藥物價格不菲，但偶然在外使用能為乳糖不耐症患者帶來一定的就餐自由。

命名: 根據科學家研究(Heyman 2006)世界上80%的成年人都存在乳糖不耐的情況。因此有些人認為乳糖耐受性不是一個標準，相對著應該把剩下的20%少數人命名為乳糖耐受群體。