Updated: August 26, 2011

The truth is, all diets boil down to a simple formula-eating fewer Calories than you burn. Break that rule and all the carb-cutting, fat-banning, low-glycemic-index-eating in the world won't make a bit of difference. This is why weight-loss experts are now advocating a back to-the-basics approach: Calorie counting.

Weren't paying attention in high school biology? No worries. This crash course on the science of Calories will catch you up fast. You'll learn to make smarter choices about the foods (and portion sizes) you put on your plate and how to maximize your daily Calorie budget. The savvier you are, the quicker you'll drop pounds.

Q: Exactly what is aCalorie anyway?

A: In science equations, a calorie is a measure ofpotential energy infood. Technically, onecalorie is the amountof energy it takes toraise the temperatureof 1 gram of water by1 degree Celsius. The Calorie we refer to generally in nutrition, which is found on food labels, is the kcal, or kilocalorie (which equals 1,000 calories). "Caloriesare like gasoline. In thesame way that gasmakes your car go, caloriesfuel your body," saysD. Milton Stokes, R.D.,president of a nutritioncounseling firm inNorwalk, Connecticut.Q: How does mybody convert Calories?

A: Enzymes in yourdigestive system breakdown the chemicalbonds that hold foodmolecules together,explains Gary Miller,Ph.D., an associateprofessor of nutrition atWake Forest University.Your digestive systemthen releases the energycontained in thosebonds, makingit available for use.Q: What does my bodydo with that energy?A: It fuels everythingfrom basic activities likebreathing, thinking, andgrowing hair to biggertasks, like carrying apregnancy or runninga marathon. However,when you don't usethe Calories you'veconsumed (maybe youdecide to skip the gymtoday), those Caloriesget shuttled to yourliver to refill your glycogenstores. Glycogenis your body's quick,easy-access energyreserve. Your ability tostore it means youdon't have to eat continuouslyto keep yourbody revved up. Still, itgets depleted everythree to four hours.When the liver is holdingas much glycogen asit can, some of it ispassed on to muscles forshort-term storage (tobe used as needed tomove your body and getyou through a workout).Between your liverand muscles, you have aready supply of Calories(roughly 300 to 400,depending on yourweight and metabolism)that you can access asnecessary throughout theday. When you eat morethan you can save in thesetemporary "accounts,"the Calories get convertedto fat and distributedthroughout your body.Q: How many Caloriesdo you have to burn tolose a pound?A: About 3,500. Thatmeans you can eat 100fewer Calories a day for35 days, or 500 fewercalories for seven days,or walk an hour a day for22 days, or do a combinationof the two byeating less and movingmore. Remember, even ifyou're exercising morethan usual, the Calories-in,Calories-out rulestill applies: If you takein more than you burn,you'll gain weight.

Q: How do scientistsdetermine the numberof Calories in a food?A: Typically, they usedifferent laboratorymethods to separate thecaloric components offood-the macronutrientsprotein, fat, and carbsfrom the noncaloricsubstances like water andminerals. They figure outthe weight of each macronutrientin grams andthen multiply by the followingto get the Caloriecount: fat, 9 Calories pergram; protein and carbs,4 Calories per gram.Q: Can I have a foodanalyzed to see if theCalorie count listed onthe label is correct?A: Yes, but it's expensive-about $90 anitem. Contact BodycoteFPL at fplabs.com formore information.Q: How do scientistsfigure out how manyCalories a personburns during exercise?A: They place a plastictube over the exerciser'smouth and measure theamount of oxygen sheinhales and uses whileengaging in a specificactivity (e.g., walking,rowing, biking). Everyliter of oxygen consumedis equivalent to 4.8Calories burned. As yourfitness level increases,you're able to take in anduse more oxygen, whichlets you work out harder,longer, and burn moreCalories (and fat).Q: How much do I reallyneed to eat every dayto maintain my weight?A: It's a complex formulathat depends on yourweight and height (theheavier you are, the moreCalories you'll burn), age(the older you are, thefewer you need), andactivity level (computer solitairefans utilize far less than marathoners).For example, a 25-year-oldwoman who is 5'9",150 pounds, and veryactive (i.e., exercising sixdays a week) requiresroughly 2,570 Calories aday to maintain herweight, while a 40-year-oldwoman who is 5'4",130 pounds, and onlymoderately active(working out three daysa week) needs 640 fewerCalories (1,930). For anestimate of your Calorieneeds, go to caloriesperhour.com/index_burn3.html.Q: Can I bank Caloriesso I can eat moreon a special occasion?A: In theory, yes. If youknow you're going to aparty on Saturday nightand want the freedomto indulge a little, you canshave, say, 100 Caloriesfrom what you normallyeat Sunday throughFriday and have an extra600 Calories to play withby the time the partyrolls around. At theend of the week, yourCalorie intake-and yourweight-stays the same.In practice, though, thisstrategy may backfire.It's tricky to cut enoughcalories to matter, butnot so many that yougo hungry or affect yourmetabolism. "Really,what you're trying to dois lose a little weightso you can gain it back,"says Michael B. Zemel, M.D.,professor of nutritionand medicine at theUniversity of Tennessee."But when you depriveyourself, you typicallycompensate by eatingway too much later-and take in more Caloriesthan you banked."Instead of saving upCalories, on days whenyou know you may wanta little more diet wiggleroom, eat foods that arelow-calorie but filling.For instance, if you usuallyhave a turkey sandwichfor lunch, try a salad withsliced turkey. You'll takein fewer Calories by skippingthe bread but feeljust as satisfied becauseof the fiber in the greensand other veggies. Thenyou won't arrive at yourevent famished, andyou can afford to have alittle of your splurgefood. An even smarteroption: "Work out a littlemore before and afterthe party," says Zemel."Every extra mile youwalk burns 100 Calories."

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