For many weight watchers, keeping a track record of calorie intake and output is part of their daily life. Eating a bowl of rice might give them 260 calories, while jogging for 30 minutes will burn off 300. Or so they think.
In fact, counting calories in food can be much more complicated. The calories in an apple, for example, vary depending on where it was planted and the type of seed.
When it comes to meat, whether the skin is eaten and the kind of sauce served with it can greatly alter the number of calories.
Wendy Ma, programme director of the College of Life Sciences and Technology, HKU Space, and an accredited dietitian, says that the numbers in a food calorie table are simply a rough estimate, with many factors not taken into account.
"Take a grilled steak, for example, she says. "The differences in cooking method, portion size, origin of ingredients all affect the amount of calories, yet the food energy table is not able to take note of these factors. The most accurate way to count calories in a food is to take out a portion of it, then burn it in a calorimeter in a laboratory."
Registered dietitian Dr Susan Chung, sport nutrition and monitoring manager at the Hong Kong Sports Institute, says that counting calories is not the right way to achieve a healthy diet.
"Some people think that if they have a bag of crisps and then skip lunch, that will be OK, because it keeps their calorie intake at an acceptable level," she says. "They forget that, besides calories, there are important nutrients, such as protein, carbohydrates, dietary fibre and vitamins that the body requires to stay healthy."
On the other hand, Chung says that calorie trackers are already conscious of what are the fatty food items, giving them a foundation to learn to eat healthily. "I think they will find that fried chicken has a lot of calories and this may lead them to go for a healthier choice, like steamed chicken," she says. "Tracking calories makes one aware of healthy food items."
Besides counting calorie intake, many people like to gauge their output when they go to the gym. But Chung says the calorie calculator on a treadmill is not accurate.
"The user is asked to put in his or her weight and the calculator will count the calories burned during the workout," she says. "But people with different fitness levels burn calories at a different rate.
"Athletic people with a good fitness level will burn calories more quickly than those who are overweight. A person's fitness level is not something the machine is able to keep track of."
Both Ma and Chung stress the importance of a healthy diet and regular workout. The fitter and stronger you become, the faster the calories will burn off.
The key to staying healthy is to keep a keen eye on what you eat and how hard you work out, rather than how many calories you eat.