Limits to the Human Population

In Human Biology: Concepts and Current Issues, 5th ed. we describe the theoretical limits on any population’s growth (Figure 23.2) and show the growth of the human population since the dawn of human history (Figure 23.3). How close is the current human population of 6.8 billion to Earth’s human carrying capacity? According to Jeffrey Sachs, Director of the Earth Institute at Columbia University, “We still do not really know”.

See “The Specter of Malthus Returns”. Scientific American September 2008, p. 38. Your students could read and appreciate Dr. Sachs’ one-page opinion piece on the subject. In my class the article led to a spirited discussion of the wide disparities in resource utilization by different countries.

Measles is on the Rise

According to the Centers for Disease Control and Prevention (CDC), there were more cases of measles in the U.S. in the first 7 months of 2008 than at any comparable time in the past 12 years. There have also been measles outbreaks in Switzerland, Italy, Israel, and Britain in recent years. The most likely reason is that an increasing number of children have not been vaccinated against the disease because some parents believe that vaccinations cause autism. Health officials contend that there is no connection between vaccinations and autism, but many parents remain unconvinced.

Measles is highly contagious, so it is no surprise that it may be among the first vaccine-preventable diseases to reappear when vaccination rates decline. Fortunately, measles is not very virulent; most patients are treated at home and recover without any long-term consequences. But if the return of measles is an early indication of lower vaccination rates, it may only be a matter of time before other vaccine-preventable diseases return as well. And that has health officials worried.

How do you feel about childhood vaccinations?

Sensing Danger in the Air

Scientists have long known that some animals can sense danger in the air, but little was known about how they did it. Now they have a clue. According to a report in Science, specialized neural cells in mice that are part of the olfactory system can detect unidentified alarm pheromones given off by other mice under stress. The scientists collected air from around stressed mice and then exposed other mice to it. Normal mice froze (a typical danger reaction in mice) when exposed to the alarm-pheromone scented air, whereas mice whose special olfactory neural cells had been destroyed did not respond to alarm-pheromone scented air at all.

It would be interesting to know whether humans also give off alarm pheromones. Could this be an explanation for why people seem to internalize the stress of others around them?

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.

Testing Fingerprints for Illicit Substances

Could your fingerprints tell more about you than just who you are? A brief report in Science this month describes a technique that could allow crime investigators or even employers to analyze fingerprints for a variety of chemicals, including explosives and substances of abuse, such as cocaine. And since the chemicals would be determined from a single fingerprint, there would be no doubt as to who had handled the substances.

Law enforcement officials and crime investigators will welcome the technique as another weapon in their arsenal against crime. But the ability to test fingerprints for certain chemicals could also raise some privacy concerns for us all. Would we consider it ethical for an employer to enter workers’ offices to check for fingerprints containing traces of illegal drugs, without the employee’s knowledge or consent?

Reference: “Latent Fingerprint Chemical Imaging by Mass Spectrometry”. Science 321:805, August 8, 2008.

A Daily Pill Against HIV Infection

First came the news that several AIDS vaccine trials had to be halted because the vaccines just didn’t work. Then last week the CDC admitted that number of people newly infected with HIV in the U.S. each year is nearly 40% higher than previously reported. Is there ANY good news on the AIDS front?

Well, yes…..maybe. A document released this month by the AIDS Vaccine Advocacy Coalition describes a potentially powerful new HIV prevention method called pre-exposure prophylaxis (PrEP.) It’s a simple notion, really – that a daily pill consisting of one or more of the current AIDS treatment drugs might prevent HIV infection from occurring in the first place. There are at least seven clinical trials of PrEP either planned or underway using the drugs tenofovir and emtricitabine, both of which are already approved for treating people who are currently HIV-positive.

No one knows for sure whether PrEP will work, but there are some promising signs that it might. But even if PrEP does prevent HIV infection, there would still be the issues of access and cost. In order to prevent HIV infection the drug would need to be available to a lot of healthy people, rather than just the few who are already infected.

A report on the status of PrEP research and some of the issues related to its use in AIDS prevention can be accessed at www.avac.org/prep08.pdf.

HIV/AIDS Incidence Revised Upward

The number of new HIV infections per year in the United States has been grossly underestimated for the past 20 years, according to an article published in today’s Journal of the American Medical Association (“Estimation of HIV Incidence in the United States.” JAMA 300:520-529, August 6, 2008.) The new estimate for HIV incidence is 56,300 new cases in 2006, 40% higher than previously reported by the CDC. The new data also show that the incidence of HIV has not declined at all since 1991, in sharp contrast to the previously estimated 50% decline since that time (compare Figure 1 in the JAMA article with Figure 9.24 in Human Biology, 5th ed., taken from official CDC data available at the time.) The total number of people living with HIV/AIDS is also expected to be revised upward, but those numbers will not be available until later this year.

How could the numbers have been so far off? For one, the new data are based on better testing methods that more precisely differentiate new HIV infections from long-standing ones. In addition, HIV infection rates are notoriously hard to come by, especially back in time. Even the new estimates are based on extrapolations using data from only 22 states. The CDC does the best it can do with limited data; the rest is an educated estimate.

Officials emphasize that this does not mean that there actually were more new cases of HIV – rather, we now have better estimates of the actual rates of new infection that existed at the time, regardless of whether or not they were accounted for. Nevertheless, some Democrats are criticizing the Bush administration for not doing enough to combat HIV/aids in this country. Senator Waxman of California released a statement last week in which he pointed out that the CDC budget for prevention has actually shrunk by 19% since 2002, and that the president recently requested a reduction in funding for HIV prevention at the CDC. Given that the incidence of HIV has not declined at all over the past 15 years, Senator Waxman may have a valid concern.

Students may react with a "So what are we supposed to believe, when even scientists can't get it right?" attitude. They'll need convincing that this kind of "flip-flop", as it would derisively be labeled in politics, is actually a normal part of a healthy scientific process.

Enhancing Taste Sensations

Scientists at San Diego-based Senomyx are searching for compounds that specifically magnify just one of the taste sensations we especially prefer (sweet, salty, or savory) or that block the sensation of bitterness. After searching through tens of thousands of synthetic and natural compounds, they now have several specific “flavor modulator” compounds that appear to work.

The commercial possibilities and health implications are almost limitless. If food product manufacturers could use less sugar in their products and still satisfy our craving for that sweet taste, perhaps people would be able to diet more effectively. Caloric intake might decline, leading to a reversal of the obesity epidemic. Less salt in our food products might mean less cardiovascular disease. Block the bitter taste receptors and children would eat more vegetables. Bitter medicines would be more palatable, improving patient compliance in taking them.

Some big companies are interested, including Coca Cola, Nestle, and Cadbury. Nestle is already using a flavor modulator from Senomyx in some of its products. Read about it in this month's Scientific American (“Magnifying Taste.” Scientific American August, 2008, pp. 96-99.) You’re likely to see “flavor modulators” or “taste enhancers” listed among your favorite products’ ingredients in the near future.

California Bans Trans Fats

The State of California has banned the use of trans fats in most food products. The ban goes into effect for all restaurant products in 2010 and for all retail baked goods in 2011. Packaged foods will not be affected, however.

Trans fats are created by bubbling hydrogen through liquid oil at high temperature. The resultant partially hydrogenated oil is a solid at room temperature. Trans fats prolong the shelf life and (some say) improve the flavor of foods. They were popular as a deep-frying oil until it became apparent that they raise the levels of low-density lipoproteins (the bad cholesterol), thereby potentially contributing to heart disease. Some restaurant chains, most notably McDonalds, have already discontinued the use of trans fats in their deep-fryers, and other chains are following suit.

The California ban raises an interesting question: Whose responsibility is it to legislate our health? The California Restaurant Association argued (unsuccessfully) that it should be the federal government, not the states – otherwise, restaurants with outlets in many states could face a wide array of different rules.

But if the government won’t act, should the states be allowed to? Until California’s law is challenged in federal courts, the answer is “Yes”. What do YOU think?