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When I look at my 'calorie counter app' it says that two slices of Peanut Butter Toast (assuming 200g) are roughly 1427 kJ of energy.

Then when I look at this post - I see a rule

There are a couple of useful observations at this point. First, the “3500 calories per pound” rule of thumb is perfectly valid… as long as it is applied correctly.

So if we convert that to metric we get

32.3 kJ / kg

So very roughly - if I have a 32KJ deficit at the end of the day - that should correspond to a 1 Kg weight loss.

Now perhaps the answer is energy density, peanut butter toast has a high energy density per gram and stored body fat has a low energy density per gram. But something still seems fishy.

Is my mathematics completely incorrect? My question is: How does 200g of toast relate to 500g of weight loss?

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  • Unfortunately, I'm horrible at conversions. I think the point, though, is this: Just because you are at a calorie deficit of 1750 Calories (or half a pound, for example) does not mean you are losing half a pound. Your math (again horrible at conversions so I am assuming) might be correct; but, physiologically it doesn't happen that way. – BryceH Oct 31 '14 at 12:49
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Your math is way off, because it's not being applied properly. First, a pound of fat contains 3500 Calories. Most healthy individuals do not eat that in a day unless they are high level athletes who use all of those calories. Never think you are going to lose a pound of fat a day.

So we have 1 pound which is roughly equivalent to 454 grams of body weight. An equal mass of fat will require 3500 Calories to sustain it.

If you want to gain or lose that amount, you have to remove or add that amount to your weekly net energy expenditure. To understand this, you have to know how many calories are required to maintain your current body mass.

Example

We have an active person with a decent build, no six pack, and weighs around 80 kg (176 lbs). After some experimentation they find that it takes 2400 Calories a day to maintain their body mass.

In order to figure out how much they need to adjust their diet to lose 454 g (1 lb) you need to look at things over the week:

  • 2400 Calories X 7 days = 16,800 Calories a week
  • To lose 454 g (1 lb) you need to take away 3500 Calories for the week
  • 16,800 Calories - 3,500 Calories = 13,300 Calories for the week
  • To find out what that means each day, we have to divide by 7 again
  • 13,300 Calories / 7 days = 1,900 Calories a day

To simplify that, taking 500 Calories out of your daily diet will have you losing 1 lb a week.

Your conversion is flawed

According to this article, one calorie (note the small 'C') contains 4.184 Joules. One kcal, or more commonly written as a Calorie (note the capital 'C') contains 4.184 kJ.

1427 kJ / 4.184 kJ/kcal = 341 kcal

NOTE: from chemistry conversions we learn that units matter and when we divide units they cancel out. To simplify this the progression gets expanded like this:

  • a kJ / b kJ/kcal = a kJ X 1/b kcal/kJ
  • the kJ units cancel out, leaving
  • a X 1/b kcal

This is how unit conversions work. Of course you do need the conversion factors to do this properly.

When talking about food, Calories are always kcal in scientific terms. Not everyone is careful enough to handle the capitalization correctly.

The body is a complex system

When everything is operating normally, the calories in vs calories out model works well enough. It's when you get to extreme cases of trying to lose massive amounts of weight that your body doesn't get enough to survive that we run into problems.

Many things work together, so we need an appropriate amount of protein, fat, and carbohydrates to function well. Completely disregarding one of these macronutrients can cause health problems in the long term. A short term (6-8 weeks) ketogenic diet (no or very little carbs) can work very well if you have enough protein, but any longer than that and weight loss slows to a crawl as other hormones come into play. A long term plan always ensures your necessary nutrition is present.

Don't starve yourself, and don't engorge yourself. Small changes over time add up.

NOTE: you may want to check out this article which provides the number of Calories and kJ in a pound and a kilogram of fat: http://www.caloriesperhour.com/tutorial_pound.php

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    I will add that 3500 calories per pound lost is an estimate, and should be treated as such. Don't get too caught up trying to measure and map out every single lb., and just focus on eating a healthy and sustainable diet. Falling off the bandwagon and putting back on all the fat you lost happens to upwards of (conservatively) 75% of all dieters. I would be far more concerned with keeping motivation up and adhering to the diet than anything else. – Moses Nov 5 '14 at 4:22
  • Thanks for this fantastic answer. I've saved it as a pdf for my own future reference. My question doesn't seem to be a good fit for this site and people are voting it down. I'm considering deleting the question. – hawkeye Nov 6 '14 at 8:19

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