DEAR DR. BLONZ: How long will I be affected by the caffeine I consume? If I have it at lunch, is there any reason it would cause sleep problems at night? -- J.C., San Jose, California

DEAR J.C.: Caffeine is a general stimulant that increases heart rate, revs up the release of stomach acid, speeds the transport of food through the digestive system, and relaxes the smooth muscles, such as those found in the lungs. Once in the body, caffeine goes just about everywhere. Because it's a foreign substance, the body starts to eliminate caffeine as soon as it appears.

The answers to your questions depend on the amount consumed, how regular your caffeine intake is, and your particular degree of sensitivity.

The "half-life" is a common way to consider how long the effect of a substance lingers. This is the length of time it takes the body to metabolize and eliminate half the amount present. The amount left after a half-life can still be buzz-worthy: A 16-ounce cup of coffee can contain well over 300 milligrams of caffeine, and after one half-life, there would still be 150 milligrams -- the amount in a double espresso or a regular-sized cup of coffee.

Caffeine's "half-life" for an average healthy individual will be about five to seven hours, but the range is extensive when considering a more general population. Those who break down caffeine fastest are smokers and, strangely enough, children. It takes them about three hours to eliminate half their body's caffeine. For women taking birth control pills, the rate increases to 13 hours. The half-life in pregnant women is 18 to 20 hours, but the rate of breakdown returns to normal within a month after delivery. A newborn does not gain any real ability to metabolize caffeine until he or she is several days old. Nursing mothers need to be aware that any caffeine received through breast milk during this period has a half-life of about three to four days.

Another consideration is the element of tolerance. After about three to five days of regular use, the normal, healthy human body adapts to the presence of caffeine. That means that the buzz you get on Day 1 is not the same buzz you will get after several days of habitual intake. The process of adaptation involves adenosine, a neurotransmitter that conveys directives to slow down and relax. Caffeine binds with adenosine receptors, preventing them from responding to normal directives, with the net effect that the body stays more alert. But after a number of days of regular caffeine consumption, the body compensates by adjusting the number and sensitivity of its adenosine receptors. Some people are better at this than others, which explains how some people can have coffee with dinner and suffer no effect on sleep.

If a habitual caffeine user abruptly stops, it leaves their body hyper-responsive to adenosine, because caffeine is no longer present to moderate adenosine's effects. This circumstance is believed to be responsible for the headache, malaise and flu-like symptoms often associated with an abrupt cessation of caffeine intake. Those who fast for blood tests, medical procedures or religious holidays are familiar with the sensation. Having caffeine reverses these effects. (It has been argued that the continued consumption of caffeine may depend as much on the avoidance of the withdrawal effects as on any enjoyment of the caffeine beverage itself.)

If one were to stop caffeine intake entirely, the adenosine receptors in the body would shift back to their pre-caffeine state over a few days.

Send questions to: "On Nutrition," Ed Blonz, c/o Universal Uclick, 1130 Walnut St., Kansas City, MO, 64106. Send email inquiries to questions@blonz.com. Due to the volume of mail, personal replies cannot be provided.

DEAR DR. BLONZ: Could you give me some information, and your opinion, on DHEA being promoted as an anti-aging hormone? I'm a nurse of 26 years and a nutritionist, and have heard very little information on this. -- S.T., Phoenix

DEAR S.T.: DHEA is the abbreviation for a hormone with the tongue-twisting name dehydroepiandrosterone. It is made by the adrenal glands, which sit on top of the kidneys. DHEA, while a hormone itself, serves in the formation of a number of other hormones, including estrogen, testosterone and adrenaline.

Popular interest in DHEA stems from some recent research that has connected higher levels of DHEA with a lower risk of some age-related problems. There was encouraging research in experimental animals, but research using people is limited and inconsistent at best. Despite this, the idea took shape that DHEA might be a miraculous fountain-of-youth "drug." Before you knew it, DHEA supplements began to appear in health food stores and as a keystone product in some multilevel marketing enterprises.

Anyone who is considering DHEA should speak with their physician. It is important to appreciate that DHEA is not a vitamin or a mineral; it is a powerful medicine -- a hormone that can bring forth undesirable as well as desirable effects. This is especially true if the levels used in the animal studies were to be scaled up for our larger bodies. For example, those at risk for prostate cancer could, by taking DHEA, be increasing the amount of testosterone in the body -- the very hormone associated with the spread of prostate cancer.

Our level of naturally produced DHEA decreases as we age, but that does not mean that providing more will reverse the effects of aging. Let's face it: We currently know very little of what DHEA can do, whom it may help and whom it may harm. Until more research is done, taking DHEA is nothing more than a risky roll of one's hormonal dice.

DEAR DR. BLONZ: There are definite concerns about heart disease in our house. Please settle a dispute: Does removing the skin from chicken before you eat it get rid of the cholesterol? -- F.F., San Jose, California

DEAR F.F.: No. Although there is a small amount of cholesterol in the skin and fat of a chicken, most is found in the light and dark meat. This holds true for beef, pork and lamb, as well. And buying leaner cuts does little to reduce the cholesterol.

I would not be that concerned, as research indicates that our intake of dietary cholesterol is not as much an influence on blood cholesterol as is the overall quality of the diet. Keeping your intake of fat to less than 30 percent of your calories, and making sure your diet is balanced with a good proportion of fruits, vegetables, grains, nuts and seeds, are the best dietary ways to move toward preventing heart disease.

Send questions to: "On Nutrition," Ed Blonz, c/o Universal Uclick, 1130 Walnut St., Kansas City, MO, 64106. Send email inquiries to questions@blonz.com. Due to the volume of mail, personal replies cannot be provided.

DEAR DR. BLONZ: A tomato carton at a local produce market stated that the tomatoes contained a number of "non-tomato ingredients." It listed petrolatum, paraffin, carnauba wax and food-grade mineral oil. The carton also stated that the fruit had been treated with ortho-phenylphenol to inhibit mold. Are these substances in or on the tomatoes, and are they dangerous to eat? -- P.P., Vallejo, California

DEAR P.P.: Petrolatum, paraffin and carnauba are different types of waxes. Produce growers apply waxes to their fruits and vegetables to retain moisture, slow flavor loss, enhance appearance and prevent mold during packing, shipping and selling. Petrolatum and paraffin are byproducts of petroleum (oil). Carnauba, which comes from the wax palm of Brazil, is also used in car wax.

Waxes are applied to a wide variety of produce, including tomatoes, apples, bell peppers, avocados, cucumbers, sweet potatoes, citrus fruits, peaches, pumpkins, eggplants, squash and hardshell nuts. Waxes are only used in small amounts, and are not considered harmful. Experts estimate that one pound of wax will cover 160,000 pieces of produce.

One potential cause for concern with waxes stems from the possible presence of pesticide and fumigant residues on the surface of the produce before the wax is applied. (In some cases, the pesticide or fumigant is mixed with the wax.) Ortho-phenylphenol is a type of anti-mold fumigant that's often used this way.

Chemicals cannot be washed off if embedded in a wax, but the health risk posed by the presence of such wax-bound pesticides or fumigant residues should be minimal if the chemicals are applied according to regulations. And, of course, it's only an issue for produce in which the skin is consumed.

Although you can try scrubbing these waxes off, it takes more than water to do the trick. A mild detergent, or products that claim to clean the wax off of produce (available at natural food stores) may be helpful. Without these, the only real way to "de-wax" the produce is to take off the peel.

It is important to understand that both conventional and organic growers can apply waxes as a protectant before shipping. All waxed produce should be labeled as such. Organic growers' waxes come from natural sources and do not contain any synthetic pesticides or fungicides. The presence of waxes is yet another reason why it's always best to consume a variety of fruits and vegetables, and change your selection with the seasons, rather than eating the same foods all the time.

Send questions to: "On Nutrition," Ed Blonz, c/o Universal Uclick, 1130 Walnut St., Kansas City, MO, 64106. Send email inquiries to questions@blonz.com. Due to the volume of mail, personal replies cannot be provided.