The ‘secret’ to weightloss

While we’re not suggesting you become kilojoule-obsessed, it does help to know what they are and how many you’re having.

Q. What is a kilojoule?

A. It’s a measure of potential energy in food. “Kilojoules are like petrol. In the same way that petrol makes your car go, kilojoules fuel your body,” says US dietitian Milton Stokes.

Q. How does my body convert kilojoules into energy?

A. Enzymes in your digestive system break down the chemical bonds that hold food molecules together, says US nutritionist Gary Miller. Your digestive system then releases the energy in those bonds, making it available for use. This energy fuels everything from basic activities like breathing, thinking and growing hair to bigger tasks such as being pregnant or running a marathon. However, when you don’t use the kJ you’ve consumed (for example when you skip your gym session), those kJ get shuttled to your liver to refill your glycogen stores. Glycogen is your body’s quick, easy-access energy reserve. Your ability to store it means you don’t have to eat continuously to keep your body revved up. Still, it gets depleted every three to four hours. When the liver is holding as much glycogen as it can, some of it is passed on to muscles for short-term storage (to be used to move your body and get you through a workout). Between your liver and muscles, you have a ready supply of kJ (roughly 1250–1700, depending on your weight and metabolism) that you can access as necessary throughout the day. When you eat more than you can save in these temporary “accounts”, the kJ is converted to fat and distributed throughout your body.

Q. How many kilojoules do you have to burn to lose weight?

A. “There are 32000kJ in a kilogram. These can be sourced by decreasing your food intake to supply less kilojoules than your body requires, or by increasing your output through exercise to burn more energy each day. The best strategy is a combination of both,” says Durban dietitian Carol Browne. Be sure to aim for a realistic and healthy target that you can sustain in the long-term. “A healthy weight-loss goal of half a kilogram a week represents an energy deficit of about 2000kJ a day, which is achievable and sustainable for most people. A person with more to lose, or who is prepared to be really active, can expect to lose more, and for it to be mostly fat loss. You should be realistic in your expectations though, as even excellent eating and exercise habits can result in a week with no weight loss. But this shouldn’t stop you focusing on a long-term goal of health and weight loss. There’s no quick fix, weight takes years to accumulate and will take time to be reduced,” says Browne.

Q. How do scientists determine the number of kJ in food?

A. Typically, they use different laboratory methods to separate the kJ components of food — macronutrients, protein, fat, and carbs — from water and minerals. One method is to work out the weight of each macronutrient in grams and then apply a formula. Another is a machine called a bomb calorimeter, which burns a small amount of the food and measures how much heat is produced, and that’s the amount of kJ it contains.

Q. How do scientists know how many kilojoules are burnt during exercise?

A. The amount of oxygen inhaled and used while exercising is measured. Each litre of oxygen consumed is equivalent to about 20kJ burnt. As your fitness level improves, you’re able to take in and use more oxygen, which allows you to work out harder, and longer, and burn more kJ (and fat).

Q. What’s the difference between calories and kilojoules?

A. Kilojoules are the standard international and more scientific measure of food energy, says Browne. The term calories is commonly used in the US. One calorie is equivalent to 4.18 kilojoules.

Q. How much do I need to maintain my weight?

A. It depends on your weight and height (the heavier you are, the more kJ you’ll burn), age (the older you are, the fewer you need), and activity level. For example, a 25-year-old woman who is 1.75m, 67kg and very active (exercising six days a week) requires roughly 10700kJ a day to maintain her weight, while a 40-year-old woman who’s 1.6m, 58kg and moderately active (three days a week) needs 8000.

Q. Can I bank kilojoules so I can eat more on special occasions?

A. In theory, yes. But in practice, this can backfire. It’s tricky to cut enough kilojoules to matter without going hungry or affecting your metabolism. “When you deprive yourself, you typically compensate by eating too much later — and take in more kJ than you banked,” says Prof Michael Zemel. Instead of saving up kJ, on days when you know you may want a little more diet wiggle room, eat foods that are low in kJ, but filling. An even better option: “Exercise a little more before and after the party,” says Prof Zemel.

Turn off the TV!

Researchers at the University of Alabama (US) found that people who were glued to the box while eating consumed nearly 600 more kJ per day, which adds up to an extra 6kg a year!

4 savvy eat-out tips

Restaurants can be a kilojoule minefield. Even if you order carefully, choosing baked instead of fried, or wholewheat rather than white, you may still find yourself with a dish that contains a full day’s worth of kJ. According to a new study by researchers at the University of Arkansas in the US, most people underestimate the kilojoule content of restaurant meals by about half, which can add up to significant weight gain, especially if you eat out several times a week. But with these strategies, you can avoid putting on kilos:

1. Order an appetiser
Starting with a green salad or vegetable-based soup will take the edge off hunger so you won’t gobble up all the bread or down your entire entrle, says Stokes.

2. Don’t drink and dine
“Alcohol lowers your inhibitions, so you’re more apt to have another drink (or two) and mow through half a bowl of nuts before you even sit down to eat,” he says.

3. Don’t supersize
Smaller portions save you kilojoules, and let you sample foods without feeling deprived.

4. Fresh is best
“Many convenience foods are very kilojoule dense and make passive overeating (eating without realising how many kilojoules you’re consuming) an everyday habit for most people,” warns Johannesburg dietitian Claire Julsing, of Anne Till and Associates. Leaving off the cheese, rich toppings and having sauce on the side helps.