The 2008 Physical Activity Guidelines for Americans from the U.S. Department of Health and Human Services (http://www.health.gov/ paguidelines/) recommend that adults get at least 2.5 hours, and preferably at least 5 hours, of moderately intense physical activity each week. Anything that gets you moving is fair game, but a combination of muscle-strengthening and aerobic exercise is ideal. According to the guidelines, episodes of aerobic activity should be at least 10 minutes long.

Pounding the pavement When walking/jogging through our suburban neighborhood, I stay on the sidewalk for safety. My wife claims that walking/jogging in the street is easier on the joints, because it is a "softer" surface. a.s.suming that she is not hit by a car first, is there really any difference to an average recreational jogger/walker between concrete and blacktop?

According to conventional wisdom, concrete is a more damaging running surface than asphalt. A runner strikes the ground approximately 1,000 times per mile. Therefore, anything that reduces the impact of each foot strike, even by a small amount, should decrease stress-related injuries.

In May 1997, Runner's World Runner's World magazine rated running surfaces from worst (1) to best (10). Here are surfaces and their ratings: snow (2), concrete (2.5), sand (6), asphalt (6), treadmill (6.5), synthetic track (7), cinders (7.5), dirt (8), wood chips (9), and gra.s.s (9.5). magazine rated running surfaces from worst (1) to best (10). Here are surfaces and their ratings: snow (2), concrete (2.5), sand (6), asphalt (6), treadmill (6.5), synthetic track (7), cinders (7.5), dirt (8), wood chips (9), and gra.s.s (9.5).

Some articles in popular magazines cite clinical studies that claim improper running surface is a leading cause of stress-related injuries. However, studies of injured runners visiting clinics cannot prove that running on a particular surface caused an injury. It is also necessary to determine how many people run on that surface without pulling up lame.

A search turned up four studies in the medical literature that compared injuries in people who ran primarily on concrete versus those who ran primarily on asphalt. In total, more than 4,600 recreational and compet.i.tive runners were surveyed and resurveyed over a period of 2 to 12 months. Three of the studies found that running surface made no difference in the number of injuries sustained.

The fourth study found that running surface made no difference in injuries to male runners, but that female runners who ran on concrete more than two-thirds of the time had more injuries than those who ran primarily on asphalt. This was the smallest of the four studies, and it had only 15 female runners who ran primarily on concrete. It is possible that something else about these runners predisposed them to injury.

An interesting biomechanics study, published in Sports Medicine Sports Medicine in October 1986, may help explain why runners' injuries do not seem to be related to surface hardness. The study's author found, unexpectedly, that the peak value of the vertical force caused by a foot striking concrete was actually lower than on asphalt or gra.s.s. The runner's foot also remained in contact with the concrete a few milliseconds longer than with the other surfaces. in October 1986, may help explain why runners' injuries do not seem to be related to surface hardness. The study's author found, unexpectedly, that the peak value of the vertical force caused by a foot striking concrete was actually lower than on asphalt or gra.s.s. The runner's foot also remained in contact with the concrete a few milliseconds longer than with the other surfaces.

This led to the conclusion that just before striking the surface, the runner subconsciously adjusts leg stiffness based on perception of surface hardness to cushion the landing.

Totally radical I understand that antioxidants decrease the number of free radicals in the body. How do you determine your number of free radicals?

Free radicals (the kind generated in chemical reactions, not in Berkeley in the '60s) are molecules with unpaired electrons. Electrons like to hang out in pairs, and when they find themselves solitary, they try to break up happy electron couples in other molecules.

DNA, proteins, and fats can all be damaged by free radicals, and free radicals have been implicated in a wide range of diseases, including cancer, Alzheimer's disease, and heart disease.

Free radicals' bad rap is not entirely fair, however. They are produced as a normal part of many chemical reactions in our cells, and they play a number of important roles in the body. For example, our immune system uses free radicals as weapons against invading bacteria and viruses.

It is not possible to find out how many free radicals you have. Doctors don't test for free radicals because the complexity of different body tissues makes these tests impractical, according to Joseph Scherger, a professor and physician at the University of California San Diego School of Medicine.

Scherger points out that although elevated levels of free radicals cannot be measured directly, their effects can be measured. For example, free radicals can cause inflammation in the blood vessels, which leads to atherosclerosis, or clogging of the arteries. Inflammation increases the amount of a chemical in the blood called C-reactive protein (CRP). So levels of CRP provide indirect information about free radical activity.

Someday, it might be possible to scan for free radical activity in particular organs or tissues. Electron spin resonance-a measurement technique that detects free radicals based on how they behave in a magnetic field-has been used to detect free radicals in small animals. Several challenges to extending this technique to humans exist. For instance, it involves administering chemicals that are unsafe to humans to trap the radicals so that they can be measured.

It might be difficult to draw conclusions from a free radical test, since levels of free radicals are dynamic. Elevated free radicals might be a sign of a chronic problem or your body's normal response to a temporary infection.

So even with such a test, advice from doctors on how to keep a balance between free radicals and antioxidants would still be to avoid smoking, minimize intake of trans fats, and keep your diet rich in fruits and vegetables.

It's elemental What can you tell me about indium-element 49-and its role, if any, in human nutrition? Are there any websites I can look at?

An Internet search turns up some amazing claims about the health benefits of indium. Among the plethora of ailments indium is purported to cure are addictions, hair loss, the appearance of aging, cancer, birth defects, low and high blood pressure, and weight problems. Most of these claims are presented without any supporting evidence, but a few sites refer to-but distort-scientific studies.

For example, one site claims that in 1971, Dr. Henry Schroeder discovered that indium supplements resulted in a lower body weight, especially in females, and may give women the extra boost to burn more calories and lose weight. Schroeder published a paper that year, in The Journal of Nutrition The Journal of Nutrition, describing the effects of low doses of indium (as indium chloride) on the growth and life span of mice. However, he reported that indium stunted the growth of mice, especially females, not that it turned them into fat-burning machines.

Also, in contrast to what one would expect if indium really were a cure-all, Schroeder found no statistically significant differences in the life spans, or numbers of tumors, in mice getting indium supplements compared to controls.

A radioactive isotope of indium is used in medicine, including in cancer treatment, but these uses exploit the radioactivity of the particular isotope, not indium's purported nutritional value.

An increase in the use of indium (nonradioactive, of course) in the electronics industry (for example, in semiconductors and solar cells), and concerns over possible health risks to workers, have stimulated a few recent studies on indium exposure. Animal studies have shown that, in high doses, indium can have adverse effects on the liver and kidneys and on fetal development.

Because many essential minerals are toxic in large doses, the adverse effects of high doses of indium do not disprove the benefits of low doses. However, indium has no known biological function, and the scientific literature does not support the claims about indium's benefits on health.

Color me young I have heard that middle-aged people can prevent their hair from turning gray by taking a vitamin B complex containing para-amino benzoic acid (PABA). Is there any truth to this?

PABA probably is most familiar as an ingredient once widely used in sunscreens, but bacteria in our intestines also make it. Although PABA is sometimes called vitamin Bx and is found in foods such as brewer's yeast, liver, and whole grains along with other B vitamins, it is not officially cla.s.sified as a vitamin because its intake is not essential for human health. and is found in foods such as brewer's yeast, liver, and whole grains along with other B vitamins, it is not officially cla.s.sified as a vitamin because its intake is not essential for human health.

The claim that PABA can prevent graying of hair has roots in studies conducted in the 1940s and '50s. They concluded that PABA consumed in large doses caused darkening of hair in some people with white or gray hair. The length of time the hair was gray before PABA treatment began did not appear to influence the darkening effect. One study also noted a darkening of hair in individuals with nongray hair.

The dosages of PABA used in the studies were high, from hundreds to thousands of milligrams per day. Much lower doses (30 milligrams or more) can cause nausea, fever, rashes, and liver toxicity, according to the 2007 edition of Dietary Supplements Dietary Supplements, by pharmacist and nutritionist Pamela Mason.

In addition, the outcomes of the PABA studies were highly variable. In some, the majority of people taking PABA did not have a change in hair color. It is not clear what factors may have led to the inconsistent results, or even how PABA could reverse graying.

Severe malnutrition can cause graying of hair, as can large deficiencies of individual nutrients, including copper, zinc, and folic acid. Nonetheless, genetics appears to be the dominant factor that determines when an individual's hair turns gray.

Hair color depends on the presence or absence of the pigment melanin, which is produced in organelles called melanosomes within cells called melanocytes by the process of (don't worry, tedium does not cause gray hair) melanogenesis.

Gray hair has a marked reduction in the number of active melanocytes within the hair bulb. As a result, fewer melanosomes are incorporated into the growing hair shaft.

The pigment changes are accompanied by alterations in the hair structure as the central, or medullary, layer of the hair thickens and the surrounding cortical layer thins. What triggers the decrease in melanogenesis, and how the pigment changes relate to the changes in the structure and texture of gray hair, are not yet understood.

Vitamin virtues Does any scientific evidence show whether purchased vitamins (multi- or individual) are effective?

For people with special nutritional needs or vitamin deficiencies, vitamin supplements can be beneficial. An example is supplementation with folic acid before and during pregnancy, which significantly reduces various birth defects, especially spinal deformities.

Even in developed nations, severe vitamin deficiencies are not entirely a thing of the past. Cases of rickets-slowed growth and bone deformities caused by vitamin D deficiency-crop up regularly in small numbers of infants in the United States. Vitamin deficiencies are also common in people over age 65.

More than one-third of U.S. adults take multivitamins, and nearly three-quarters use nutritional supplements of some kind. After multivitamins, the most popular nutritional supplements are calcium, vitamin E, and vitamin C.

Yet, although many studies have examined the effectiveness of multivitamins and individual nutritional supplements on preventing a range of ailments, including cancer, cardiovascular disease, and age-related cognitive declines, results are highly variable. Some show increased risk, others show decreased risk, and still others reveal no effect.

Several comprehensive reviews have concluded that the overall quant.i.ty, quality, and consistency of evidence is weak that nutritional supplements benefit the general adult U.S. population. They also call for future research to better control for prior nutritional status of study partic.i.p.ants. Furthermore, supplementation studies lasting a few months, or even years, may be inadequate, because chronic diseases can take more than a decade to develop.

Food and Drug Administration oversight of nutritional supplements is loose. Supplements are categorized as food rather than as drugs, which have tighter oversight, and supplements sometimes contain contaminants. For instance, a study by the International Olympic Committee showed that some supplements for athletes contained undeclared steroids.

Products that have the ConsumerLab.com "CL Seal" have been tested for product label accuracy and ingredient quality. Similarly, the verification mark of US Pharmacopeia (USP), a nonprofit, nongovernment organization, signifies that the supplement was produced through USP-verified good manufacturing practices. "CL Seal" have been tested for product label accuracy and ingredient quality. Similarly, the verification mark of US Pharmacopeia (USP), a nonprofit, nongovernment organization, signifies that the supplement was produced through USP-verified good manufacturing practices.

The FDA is phasing in new regulations that require manufacturers to evaluate the composition of their supplements. However, none of these oversights ensures that the product works. Without FDA review, labels are allowed to claim that a supplement affects a body structure or function (but not that it prevents or treats disease).

Fuel economy What is a person's metabolic rate based on?

Metabolic rate has three components: resting metabolic rate (the energy it takes just to be alive-to breathe and for our cells to go about their daily business), the energy expended on eating (digesting, absorbing, and storing food), and the energy required for all other activities.

Resting metabolic rate accounts for approximately 60 percent of the calories we expend every day. Eating (excluding the calories burned getting to the nearest burger joint) makes up about 10 percent of daily energy expenditure. The remaining 30 percent of calories are burned as a result of activity.

Activity can be divided into exercise and nonexercise activity thermogenesis (NEAT). NEAT is the energy burned during daily activities that are not fitness-related, such as standing, ambulating, and fidgeting. Researchers discovered, by having people wear motion-sensing undergarments, that lean, self-proclaimed "couch potatoes" engage in approximately two hours more NEAT behaviors each day than their obese counterparts. The differences in NEAT meant that the obese people burned 350 fewer calories per day than the lean people. Interestingly, even when the obese people lost weight, they did not increase their NEAT.

In fact, another study showed that even moderate weight loss (15 to 20 pounds, or 7 to 9 kilograms) actually decreases metabolic rate. This finding explains why it is difficult to maintain weight loss through dieting alone: the body burns fewer calories at the new, lower weight. On the other hand, exercise burns calories in the short term and can crank up metabolic rate in the long term by building muscle.

Muscle ma.s.s determines a large proportion of the individual differences in metabolic rate, because, even at rest, muscle tissue consumes more fuel than fat. Differences in muscle ma.s.s explain why women have, on average, a 10 percent lower total daily energy expenditure than men. Also, metabolism tends to slow down with age because of loss of muscle ma.s.s, not just because of reduced activity.

Metabolism is regulated by intricate feedback mechanisms between the body and the brain. For example, during starvation, certain thyroid hormones drop rapidly, leading to a 40 percent decrease in resting metabolic rate. The thyroid gland is under the influence of the pituitary gland in the brain, which receives orders from a brain region known as the hypothalamus. The hypothalamus is influenced by leptin, which is produced by fat cells. When it was discovered a decade ago, leptin (from the Greek leptos leptos, meaning thin) was thought to have potential as a magic skinny pill, but alas, controlling metabolism is not so simple.

Fit to be sweaty I read somewhere that people who are aerobically fit sweat more than people who are less fit. Is this true?

Studies have shown that people who are aerobically fit do sweat more, and begin sweating more quickly, than people who are less fit when they exercise at similar relative relative intensities. intensities.

"Relative intensity" means a fixed percentage-say, 80 percent-of individuals' maximal aerobic power, which is ascertained from a person's oxygen uptake and carbon dioxide production during exercise. To get a fit person to exercise at 80 percent of his or her maximal aerobic power, experimenters need to crank up the tension on an exercise bicycle, or the incline or speed of a treadmill, compared to the setting that gets a nonfit person exercising at 80 percent of maximal power.

Exercise physiologists compare people who are exercising at the same relative intensity, rather than doing an identical task, because they are trying to understand how the body adapts to training, and what happens to sweating, heart rate, oxygen consumption, and so on as people get close to their physical limits, whatever those limits are.

Therefore, a couch potato probably would sweat more than a marathoner when trotting 100 feet to the mailbox. But fit people get more sweaty, more quickly when they push themselves equally hard with respect to their own physical limits. Other individual differences, including gender (on average, men sweat more than women), also influence sweating.

Red and white My daughter does not eat red meat. I've seen the TV commercial from the pork industry that calls pork "the other white meat," suggesting that it compares to chicken as far as nutrition is concerned. How does pork compare to beef?

This advertis.e.m.e.nt is a clever marketing ploy by the pork industry, which is attempting to piggyback on the growing popularity of chicken. Since the 1970s, per-capita consumption of chicken has increased, while consumption of beef has declined. Pork consumption has held relatively steady, at 50 pounds per person annually.

Although pork is paler than beef, the U.S. Department of Agriculture cla.s.sifies all meat from livestock-including pork, veal, beef, and lamb-as red meat. The red color comes from myoglobin, which is an iron-containing protein that holds oxygen in muscle. Pork has less myoglobin than beef, but more than the white meat of chicken.

Hogs are leaner than they used to be due to improved breeding and feeding, but clearly fat content and nutritional value also depend on the cut of pork selected and how it is cooked. Studies have shown that cholesterol and triglyceride levels in consumers of lean pork, lean beef, or white meat (chicken or fish) following a fat-controlled diet are similar. This indicates that these levels depend on the fat content, not the protein source itself.

Meat is a good source of minerals and B vitamins. On average, pork has less iron and zinc than beef, but about the same amount of copper. In terms of the B vitamins, pork has more thiamine than beef and about the same amount of niacin and riboflavin.

Your daughter may be concerned about fat intake, or studies that have linked consumption of red meat to increased risk of certain types of cancer, including colon and breast cancer. The exact relationship between red meat consumption and cancer risk is uncertain because consumers of red meat and nonconsumers usually have other differences in diet. For instance, people who abstain from eating red meat may consume more fruits and vegetables high in antioxidants.

People avoid certain types of meat for a variety of reasons besides health concerns. Certain animals may be labeled sacred or unclean by their religion. They may feel that it is unethical to eat mammals or any animal. They may also be concerned about the environmental impact of factory farms, or that more resources are needed to produce meat than an equal amount of calories from a plant source.

Hold the sunny side Since the medical profession touts the need to avoid excess egg consumption due to the yolks, I've been wondering why science has not made any effort to create a smaller yolk content. Or has this been attempted?

Chickens lay the occasional yolkless egg, but hens that consistently produce meringue-ready eggs would be expensive and technically difficult to breed. After all, the yolk is not just a critical ingredient in hollandaise sauce, but it also provides nourishment for a developing chick. So eggs with no yolk, or a very small yolk, would be sterile.

Each egg starts out as a single cell in the ovary of the mother's body. The egg cells are already present when a female bird hatches. When she is a few months old, yolk is added to one of these cells. A surge in estrogen stimulates the liver to produce vitellogenin, the major protein in egg yolk. Vitellogenin is transported to the oviduct-the tube that leads from the ovary-via the bloodstream.

The finished yolk pa.s.ses down the oviduct to the place where the alb.u.men, or egg white, is produced. The alb.u.men is added in layers, and the yolk ends up floating in a watery layer of alb.u.men surrounded by a thick, tough layer of alb.u.men that acts as a shock absorber. The motion of the egg twists the alb.u.men at either end, producing the white stringlike anchors-chalazae-that keep the yolk centered.

If everything is functioning normally, the outer membrane and sh.e.l.l are added further down the oviduct, and the hen lays a perfect egg. When things go awry, eggs can end up with double yolks or no yolks. The oviduct is an a.s.sembly line with multiple eggs in progress at once. If two yolks drop into the oviduct at the same time, they may end up encased in the same alb.u.men and sh.e.l.l. Conversely, if something interferes with yolk production, the hen may lay an egg containing alb.u.men only.

The size of the yolk relative to the alb.u.men increases as hens age. Also, across different breeds of chickens, a moderate amount of natural variation occurs in the ratio of yolk to alb.u.men. Theoretically, yolks could be made even smaller by tinkering with one or more of the at least four genes involved in vitellogenin production.

Egg yolks have gotten a bad rap because of their cholesterol content, but the yolk has a richer concentration of vitamins and minerals than the white. Studies have shown that eating an egg or two a day does not increase heart disease risk in healthy individuals.

Grain of salt Recently, some TV commercials have claimed that their products contain "sea salt that contains less sodium than regular table salt." Aren't they both sodium chloride (NaCl)? And don't we get some of our table salt from seawater? Also, it has become very difficult to avoid excess salt. Some products contain more than 1,000 mg of sodium per serving. Is there any way to get these manufacturers to use a lot less salt?

For people competing in endurance events, hyponatremia-an abnormally low concentration of sodium in the blood-is a real danger when they drink too much water without replenishing the sodium lost in sweat. However, in most countries the average salt (sodium chloride) intake is at least double the maximum 5 grams (about one teaspoonful, or 2,300 milligrams, of sodium) per day recommended by the World Health Organization.

Multiple sources of evidence show that high salt consumption can increase blood pressure. Elevated blood pressure is the single most important cause of heart attack and stroke. However, studies reveal considerable individual variation in the effect of salt consumption on blood pressure.

Salty foods irritate the stomach lining, and high consumption has been linked to stomach cancer. Some evidence also suggests that high salt intake can lead to water retention, increase the risk of kidney stones, contribute to osteoporosis, and worsen asthma symptoms.

Over 85 percent of the mineral composition of seawater is sodium chloride. The purer the sea salt, the more sodium chloride it contains. The label on my inexpensive bottle of sea salt says that it is more than 99 percent sodium chloride. That is about the same as regular table salt, which is mostly mined from deposits left by ancient salt lakes. Low-sodium salt is sodium chloride mixed with another mineral salt, such as pota.s.sium chloride.

Most dietary salt comes from processed food, so check the sodium content on the label, because the manufacturer's claims may be misleading. Salt has been used as a preservative for thousands of years and has other roles in cooking, but it is often added to make poor-quality ingredients palatable. Unfortunately, salty food desensitizes the tongue to salt.

A gradual reduction in salt exposure across the diet can be achieved without affecting consumers' taste perceptions; this strategy has been effective in several countries. Education, lobbying, and consumer demand would drive manufacturers to make more changes. A creative way to reduce salt intake and high-calorie, low-nutrient processed food might be a family a.s.sembly line that makes meals from good-quality ingredients and then freezes homemade TV dinners for those rushed days.

Quicksilver Is there a difference in the amount of mercury in fish, whether you eat it raw or cook it? Is it possible to avoid the mercury in a fish by how you prepare it?

Mercury in fish is tightly bound to protein and is not removed during cooking processes such as smoking, broiling, baking, boiling, pan frying, and deep frying. Nor does the addition of lemon juice release mercury from its bound state. On the other hand, the cooking method affects the health benefits of fish, and the mercury concentration is strongly dependent on the type of fish.

Mercury originates from natural sources (volcanoes) and human sources (coal-fired power plants, waste incineration, gold mining). Organisms do not readily absorb mercury in the form in which it is usually released into the environment-metallic or inorganic mercury. Once rainwater carries inorganic mercury into lakes and oceans, microbes convert it into methylmercury, or organic mercury. (In chemistry, "organic" refers to carbon-containing compounds and has nothing to do with organic agriculture.) Organic mercury is readily absorbed by organisms and acc.u.mulates in their tissues. It bioacc.u.mulates in the aquatic food chain. In other words, short-lived species low in the food chain (such as sh.e.l.lfish and salmon) have low concentrations of mercury, while longer-lived predators (such as swordfish and shark) have high concentrations. The levels in albacore tuna are lower than those in swordfish but higher than those in salmon.

Industrial catastrophes that have resulted in ma.s.s consumption of high levels of mercury reveal that it is toxic to nerve cells, especially in children exposed during their early development. Studies of the effects of exposure to lower levels of mercury have been conflicting, but based on the possible risks, the U.S. Environmental Protection Agency and the U.S. Food and Drug Administration have issued advisories for women of childbearing age, pregnant women, nursing mothers, and young children.

At the same time, studies suggest that intake of fatty acids in fish by pregnant and nursing women is beneficial for the development of brain cells in infants. In addition, fish, except deep-fried fish, has well-doc.u.mented cardiovascular benefits. For example, omega-3 polyunsaturated fatty acids in fish decrease the risk of heart attack by improving the fluidity of heart cell membranes.

In response to the confusion about the role of fish in a healthy diet, a 2006 article in the Journal of the American Medical a.s.sociation Journal of the American Medical a.s.sociation concluded that, with the exception of a few fish species, the benefits of moderate fish consumption (two servings per week) outweigh the risks. The article recommends that nursing mothers and pregnant women avoid shark, swordfish, golden ba.s.s, and king mackerel; limit intake of albacore tuna to 6 ounces per week; and consult advisories for locally caught fish. But they should get at least 12 ounces per week of other fish and sh.e.l.lfish. See concluded that, with the exception of a few fish species, the benefits of moderate fish consumption (two servings per week) outweigh the risks. The article recommends that nursing mothers and pregnant women avoid shark, swordfish, golden ba.s.s, and king mackerel; limit intake of albacore tuna to 6 ounces per week; and consult advisories for locally caught fish. But they should get at least 12 ounces per week of other fish and sh.e.l.lfish. See http://www.epa.gov/waterscience/fish/ for a list of mercury levels in different species and local fish advisories. for a list of mercury levels in different species and local fish advisories.

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SHERRY S SEETHALER, a science writer and educator at the University of California, San Diego (UCSD), works with scientists to communicate their discoveries to the public. Seethaler also writes a weekly column for the a science writer and educator at the University of California, San Diego (UCSD), works with scientists to communicate their discoveries to the public. Seethaler also writes a weekly column for the San Diego Union-Tribune, San Diego Union-Tribune, answering readers' questions spanning nearly every imaginable science topic. answering readers' questions spanning nearly every imaginable science topic.

She holds a Ph.D. in science and mathematics education from the University of California, Berkeley. Her dissertation examined how students reason about scientific controversy. She designed and taught the innovative course, Teaching Contemporary Scientific Controversies, and helped design UCSD's California Teach program, which prepares science and math students to teach.

Seethaler is author of Lies, d.a.m.ned Lies, and Science Lies, d.a.m.ned Lies, and Science (FT Press Science, 2009). (FT Press Science, 2009).

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