Lactase Persistence A Model For Antagonist Pleiotropy Usa Biology Essay


In the United States we are taught that "milk does a body good" and to consume at least 3 servings of dairy a day. However, this dairy-centered idea of health and nutrition is not common around the world. In fact, 70% of the globe is lactose intolerant, having no lactase enzyme in their gut to digest lactose (the sugar in dairy) past the age of infancy. Lactose intolerance is the most common carbohydrate intolerance in the world and occurs most often after weaning, or within the first few years of life (Manhan & Escott-Stump, 2008). Some say that we have lactase so we can digest cow's milk, a necessary and healthful substance. Others argue that we were never meant to consume dairy past infancy and that lactase persistence (lactose tolerance) is actually harmful to one's health. Because lactose intolerance is such a widespread "disease", it has been studied intensely for decades, with adequate attention given to the evolutionary history of human lactase activity and lactose consumption. Multiple hypotheses exist explaining why some humans have lactase and others do not. We will also investigate modern chronic diseases and their association with dairy consumption.

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The mechanism of lactose digestion in humans is fairly simple. It involves the enzyme lactase, which breaks the disaccharide lactose into the monosaccharides galactose and glucose in the upper small intestine. If lactose is not hydrolyzed into galactose and glucose (as is the case with lactose intolerance), then lactose will pass intact into the large intestine, where colonic bacteria break it down into short chain fatty acids, carbon dioxide, and hydrogen gas. These substances can cause bloating, flatulence, diarrhea, and abdominal pain. The number one complaint of individuals with lactose intolerance is gastrointestinal (GI) discomfort after consuming dairy. In fact, a history of GI discomfort after consuming something containing milk is the key to diagnosing lactose intolerance. Clinical diagnoses are made as well, including abnormal hydrogen levels in the breath and an abnormal lactose tolerance test. When an individual with lactose intolerance consumes lactose, the increased hydrogen produced in the large intestine diffuses into the bloodstream and is then exhaled through the lungs. Increased hydrogen in the breath is a good indicator of lactose intolerance. Also, if a blood glucose level of <25 mg/100 mL above fasting level is measured after consuming dairy, this indicates the patient likely has a lactase deficiency (Manhan & Escott-Stump, 2008). Consequences of lactose intolerance are minimal, and usually extend no further than occasional inconvenience and GI discomfort.

Lactose intolerance is virtually incurable. While small amounts of lactose (6-12g, or the amount in about 4-8 fl oz milk) may cause little to no GI distress, symptoms of lactose intolerance rarely disappear altogether (Manhan & Escott-Stump, 2008). If an individual (in modern society) with insufficient lactase wishes to consume products containing lactose, they may take a Lactaid pill, a pharmaceutical containing lactase, along with the lactose-containing food or drink milk containing supplemental lactase. There are also plentiful dairy alternatives, such as plant-based (soy, almond, hemp, rice, oat, cashew, etc.) milks, yogurts, spreads, cheeses, and ice creams. Life without lactase does not have to be, and is rarely limiting or disadvantageous.

We need to distinguish between human infants and human adults in regards to lactase. Human babies have lactase so that they can digest their mother's milk, a necessary substance (and usually the only substance consumed for the first months of life) for infantile growth and development (Manhan & Escott-Stump, 2008). Traditionally we would see a decrease in lactase production once a child is weaned, and that child would no longer be able to digest mammalian milk (what we consider to be lactose intolerance in the modern world) (Gluckman, 2009; Swallow, 2003). Lactase persistence is a fairly recent human development on the evolutionary time scale. Anthropologic data display a correlation between the onset of pastoralism (the beginning of animal agriculture) and lactase persistence in both European and African populations about 7,000 years ago. This indicates convergent evolution, suggesting that lactase persistence was only beneficial to those populations that had domesticated animals (Tishkoff, 2006). We find that populations elsewhere in the world, specifically Asia (have cows but do not milk them), have a high prevalence of lactose intolerance (Gluckman, 2009). It is clear that the lactase mutation has been selected for over time, most likely due to its nutritional value early in life (we will explore multiple hypotheses). As was mentioned previously, 70% of the world does not produce lactase and experiences the environmental mismatch between their digestive enzymes and the cultural trend of widespread dairy consumption (at least in the Western world) (Manhan & Escott-Stump, 2008).

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There are 3 main hypotheses for why humans began producing lactase as presented by Holden and Mace from the department of anthropology at University College London: adaptation to drinking milk, increased calcium absorption at high latitudes, and fluid regulation in dry climates. The first selective pressure for drinking milk is simple: it was there. Those who had the mutation to keep their lactase stores past infancy and had domesticated mammals may have drawn nutritional benefits (especially early in life, as infants and children are especially susceptible to malnutrition) from being able to digest milk while those who did not suffered from the aforementioned GI complications upon consuming milk. Holden and Mace also point out that we would not expect to see lactase persistence in those populations that raised livestock but did not consume their milk (some parts of Asia and Africa). Not only did they find this to be true through their research of 62 different cultures around the world, but their analysis showed that "the evolution of high lactose digestion capacity is strongly associated with the presence of pastoralism and further that pastoralism is always adopted before high lactose digestion capacity evolves" (Holden & Mace, 2009). In other words, it is not likely to see the lactase mutation selected for in populations that did not domesticate mammals.

The second hypothesis Holden and Mace tested was the high latitude theory. This hypothesis relies on the notion that those residing at high latitudes do not absorb as much vitamin D as lower latitude inhabitants (think Scandinavia vs. the tropics) due to limited sun exposure. Vitamin D is imperative for calcium absorption in the body; it has been hypothesized that lactose also aids in calcium absorption. The northern European lactase persistence trend fits into this hypothesis because they reside at high latitudes. Data show populations with the highest lactose digestion capacity originating in Europe and northern Africa, and some "independent instances" of high lactose digestion capacity in sub-Saharan Africa, disproving the latitude hypothesis for these populations as some of these areas are obviously at lower latitudes (Holden & Mace, 2009). Tishkoff also found high rates of lactase persistence in low-latitude Africa (Tishkoff, 2006). If this hypothesis is true, it provides evidence for the convergent evolution theory.

The final theory tested by Holden and Mace was the fluid maintenance hypothesis in arid regions. It has been hypothesized that fluid milk served as an important water source for those residing in deserts or similar environments and that individuals who were lactose intolerant suffered from diarrhea and subsequent fluid loss. The sub-Saharan Africans and pastoralists in the Middle East fit into this hypothesis. Holden and Mace took aridity estimation data from these areas and compared it to the worldwide prevalence of lactose digestion capacity. They found that "neither solar radiation nor number of dry months per year is significantly associated with the variation in the frequency of lactose digestion capacity in any of the regressions" (Holden & Mace, 1997). The issue with their analysis is that they performed these regressions for the sum of the populations with the lactase mutation; it may be possible that this hypothesis is true only for those living in arid regions (but obviously not for those lactase persistent populations in non-arid environments).

Based on the research that Holden and Mace performed, we can conclude that lactase persistence likely arose because individuals began consuming non-human milk after infancy and those who had the mutation for lactase persistence continued to reproduce (due to some benefit of drinking milk) and their offspring continued to consume milk and reproduce and so on. However, based on Tishkoff's research, we can assume that lactase persistence is a product of convergent evolution. Therefore, it could be possible that lactase persistence was selected for for a variety of reasons in independent populations. In other words, just because not all lactase persistent populations lived at high latitudes does not mean that the high-latitude theory is incorrect. The high-latitude theory may be correct for those living at high latitudes, and the aridity theory may be correct for those living in dry regions. The point is that it appears there were multiple benefits of fluid milk consumption, and it is possible that the convergent evolution of lactase persistence developed for a variety of reasons.

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Let us proceed through the adaptationist program as described by Nesse and Williams (Nesse & Williams, 1994). We will attempt to answer the questions of the adaptationist program as it pertains to lactase persistence to determine if it truly is an adaptation by natural selection. First, why does the trait exist? As evidenced by Holden and Mace's analyses and Tishkoff's findings, lactase persistence exists because early pastoralists began consuming milk, reaped possible nutritional benefits, did not suffer reproductively, and continued to pass on their gene for lactase persistence. Those that did not have the dominant lactase persistence trait who consumed milk suffered from GI complications and possible dehydration and death from diarrhea. Second, what is the function of lactase persistence? Lactase persistence allows humans to digest mammalian milk after weaning from the human breast. It is debated whether or not milk is nutritionally beneficial (we will explore the costs of lactase persistence). Third, how has this trait contributed to reproductive success? There seems to be no correlation between reproductive success and lactase persistence except in those individuals who are undernourished, receive nourishment from milk, and therefore live healthfully enough to reproduce. For example, those living at high latitudes and arid environments may have benefited from milk consumption whereas those living in temperate regions, with plenty of sun exposure and adequate nutrition may not have particularly benefited from drinking milk past infancy. Remember, almost three-quarters of the world is lactose intolerant, most of which exists healthfully on a lactose-deficient diet (Manhan & Escott-Stump, 2008). Finally, is lactase persistence part of some other adaptive machinery? This question is yet to be answered. The lactase-lactose relationship is fairly simple and does not appear to affect or be affected by any other adaptive machinery.

Let us now view this issue from a different lens-from the perspective of lactose intolerance (in individuals not experiencing lactase persistence). We will analyze current research related to the costs of lactase persistence and then apply the adaptationist program to lactose intolerance. Just because milk is widely consumed, and highly advertised, does not make it a healthful substance to consume. Unless an individual is starving or severely undernourished, there is no nutritional need to consume milk past infancy and it could be potentially dangerous to one's health.

First, we will address the greatly misunderstood concept of osteoporosis as it pertains to milk consumption. Read this carefully: cow's milk does not prevent osteoporosis; it increases the risk for osteoporosis (Campbell, 2006; Lanou, 2005). This is a strong and likely shocking statement, as most Americans grew up in a dairy-obsessed nation promoting consumption of fluid cow's milk, cheese, yogurt, and many other products that contain dairy. The key misunderstanding pertains to calcium. We know that we need calcium to build and maintain bone mass. We also know that cow's milk contains a significant amount of calcium. What most people do not care to realize is that cow's milk is also high in animal protein, specifically casein. Casein contains a significant amount of the amino acid methionine, a sulfur-containing compound that when broken down in the body is extremely acidic (production of sulfuric acid) (Campbell, 2006; Champe & Harvey, 2008). This acidifies the blood, which must be immediately neutralized before becoming deleterious to other body systems. The body neutralizes the acidic blood by leaching calcium from the bones-the most readily available and highly basic compound it has. The key is this: although cow's milk is high in calcium, most of that calcium rarely finds its way into your bones. In fact, only about 32% of calcium from cow's milk is absorbed, whereas 40-64% of the calcium present in plant-based foods such as beans and dark leafy greens is absorbed (Keller, 2002). Because the body leaches calcium from the bones to neutralize the blood, cow's milk causes bone calcium loss, weakening the bones, and increasing one's risk for osteoporosis (this is why serum calcium levels are not a good indicator of bone health) (Campbell, 2006). We can confirm this biochemical mechanism by examining global osteoporosis and hip fracture trends. Dr. T. Colin Campbell published The China Study: The Most Comprehensive Study of Nutrition Ever Conducted in 2006. What he found was astonishing: a statistically significant proportionally increased risk for osteoporosis as consumption of dairy increased (Campbell, 2006). Also surprising, he found a similar trend between incidence of hip fractures and calcium consumption between selected areas (including the United States, Sweden, United Kingdom, Yugoslavia, and Hong Kong among others). There is obviously more to bone health than calcium. This research has been replicated by others, confirming the results (Abelow, 1992; Barzel, 1998; Cadogan, 1997; Lloyd, 2000; Winzenburg, 2002). The common notion that milk and/or calcium prevent osteoporosis is not as clear as many think it to be.

Next, we will investigate how dairy may affect other diseases such as cancer and diabetes. The clearest correlation is seen with breast cancer. It is an accepted fact that IGF-I, insulin-like growth factor I, is a breast cancer risk-increasing compound (Voskuil, 2005). We need not go into the mechanism here because "most studies indicate that IGF-I is a potent stimulus for breast cancer cell proliferation..." (Outwater, 1996). Milk contains IGF-I, and the hormones in milk (given to the cows to encourage rapid growth), specifically bovine growth hormone, increase the IGF-I levels of milk (Outwater, 1996; Voskuil, 2005). Dairy may also increase the risk for breast cancer a second way: dairy contains relatively high amounts of estrogen, a hormone associated with increased breast cancer risk. Not only do estrogens themselves increase the risk, but they also stimulate IGF-I expression, for a two-fold effect (Outwater, 2006).

We also see an increased risk for prostate and ovarian cancers as milk consumption increases in countries around the world in epidemiologic studies (Chan, 2001; Fairfield, 2004; Ganmaa, 2002; Giovannucci, 1998; Larsson, 2004). For instance, Fairfield et. al found that one or more servings of skim or low-fat milk per day was associated with a 32% increased risk for ovarian cancers (2004). An increased risk for autoimmune diseases such as type 1 diabetes and multiple sclerosis has been correlated with dairy consumption as well (Banwell, 2008; Dahl-Jorgensen, 1991; Malosse, 1992). In fact, The American Academy of Pediatrics advises against the introduction of cow's milk to an infant's diet (Scott, 1995). And finally, we see an increased incidence of heart disease and obesity with dairy consumption (Barnard, 2005; Newby, 2009; Ornish, 1998, 2001). Milk contains sugar and cholesterol, often displacing calories that could have been consumed from more nutrient-dense, fiber-rich foods. It is obvious that there are serious risks associated with consuming milk and dairy. There are many factors that play into why it is so heavily promoted and consumed, but we are only concerned with evolutionary hypotheses here.

Let us now apply the adaptationist program to lactose intolerance (Nesse & Williams, 1994). Lactose intolerance exists because after weaning, there is no medical or nutritional need for a human (or any other animal for that matter) to consume milk. The function of lactose intolerance is to discourage dairy consumption by manifesting distressing symptoms after consuming milk. When humans began exploiting animals and consuming their milk (primarily cows, sheep, and goats), we became aware of the lactase gene mutation. This is where confusion sets it-if it appears that milk consumption may do more harm than good, then why does it appear that lactase persistence was selected for? Many of the costs of dairy consumption (cancer, heart disease, and osteoporosis) usually do not kill a person until after the age of reproduction (Centers for Disease Control and Prevention, 2009, 2010). Therefore, lactase persistence probably does not interfere with reproductive success and is not selected against. This is quite unfortunate, for in today's world, people living past the age of reproduction typically die of a chronic disease, among which are cancer and heart disease.

This notion can be viewed through the framework of antagonistic pleiotropy, which describes a trait that is beneficial early in life and detrimental later in life (post-reproductive) (Gluckman, 2009). Based on the evidence discussed thus far, we can assemble the research into beneficial and negative effects of lactase persistence/milk consumption. The possible beneficial effects include improved nutriture and fluid status, while the possible negative effects include osteoporosis, breast, ovarian, and prostate cancer, heart disease, and obesity. We can assume that the beneficial effects of improved nutriture and fluid status occur early in life. We know this because lactase persistence has been selected for, and to be selected for a trait must have a beneficial effect on reproduction. It is understood that the age-specific mortality rates for cancers, heart disease, and obesity are quite high-people do not typically die from these things until later in life, past the age of reproduction (Centers for Disease Control and Prevention, 2009, 2010). Also, Europeans with the lactase mutation (those who are lactase persistent) are found to have high body mass indices (a height to weight ratio which is used to determine overweight and obesity status) whether or not they consume milk (Kettunen, 2010). It may be a feasible hypothesis that the lactase gene is antagonistically pleiotropic-it allows for lactase persistence and dairy consumption, but also increases one's risk of obesity. The issue with obesity is that it is a very complex disease, with multiple mechanisms. Perhaps the lactase mutation is one of the many genes contributing to the obesity epidemic in the United States. This theory provides a solid foundation for more insight into modern chronic diseases, and should be researched further through randomized, controlled clinical trials.

The World Health Organization describes lactose intolerance as a "metabolic disorder" (Gluckman, 2009). We now understand that lactose intolerance is not a disorder, but simply a feature of human physiology. We are intolerant to so many substances, yet because our society is not under the impression that we must consume them, we do not think of our intolerance to them as metabolic disorders. It is safe to assume that humans were likely never meant to consume milk past infancy (or milk from any other species). We know this due to the harrowing effects of dairy consumption, including increased risk for osteoporosis, breast, ovarian, and prostate cancer, type 1 diabetes, heart disease, and obesity. No one knows for sure why lactase persistence developed and why it is still around, but it is likely that the lack of deleterious reproductive effects has discouraged its removal from the gene pool. It may even be a good model for antagonistic pleiotropy. The conclusion is that whether or not you are lactase persistent, you are better off passing on the milk and getting your protein, calcium, and fluid from plant-based sources.