DEAR DR. BLONZ: Dear Dr. Blonz, How long is milk good for once opened? My mother and I disagree, and I am counting on you to settle the debate. Irrespective of when it is opened, my mom looks at the sell-by date on the carton and insists that the milk will be good for at least a week past that date if refrigerated. I disagree, saying that once the milk container is opened, that date is no longer something to really look at. New milk cartons have about two weeks, that I've noticed, to sell. I cannot imagine that milk is good for at least three weeks if opened right away. My mom gave my kids milk on April 6 that had a date of March 23 on it. She said it smelled fine, but my kids said it tasted funny. I was mad at her for even giving them such old milk. Please, who is correct? -- S.T., San Jose, CA.

DEAR S.T.: Fluid milk is a perishable food, and it will spoil. The sell-by date on the container is the last day on which the carton should be sold. Manufacturers use these dates to signal retailers how long to sell the product, and the dates include an allowance for normal home use. That means that a gallon container will typically have an earlier sell-by date than a quart container that comes from the same batch of milk.

Assuming it has been opened before the sell-by date stamped on the container, homogenized milk should keep for about 5-7 days once opened. Tell your mother that the clock begins when you open the carton. But there is some flex here. The fresher the milk, and the more time it spends at a chilled temperature, the longer it will remain wholesome. All things being equal, a carton with 10 days before its sell-by date will last longer than one opened on that date. Three weeks? That’s pressing it.

Also, keep in mind that the sell-by date is not a guarantee of freshness or safety. The breakdown of food is a gradual process and does not take place on one particular day. You always need to keep containers chilled and tightly closed. Never return unused milk to the container. Then there is the fact that the accuracy of any dating system relies on the proper handling of foods. If there's been mishandling by the manufacturer, trucker, supermarket or consumer, the life -- and safety -- of the product is subject to compromise. In the end, consumers must trust their eyes, nose and palate in addition to those numbers stamped on a carton. If you notice, as your kids did, an off taste, smell or appearance in any food product, forget the date and toss it out.

Send questions to: “On Nutrition,” Ed Blonz, c/o Andrews McMeel Syndication, 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: My question is about coffee and antioxidants. I had thought that because coffee beans are roasted, they lost their antioxidants in the process. Also, could you explain a bit about the effects of caffeine on the body and how long those effects are felt? -- M.Q., Tucson, Arizona

DEAR M.Q.: Plants produce seeds and package them with enough nourishment to keep them viable until they can begin to grow and gather nutrients on their own. Along with sustenance comes an army of substances (phytochemicals) designed to protect against attacks, whether from insects, animals or the oxidizing rays of the sun. (Interestingly, many “antioxidant” substances have bitter tastes, and this helps to discourage nibbling.)

The coffee plant is no exception to this theme. The ripe, red coffee cherry is the seed for the coffee plant, and it typically contains two green coffee beans. The coffee plant grows in tropical climates, and it contains an array of antioxidant substances for its seeds. Green coffee beans are a rich source of antioxidants. The roasting process darkens the color of the bean and develops the flavor and aroma we associate with coffee. You would think that a high-heat process would be destructive to antioxidants, and roasting does indeed affect the level and type of antioxidants in coffee. What’s been discovered, though, is that the roasting process creates a novel type of antioxidants in coffee beans known as melanoidins, which are not present in the unroasted beans. A study in the June 2002 issue of the Journal of Agricultural and Food Chemistry found that a medium-roasted coffee bean contained more antioxidants than either a light-roasted or a dark-roasted bean.

Regarding caffeine, it won’t surprise you to hear that it stimulates the brain. Its other effects include an increase in heart rate, increased acid release by the stomach, quicker transport of food through the digestive system and a relaxation of the smooth muscles, such as those found in the lungs. Caffeine is also a diuretic, which means it increases the volume of urine the body produces.

Once in the body, caffeine goes just about everywhere. In a pregnant or nursing woman, this may mean it makes its way to a developing fetus or into breast milk.

Because caffeine is a foreign substance, the body starts breaking it down as soon as it appears. The half-life of caffeine can vary. 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.

The half-life for the average nonsmoking adult is five to seven hours. For women taking birth control pills, it’s up to 13 hours. The half-life in pregnant women is 18 to 20 hours, but returns to normal within a month after delivery. A newborn does not gain any real ability to metabolize caffeine until several days old. Any caffeine received through breast milk during this period has a half-life of about three to four days -- important to consider while pregnant or nursing.

Abrupt withdrawal by a regular caffeine user can lead to symptoms ranging from a simple headache to nausea, drowsiness, depression and reduced attention span. People who drink as little as two cups of coffee a day may experience these withdrawal effects. According to a study in the October 1992 New England Journal of Medicine, half of the 62 coffee drinkers studied experienced moderate to severe headaches when they stopped drinking coffee, and about 1 in 10 reported depression and anxiety.

The symptoms were connected to the caffeine, in that those who discontinued coffee but received caffeine (not placebo) capsules did not report the same problems. However, these symptoms tend to be short-lived, and most can be avoided by cutting back gradually.

Send questions to: “On Nutrition,” Ed Blonz, c/o Andrews McMeel Syndication, 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: With the holidays here, I was wondering whether calories from alcoholic beverages such as beer, wine and liquor cause weight gain at about the same rate as calories from carbs and fats. -- F.S., Hayward, California

DEAR F.S.: Calories from alcohol tend to have a more complicated agenda than those from other food components. Much depends on your diet, your overall health and your drinking habits. Regarding strict calorie content, one gram of alcohol contains about seven calories. Compare this with carbohydrates and proteins, which contain four calories per gram, and fat, which contains nine calories per gram.

While there can be brew-to-brew variances, the alcoholic content of beer is about 5 percent by volume, except in states or counties having regulations that limit alcohol content to 3.2 percent. There will be additional carbohydrates in beer, and there can also be an additional amount in other alcoholic beverages, such as liqueurs and finished wines.

Unlike packaged foods, there is no requirement for a Nutrition Facts label on alcoholic beverages. To get a caloric breakdown for a specific beverage, check with the manufacturer. The National Institutes of Health provides a generic calculator at tinyurl.com/ybh4cynr. If you’re only interested in good ballpark figures, you can stop here.

It should be kept in mind that alcohol can become toxic in excessive amounts, so the body has specific enzymes designed to break it down. When we consume alcohol more rapidly than it can be broken down, interim metabolites remain in queue, and this is what brings on the various feelings of inebriation. (Larger people tolerate more because their “queue” is bigger.) An interesting side note to this is that calories from alcohol count more in those who don’t drink on a daily basis. This comes from the fact that when alcohol is consumed on a regular basis, such as a glass of wine a day, the body has some of the needed enzymes at the ready.

There are complexities in alcohol’s metabolic rate that have to do with its ability to proceed down different pathways according to the situation at hand, each having a different impact on the net caloric yield. It’s known, for example, that the metabolites of alcohol can slow down the burning of the body’s own fat for energy; this happens because the alcohol gets preferential treatment. The net effect can be more of a positive fat balance, and alcohol-related fat tends to be deposited in the abdominal area.

Another issue with alcohol is its effect on the heat produced by the body (thermogenesis). Metabolic processes such as digestion are not 100 percent energy-efficient, so it is normal for some dietary energy (i.e., calories) to be lost as heat. (This also explains why we get warm when we exercise.) Alcohol is known to increase thermogenesis more than other foods, but this effect is less robust in overweight individuals, or when paired with a high-fat diet.

A bottom caloric line is that there are about seven calories per gram of alcohol, a number greater than carbohydrates, but lower than fat calories. It is no mystery that the more you drink, the more calories you consume. Excess calories from any source will be stored as body fat, but the alcohol-related excess tends to find a home around the midsection.

Send questions to: “On Nutrition,” Ed Blonz, c/o Andrews McMeel Syndication, 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.