The Benefits of Human Breast Milk

Colonization of the human infants’ gastrointestinal tract by bacteria occurs only after birth. It is known that human breast milk encourages colonization of the newborn’s gut by beneficial bacteria and discourages colonization by harmful ones, but how exactly does it do that? An article in the Proceedings of the National Academy of Sciences provides a tantalizing glimpse into the likely mechanisms.

About 4-12% of the macronutrient composition of human milk consists of a structurally diverse group of complex sugars called oligosaccharides. These oligosaccharides are indigestible by the infant, and so for many years scientists thought they had no function. It turns out that they are digestible by a particular group of beneficial bacteria called bifidobacteria. The presence of these oligosaccharides in human milk gives the bifidobacteria a nutritional advantage over less desirable bacteria in the human infant’s gut. In other words, these oligosaccharides are produced in human milk specifically to provide nutrients to beneficial bacteria, rather than to nourish the infant.

Some of these oligosaccharides have another interesting property; they bind to certain harmful bacteria, thus preventing them from attaching to the epithelial cells lining the gut. Researchers think that these oligosaccharides may flush out harmful bacteria during that very vulnerable time in early life when the infant’s immune system has not yet developed fully.

It's Official: She Can Compete

It took more than 10 months for a committee of the IAAF, international track and field’s governing body, to decide that Caster Semenya could compete in track and field events as a woman. As you may recall, Ms. Semenya’s gender was called into question after she completely dominated the 800-meter event in the world championships last August in Berlin (see this blog, Sept. 7, 2009.) A statement released yesterday by the IAFF on their Web site (www.IAAF.org) reads in full:

Caster Semenya May Compete. “The process initiated in 2009 in the case of Caster Semenya (RSA) has been completed. The IAFF accepts the panel of medical experts that she can compete with immediate effect. Please note that the medical details of the case remain confidential and the IAAF will make no further comment on the matter.” (IAFF statement)

Remember the old “Where’s the beef?” commercials for Wendy’s chain of restaurants? One might ask, “Where’s the proof?” Out of fairness to all athletes in sports, sports governing bodies should develop understandable policies, guidelines, or criteria for gender assignment in sports. But it won’t be easy (gender IS ambiguous, sometimes), so I’m betting the IAAF won’t even attempt it.

The Wendy’s commercial ended with the remark, “(I don’t think there’s anybody back there…)” IAAF, are you listening?

New Stem Cell Guidelines

The National Institutes of Health (NIH) is about to change its definition of human embryonic stem cells (hESCs) in light of recent trends in stem cell research.

In March of 2009 President Barack Obama signed an executive order once again permitting the use of hESCs in research. According to the executive order, the NIH is charged with ensuring that NIH-funded research in which hESCs are used is ethically responsible, scientifically worthy, and conducted in accordance with applicable law. NIH does that by setting strict guidelines for what types of cells may be used and how they must be derived.

According to the current NIH guidelines, part of the definition of hESCs is that they are cells “derived from the inner cell mass of blastocyst stage human embryos”1. But the definition apparently had the unintended consequence of excluding some cell lines that were derived from even earlier, morula-stage cells (Review Figure 21.5 in Johnson’s Human Biology). The revised language will read, “derived from early stage human embryos, up to and including the blastocyst stage”, so that these more recent cell lines may be used in federally funded research projects.

The new guidelines do not change the rigorous ethical standards for deriving human cell lines. They just make more stem cell lines available to researchers.

1 Federal Register vol. 75, no. 35, Tuesday, Feb. 23, 2010, p. 8085-8086.

Is She a Woman?

After 18-yr-old South African Caster Semenya turned in the fastest time of the year in a women’s 800-meter event in 2009, track and track and field’s governing body ordered an investigation into her gender. They're demanding “proof” that she’s actually a female, so they’ve assembled a committee tell them. Let's see, now, a geneticist, an endocrinologist, a gynecologist, and a psychologist are getting together to determine the sex of…

It sounds like the beginning of a bad joke, but it’s not. Legitimate physiological and anatomical “gender confusion” can be caused by sex hormone imbalances, tissue unresponsiveness to sex hormones, genes that don’t turn on properly during fetal development, or even sex-determining genes located on the wrong chromosome (in rare cases, an XY male can develop the characteristics of a female).

Sports federations should develop rules that allow gender determination (for the purposes of sports competition) before an athletic event, not afterwards, in the public eye. One can only wonder about the emotional damage about to be done to young Caster Semenya.

Breast-Feeding and Breast Cancer

A recent study in the Archives of Internal Medicine raises the possibility (but does not entirely prove) that breast-feeding may lower the risk of breast cancer among women at high risk for the disease. The study gathered data from over 60,000 women who had reported at least one pregnancy by 1997. Among women with close relatives who developed breast cancer, those who breast-fed for a period of time had only 41% as many breast cancers by 2005 as women who had never breast-fed. The lowered cancer incidence was only seen among women at high risk for breast cancer; women with no family history of breast cancer did not benefit from breast-feeding.

Why doesn’t this study prove once and for all that “breast-feeding prevents cancer”? Because it only shows an association between breast-feeding and lowered cancer risk - cause-and-effect has not been proved. Perhaps women who choose to breast-feed are more health-conscious overall. Perhaps they exercise more or have a better diet, and that’s actually what lowered the cancer risk. Further research will be needed to tease out the details.

Caloric Restriction and Longevity

It has been known for some time that severe caloric restriction can retard the aging process and prolong the life of a variety of species, from worms to mice. But skeptics of the idea that caloric restriction might also slow the aging process in humans point out that the metabolisms of worms and mice are quite different from that of humans.

Now a new study reports that caloric restriction slows the aging process in primates, too. Macaque monkeys that have been on a calorie-restricted diet (by 30%) for the past 20 years are living longer and are healthier than their age-matched control counterparts. Excluding animals that died of non-age-related causes (accidents for example), 50% of the animals on a normal diet have died of age-related causes, compared to only 20% in the restricted-diet group. The calorie-restricted animals also have fewer age-associated diseases such as diabetes, cancer, and cardiovascular disease. The study is still ongoing.

Genetic Screening Tests

What a difference a decade makes. Back in 2000, most U.S. states screened newborn infants for just four genetic disorders. Today, all 50 states screen for at least 21 of the 29 serious genetic or functional disorders that the American College of Medical Genetics recommends for testing. The list includes cystic fibrosis, sickle cell anemia, hypothyroidism, phenylketonuria, and a host of other conditions so rare you’ve never heard of them.

A list of the screening programs and tests provided by all 50 states can be found on the National Newborn Screening and Genetics Resource Center website. For information about the recommended tests, go to the March of Dimes website.

Fatness, Fitness, and Health

Human Biology, 5th ed. (pp. 346-34) has a Current Issue feature on the topic of whether or not being “overweight” (BMI 25-30) is overstated as a risk factor for poor health or mortality. A report out just last week in The Archives of Internal Medicine (168: 1617-1624, 2008) supports that notion. The study documents the prevalence of six cardiometabolic risk factors (elevated blood pressure; elevated triglyceride level; decreased HDL level; elevated fasting glucose level; insulin resistance; systemic inflammation) in normal, overweight, and obese individuals. The study found that nearly 24% of all normal-weight individuals were ”metabolically abnormal” by virtue of having at least two of the six risk factors. And conversely, 54% of the overweight individuals were still metabolically healthy. The study suggests that just being overweight does not necessarily mean that an individual is at increased risk for heart disease.

A study published last December tends to support the hypothesis that being overweight is not as important a factor previously thought (Journal of the American Medical Association 298:2507-2516, December 5, 2007). In this study, 2603 adults over the age of 60 were tested for cardiovascular fitness (gentle treadmill test) and then mortality was followed for 12 years. Overweight individuals did have a higher mortality rate than did normal-weight individuals (18 vs 13 deaths per 1000 person-years). However, even more striking was the effect of fitness; mortality rate of the least-fit quintile was four times that of the most fit quintile (33 vs. 8 deaths per 1000 person-years.) Among the older generation at least, maintaining cardiorespiratory fitness may be more important than maintaining a normal body weight. Whether this is also true for younger individuals remains to be seen.

Hormone Replacement Therapy Revisited

Back in the 1990s the federal government launched the Women’s Health Initiative to investigate the health of older women. One of its goals was to determine the risks and benefits of hormone replacement therapy (HRT) in post-menopausal women. The HRT study was stopped early (in 2002) when it was discovered that HRT led to an increased risk of breast cancer, heart attacks, and strokes.

What about the women who were on HRT during the study - do they remain at higher risk even after discontinuing HRT? The good news is that apparently most of the increased risk goes away with time. The first follow-up study indicates that three years after HRT is discontinued, the increased risk of heart attacks disappears, and the increased risk of cancer declines significantly. Future reports will show whether any increased risk of cancer remains as time passes.

Current recommendations for HRT remain unchanged; women should consider HRT only if they have moderate to severe post-menopausal symptoms, and for the shortest time and lowest dose that is effective.

Reference: Heiss, Gerardo, et al. Health Risks and Benefits 3 Years After Stopping Randomized Treatment With Estrogen and Progestin. JAMA 299:1036-1045, Mar. 5, 2008.

My Mother's Cells Within Me

Nearly all of us harbor cells that come from a close relative. The phenomenon, called microchimerism, occurs because the placenta is not a perfect barrier to formed cells - sometimes maternal cells make their way into the fetus, and vice versa. Apparently some of these cells live indefinitely in their new host, which is surprising since foreign cells are usually attacked and killed.

Scientists are still working out what the foreign cells may be doing in their host. In some cases they may differentiate into fully functional tissue cells in the host. For example, genetically female heart cells (presumably from the mother) have been found in males. In other cases these foreign cells may trigger immune disorders later in life, when the immune system of the host finally recognizes and attacks the foreign cells. Diseases that may have a microchimerism link include several autoimmune inflammatory diseases of connective tissue (scleroderma, lupus erythematosus, and rheumatoid arthritis), and perhaps even Type 1 diabetes, a disease characterized by poor regulation of blood sugar.

Reference: Nelson, J.Lee. Your Cells are My Cells. Scientific American Feb. 2008, pp. 72-79.