DEAR DR. BLONZ: Calories are listed on the Nutrition Facts panel of foods, so are they considered nutrients? What's the best way of figuring out how many calories we need? And is there an explanation for why nuts and seeds have so many calories? -- F.P., Sunrise, Arizona

DEAR F.P.: Calories are not nutrients; they are a unit of energy. The calorie is the basic unit of food energy in the U.S., but other common energy units are the joule and the British Thermal Unit (BTU).

The nutritional calorie is defined as the amount of energy (heat) necessary to raise the temperature of one kilogram of water by one degree centigrade.

Fats, proteins, carbohydrates and even alcohol are complex compounds that each have caloric values because, when absorbed and metabolized, they can release energy for use by the body. On a weight basis, fats contain the most calories: 1 gram contains approximately 9 calories; 1 gram of protein or carbohydrate contains 4 calories; 1 gram of alcohol contains 7 calories. Vitamins and minerals do not provide calories.

The amount of energy your body needs depends on your age, size, body composition and how much physical work you do. It is difficult to generalize what any one individual might require. A first step is to get an estimate of your basal metabolic rate (BMR), which is the amount of energy your body requires at rest.

Your BMR changes throughout your life, being highest during times of rapid growth. After age 30, it tends to gradually decrease through the rest of one's life. One contributor to the decline is the decrease in muscle mass in relation to body weight, muscles being metabolically active tissue. As an analogy, think of how an eight-cylinder car will burn more gas at idle than a four-cylinder car.

The slowdown contributes to the tendency to gain weight as we age, particularly for those who continue to eat the same way they did when they were young. For an average adult, BMR accounts for the burning of 1,200 to 1,800 calories per day. To this number, we then add the caloric requirements for your daily activities, which for a 150-pound adult might include 1.4 calories per minute for sitting still, 5 calories per minute while walking at 3.5 mph, or 15 calories per minute during a vigorous game of soccer. The total of your BMR plus your activities is the number of calories you would need to take in every day to maintain your body weight. Online calorie calculators provide an estimate of daily calorie needs based on your height, weight, age and activity level (check goo.gl/zTLXE3).

The human body is not designed to waste resources. Calories not needed at the moment get stored for later use. Humans, like other mobile creatures, store excess energy as fat -- the most concentrated form. Plants, by contrast, don't need to be mobile, and they produce their own energy from the sun. The energy in plants is stored as carbohydrates, which allows them to get more growth "bang" for their calorie "buck."

All this changes when the plant makes its seeds: With a seed, the need is for a light, concentrated source of energy -- one sufficient to fuel the plant's growth until it can sprout its own leaves and produce its own energy. This explains why we find fats and oils in the nuts and seeds of plants.

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: 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.