Posted by: confused | 2008/01/29

Kj vs Calories

How many KJ = a calorie ?
Food info always mentions KJ but exercise machines use calories so I need the know the correlation between the two

How many calories do you burn swimming ? - for some reason swimming is not included as a sport on your "burn-o-meter", nor is weight training


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Our expert says:
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Hi confused

A Calorie is larger than a kilojoule - you multiply the Calorie amount by 4.2 (rounded off - it's 4.18). So 100 Calories (or kCal) equals 418 kilojoules

As for how many you burn swimming, depends on how fast you swim, what stroke you swim, and on genetics. But generally, you'd burn around 10 to 15 kCal per minute (so around 45 to 60 KJ). In 30 minutes, then maybe 300 to 400 kCal.

Just a word on this counting of calories, the idea is that if you burn more than you take in, you lose weight. That may be true in theory, but there are a couple of things that can go wrong if you're not careful.

First of all, it's impossible to know exactly how many calories you take in. You would have to weigh all your food, and that is a nightmare to do. You would have to sit and weigh each part of your meal, know exactly what you eat and drink and then sit with a calculator and work out how many calories you've taken it.

Secondly, it is even more impossible to know how many calories you have burned during the day. Even during exercise, it's not possible to measure with 100% accuracy how many calories you have burned, because the heart rate monitors and machines are just guesses or estimates. And that's not even talking about the other 23 hours a day, where you burn far more calories. So this makes it impossible.

And lastly, it really doesn't work as simply as this anyway. The body is so well created that it can match intake to use over many many years. If you think about it, someone who remains at a stable weight over a period of just 10 years, is probably eating an average of 8 or 9 million calories during this period. In order to remain at the same weight, the body is somehow balancing this amount by using about 8 or 9 million calories too! So, it’s staggering that the body is able to do this as efficiently as this, and for this reason, being very particular about counting calories all the time is probably a lost cause. It’s also the reason why sudden crash diets, where you cut right back on eating and live on lettuce every day, for example, will not work. The body simply adjusts its metabolism and so you use fewer calories. This also promotes weight gain when you do eventually start eating again.

So, rather than taking drastic steps, you have to realize that slow and steady progress is the key. It is a useful idea to become educated about what you are putting into your body, and then to be sure that it’s not excessive. The general recommendation is that for a sedentary, inactive person, you should aim for about 29 calories per kilogram (A 70 kg person, for example, would need about 2000 Cal per day). If you check out your diet, and read up how much you are putting in, and it’s around 2500 or even 3000, then you will never lose weight (or you would have to exercise like a demon, which is probably not an option). It’s also not a good idea to be consuming way under 2000 Cal per day, because this will cause the body to slow its metabolism down, as I mentioned earlier.

So, the key is to cut down on those calories very slightly (and this is simple enough to do if you take practical steps – slightly smaller portions, less variety, eat foods like apples and citrus fruits that are high in fibre etc.), and to increase your energy expenditure by means of exercise. Just 30 to 45 minutes, 4 or 5 times a week is enough to increase your overall energy expenditure by a large amount. For example, 30 minutes of walking burns about 300 Cal, and the effects last longer than just the actual exercise time. Therefore, if you are inactive now, and you suddenly start exercise by walking 30 minutes, you will be using 300 Cal more than you were previously. At the most simple, this means that if you diet stays the same, you will lose weight.

I hope that this has helped a little – I have tried to just give you a general idea. For a specific diet and eating plan, a dietician is far more qualified than I am to advise you, and I would suggest getting in contact with one to discuss this.

Good luck

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Our users say:
Posted by: sally | 2009/02/14

how calories should i eat per day to lose weight

Reply to sally
Posted by: Christo | 2008/06/21

Dear doc,

I would like to know how many calories I am to burn per day to be fit: I am 45 years old, weigh 109 kgs and would like to loose downs to 102kg.

I do have a no fat diet, but I just don't seem to loose weight.



Reply to Christo

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