Lactose intolerance is a common digestive condition where the body has difficulty digesting lactose, a sugar primarily found in milk and dairy products. An enzyme called lactase, located in the small intestine, breaks lactose down into simpler sugars: glucose and galactose. These simpler sugars are then absorbed into the bloodstream for energy. When there is insufficient lactase, undigested lactose travels to the large intestine, leading to various uncomfortable symptoms.
Genetic Basis and Prevalence
Lactose intolerance often stems from lactase non-persistence, an ancestral human condition where lactase activity naturally decreases after weaning. This decline in lactase production is widespread globally, affecting approximately 65-70% of the world’s adult population.
Lactase persistence, the continued ability to digest lactose into adulthood, is a relatively recent evolutionary adaptation. It arose in populations with a long history of dairy farming and consistent consumption of milk products, such as certain Northern European and some nomadic African groups. These populations developed genetic mutations that allow the lactase enzyme to remain active throughout life.
In contrast, populations with less historical reliance on dairy farming, including most Chinese individuals, typically retain the ancestral lactase non-persistence trait. Studies indicate a high prevalence of lactose intolerance among Asian populations, affecting about 90% of individuals in some groups. While genetic markers, like the C/T-13910 variant, are associated with lactase persistence in European populations, research in northern Chinese populations shows this allele is very low or absent and does not reliably predict lactose tolerance, suggesting other genetic or epigenetic factors may be at play.
Recognizing Symptoms and Diagnosis
Lactose intolerance manifests through gastrointestinal symptoms that typically appear within 30 minutes to two hours after consuming lactose-containing foods. Common symptoms include bloating, increased intestinal gas, abdominal pain, stomach cramps, nausea, and diarrhea. The severity of these symptoms can vary among individuals, depending on the amount of lactose consumed and the degree of lactase deficiency.
Diagnosing lactose intolerance involves several methods. A common diagnostic tool is the hydrogen breath test, which measures hydrogen gas before and after consuming a lactose solution. Undigested lactose fermented by bacteria in the large intestine produces hydrogen, which is then absorbed and exhaled. Elevated hydrogen levels after lactose intake indicate lactose malabsorption.
Another diagnostic method is the lactose tolerance test, where blood sugar levels are monitored after consuming a lactose-rich liquid. If blood sugar levels do not rise significantly, it suggests that lactose is not being properly broken down and absorbed. For infants and young children, a stool acidity test can be used, as undigested lactose in the stool leads to increased lactic acid and other fatty acids. An elimination diet, where dairy products are removed for a period to see if symptoms improve, and then gradually reintroduced, can also help in identifying lactose intolerance.
Dietary Management and Cultural Considerations
Managing lactose intolerance involves adjusting dietary habits to minimize symptoms while ensuring adequate nutrient intake. Many individuals can tolerate small amounts of lactose, so complete avoidance may not always be necessary. Options include choosing lactose-free or reduced-lactose dairy products, which have been pre-treated with lactase enzyme, or incorporating lactase enzyme supplements before consuming dairy to aid digestion. These supplements break down lactose in the digestive tract, mimicking the action of naturally produced lactase.
Traditional Chinese cuisine is inherently low in dairy products. Historically, fermented mammalian milks were consumed in some Asian subgroups, but the widespread uptake of liquid milk and powdered milk in Asia largely began after World War II, often influenced by Western dietary trends. This means that many Chinese individuals do not have a long ancestral history of dairy consumption, aligning with the high prevalence of lactase non-persistence in the population.
For individuals with lactose intolerance in China, culturally appropriate dairy alternatives are readily available. Soy milk is a widely consumed and traditional plant-based beverage that serves as an excellent dairy substitute. Other alternatives include rice milk, almond milk, and oat milk, which are increasingly popular. When selecting these alternatives, it is important to choose varieties fortified with calcium and vitamin D to ensure adequate intake of these nutrients, as they may not naturally be present in the same quantities as in cow’s milk.
Ensuring sufficient calcium and vitamin D intake from non-dairy sources is important for bone health, especially since dairy products are a major source of these nutrients in many Western diets. Calcium can be obtained from foods such as leafy green vegetables like bok choy and kale, fortified orange juice, tofu, and certain fish like sardines and salmon. Vitamin D can be synthesized by the skin through sun exposure, but dietary sources like fatty fish, fortified cereals, and supplements may be needed, particularly in regions with limited sunlight or for individuals with reduced sun exposure.