2. What are the health outcomes of dairy exclusion diets?
The health outcomes of dairy exclusion diets depend on whether other sources of nutrients, such as calcium and vitamin D, occur in the diet in sufficient quantities to replace dairy products as a source of these nutrients, and to what extent other components of milk are beneficial.
Calcium is necessary for normal growth and bone development as well as subsequent maintenance of bone density. The strongest argument for promotion of dairy ingestion is the beneficial effect of calcium (and fortified vitamin D in milk) on growth and development of the skeleton. Calcium is necessary for adequate bone accretion and optimal peak bone mass, which is a major determinant of risk for osteoporosis and fragility fractures later in adult life. Evidence suggests that certain age groups, such as children and teenagers, may be at increased risk for deficient bone acquisition if their diets are deficient in calcium or vitamin D. There is weak evidence that children with diets deficient in calcium have increased fracture rates. The maximal accumulation of bone mineral, and therefore the maximal calcium requirement, occurs during puberty. Although studies indicate that young children who drink milk are likely to meet or exceed the adequate intake for calcium, teenagers, as a group, tend not to take in enough calcium to meet recommended needs. This is exacerbated by dairy avoidance in individuals who consider themselves to be lactose intolerant, regardless of whether they have undergone objective testing for lactose intolerance.
Studies have demonstrated that the presence of lactose does not necessarily affect the efficiency of calcium absorption across the intestine, and that lactase nonpersisters do not have significant impairment in calcium absorption. Thus, the limiting factor in achieving optimal peak bone mass in young individuals is the intake of calcium. Similarly, in older individuals, low calcium intake rather than deficient absorption appears to be a major factor contributing to loss of bone mass. Replacement of calcium using supplements or dairy products slows the rate of bone loss in older people, possibly as a result of an overall decrease in bone turnover. Across the age spectrum, the factor limiting adequate calcium accrual in many individuals appears to be dairy avoidance.
Dairy exclusion diets may exacerbate the risk for osteoporosis for those already at greatest risk. These include women throughout the life cycle and certain racial/ethnic groups. Low intake of dairy products may place African Americans and others at risk for deficiencies of other necessary nutrients for bone health such as vitamin D, in addition to low calcium intake. Individuals with diseases that result in decreased calcium absorption due to intestinal inflammation (inflammatory bowel disease) or that require the use of corticosteroids (which in themselves directly reduce bone mass) have increased risk of osteoporosis.
Dairy exclusion diets may decrease gastrointestinal symptoms (bloating, cramps, flatus, and diarrhea) in symptomatic individuals who have lactose malabsorption or lactose intolerance. The degree of relief is likely related to the level of expression of lactase and the quantity of lactose ingested. People who remain symptomatic on a dairy exclusion diet may have other causes for their gastrointestinal symptoms, such as irritable bowel syndrome, celiac disease, inflammatory bowel disease, or small bowel bacterial overgrowth.
Dairy exclusion diets may affect other health outcomes. In several studies, individuals taking calcium supplements or increased dairy intake have decreased blood pressure. Calcium supplementation has been suggested to improve cardiac and vascular smooth muscle contractility; however, additional research is needed to clarify whether this has a significant impact on cardiovascular risk. Calcium ingestion has been associated with decreased risk of development of adenomatous colon polyps; it is not known whether this translates into decreased rates of colon cancer. One area of recent interest is the effect of lactose ingestion on colonic bacterial populations, as this may increase production of fatty acids such as butyrate, which may promote mucosal growth and reduce inflammation.
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