When I'm on a diet, I feel it's very important for me to have an idea of how much weight I should expect to lose per week. Most diets are "fixed" and promise a certain amount of weight loss per week based on standard or average measurements, but they don't take into consideration the single most important factor - that's you.

The first thing one should keep in his/her mind is how much energy you need to perform your daily duties: we'll call this your Active metabolic rate. This amounts to your basic metabolic rate plus the energy you spend in your daily activities. Contrary to popular belief, the vast majority of this energy is spent simply to keep you warm and alive, and the bigger your body is, the more energy you'll spend. That's why you can roughly estimate your energy needs with a simple formula:

    AMR = 2550 + ( weight_in_kilos - 75 ) * 17.5
This formula computes the amount of kilocalories per day required for caucasian adult males having a light to moderate activity (if you're a lumberjack in Canada, it will likely sub-estimate your energy needs). There are many other ways of computing your AMR, see for instance http://www.blaha.net/Main%20BMR%20Calculator.htm for a simple JavaScript calculator (or http://www.weight-free-lifestyles.com/metaboli.htm or see the diet node).

Now that we know how much you spend in your daily activities, we'll need to estimate the effects of a moderate intake reduction. To do this, we need to understand that:

  • A kilogram of fat stores 9000 kcal
  • Your body will consume 1300 kcal just to turn a kilogram of fat into useful sugar, so a kilogram of fat is actually worth only 7700 kcal when you need to consume it
So we can compute an average daily weight loss, in kilos, as
    WLd = ( AMR - caloric_intake ) / 7700
If you are eating more than your AMR, on the other side, this formula for weight gain will be as follows:
    WGd = ( caloric_intake - AMR ) / 9000
Real word example #1: choosing a diet
I am a healty overweight 30 year old male and my weight is circa 100 kg. I expect my AMR to be around 2988 kcal per day.
  • If I eat circa 3000 kcal per day, my body weight will stay fixed
  • If I eat circa 2200 kcal per day, I will lose 100 grams per day, or roughly 3kg in a month
  • If I eat circa 1600 kcal per day, I will lose circa 180 grams per day, or roughly 5kg in a month
The calculation for the third scenario is probably wrong and overestimates the actual weight loss because if you are hungry your body will reduce its caloric need preparing to face times of adveristies (evolutionary lore). This can lead up to 30% calories less used per day, so it's not a good idea (leaving headache and constant hunger apart). This is - in my opinion - one of the reasons why crash diets don't actually work and I don't recommend them. In my experience, the minimum required calories for me not to be hungry are around 1900-2100 per day. Your mileage may vary.

Real world example #2: Getting fat with pasta
I love pasta. There's probably nothing I love more than pasta and bread. The average suggested serving per person is about 100 grams of pasta per person. I guess that if I eat 150 grams per day nothing much will change.... right?
In the short term scenario, I am right. 100 grams of pasta are about 360 kcal, so if on a given day I eat 50 more, I'll have +180 kcal that day, and this is not much. We can estimate how much fat is that by computing its WGd - that's just 20 grams of fat added. Who cares?
In the long term scenario, still, 20 grams in excess per day lead to a whooping +7.3 kilos per year! This is quite a lot, and sure not worth the "added value" of the extra 50 grams of pasta.

Final note
You should not expect your weight to follow this weight loss in the very short term (1 to 2 weeks) because the amount of water your body stores is highly variable and depends on what you eat, on the climate, and on many other factors. Still, when depurated by the variance given by water, this approach can give you a credible expectation of your weight loss.

Personal note: I have lost nearly 40kg (nearly 90 lbs) during the last year and a half using a moderate diet (ca. 2000-2100 kcal per day), a moderate increase of physical activity and these formulas to monitor my weight loss. I think they are useful. Anyway, always consult with a physician when starting a weight loss or gain program.