Hypophagia – How much fat can I lose in a day?

In the last year I have seen some ridiculous claims about weight loss. Many of them claim that people lose incredible amounts of weight in very short periods of time. And there are some times in Ketogenic dieting where you can lose a lot of weight. Particularly in the beginning when hormones are shifting due to significant drop in Insulin levels. Much of this loss is water weight.

I’m going to risk repeating myself and made a statement that should not be in the least bit controversial. But, for some reason it is controversial.

Weight loss is only vanity.
Fat loss is what really matters.

And you can quote me on that. Don’t get me wrong. If you lose body fat more than likely you will lose weight – all things being equal. But your goal should not be to be at a particular weight. Your goal should be to be at a particular body fat percentage.

How Much Fat Can a Person Lose In a Day?

Turns out this is a known quantity called the fat oxidation rate. That’s the maximum amount of fat a person can burn in a day. And the number is also known. The amount of fat a person can lose in a day under starvation conditions is 31 cal/lb/day (SS, Alpert. A limit on the energy transfer rate from the human fat store in hypophagia. J Theor Biol. 233 (1): 1–13) (PDF). (Checking the numbers / units, the article states 290 kJ/kg-day, which is 69.26 kCal/kg-day. With 1 lb = 2.2 kg, that’s 31.5 kCal/lb per day).

Let me repeat myself – You can only lose 31 calories worth of fat per lb of your body fat weigh in one day. Note this is for a person who does “moderate activity”. Lyle McDonald looked at this number and concludes it’s a good starting point.

Someone else ran the numbers with more or less the same conclusions as I reach here and lays out their plan.

Suppose you weigh 200 lbs and have 25% Body Fat. Most high tech scales will give you your weight and body fat percentage. That’s 50 lbs of body fat (take the 200 and multiply it by 0.25 to get 50).

If you have 50 lbs of body fat that’s 50 times 31 calories of fat per day or 1550 calories from body fat you can lose right on that very day.

Note that the number of lbs that you can lose decreases constantly as you lose body fat.

Updated 2017-11-08: Alpert has reportedly updated his number from 31 to 22 with the following reported comments:

The value used of 31 kcal/d lb is correct only if there is no energy loss to activity. In the original paper which you cited. I took a value of activity losses from some of my previous work, but I have recently developed a much better way to estimate activity losses which changes the maximum practical value for the idealized value of maximum loss from fat alone to the actual loss to only 22 kcal/d lb. This mean that it is very hard to lose fat alone. That is life.

So What is Actually Lost in those Short Timeframes?

Water. And when things equalize in your body it returns. That’s why people say they tried a diet and regained it all back.

And there’s nothing wrong with losing the water and inflammation that goes along with water. It might make you healthier to lose some water especially if you get more mobile as a result.

But, if you look at the water percentages of healthy persons and compare them to obese persons the obese people do have much LOWER water percentages than healthy people. Here is what may be little known fact (How Much of Your Body Is Water?):

Body composition varies according to gender and fitness level because fatty tissue contains less water than lean tissue.

As you lose weight your fancy scale will tell you that you have gained a serious amount of water as a percentage. Here’s my own numbers (per my high tech scale):

  • At 248 lbs I had 69.5 lbs of water.
  • At 197 lbs I have 74.9 lbs of water.

if you do the math I have more water now than when I was 49 lbs heavier. My percentage of body that is water has gone from a very unhealthy 28% (bottom of the scale readings) to a less unhealthy 38%. Older men are supposed to be closer to 50%.

So as your weight drops your body fat percentage goes up. That’s because you really still need more or less the same amount of water in your body for blood, tissues, etc. You don’t need it in the fat cells (around 10% of a fat cell is water) but there’s some thought that the fat cells fill with water as the fat empties. Might well be the case (look up whooosh on the search box above).

Author: Doug

I'm an Engineer who is also a science geek. I was pre-diabetic in 1996 and became a diabetic in 2003. I decided to figure out how to hack my diabetes and in 2016 found the ketogetic diet which reversed my diabetes.

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