Walk Talk Nutrition: 3500 calories per lb

Weight loss predictions: Blue: 3500 rule; Red: actual

Weight loss prediction from cutting 140 calories/day: Blue: 3500 rule; Red: actual

The Walk Talk Nutrition RD team takes on the 3500-calorie-per-pound weight loss rule.  Turns out it doesn’t work.  Well, if you’ve ever been on a diet, you already knew that.  Now we have a better idea of why it isn’t working.

Our discussion focuses on 3 key articles:

  1. the original calculations by Wishnofsky, done in 1958, showing that 1 lb of fat tissue has about 3500 calories of lipid: “Caloric Equivalents of Gained or Lost Weight”
  2. a consensus statement by a group of researchers on the current (sorry) state of our understanding of energy balance and obesity.
  3. an article discussing new equations that predict weight loss more accurately. (subscription required)

So why isn’t weight loss a simple math problem: for every 3500 calories you subtract from your food intake, you lose 1 lb of fat.  It turns out, there are plenty of other metabolic changes that come along with weight loss, that impact your calorie requirements.  In general, as you lose weight, your calorie needs go down along with your weight.  Meaning you now need less energy for everything, from the basics like heart beat and breathing, to food digestion to moving your smaller body around to your daily 1 hour walk.

The authors of the 3rd article give a great example of how this works.  Say you ask an obese woman to cut just one soft drink per day, substituting water.  That’s a 140 calorie reduction in intake.  If you do the 3500 calories/lb rule math, you might predict that she will lose 15 lbs in one year just from that one simple change.  But if you plug all her numbers (body weight, height, gender, calorie intake, calorie reduction….) into a new predictive equation, it turns out she’ll only lose less than 6 lbs in a year from that change.  It’s still weight loss, but much less than she may have anticipated.  If she stuck to her plan, expecting to see a 15 lb loss, she might end up disappointed and frustrated at the real results.

Here’s another common weight loss recommendation: Burn an extra 100 calories/day by exercising and you’ll lose 50 lbs in 5 years.  That sure sounds like an attractive option.  Burning an extra 100 calories/day with activity is pretty simple.  Except that this prediction — based on that 3500 calorie rule — is wrong.  In fact, according to the new simulators, you’d only lose about 10 lbs in those 5 years from making this change.

Here’s a link to one of the new weight loss prediction simulators.  The introduction says it’s intended for research purposes, and there are a lot of variables you can change.  If you really want to geek out about this issue, you can click on the link to the Dynamic Mathematical Model equations — lots of algebra/calculus.

The take away message:

  • Weight loss is not linear.  It doesn’t go in a straight downward line (the blue line above) based on 3500 calories = 1 lb body fat.
  • Most diets — whether fad diet or sensible diet — and most fitness gadgets base weight loss predictions on that model.  If you’d rather avoid the frustration caused by that outmoded belief system, tune out those optimistic weight loss predictions.
  • You can give a slight boost to the energy demands of digestion by eating more whole and plant-based foods.  They’re harder to digest and aren’t always completely digested, causing some calorie waste.
  • Initial weight loss on most diets looks dramatic.  You’re burning off stored glycogen, and the water stored with that glycogen is lost, making the scale go down.  That rate of loss never lasts long.  Losing fat weight is a slow process.  As long as the average trend is down, ignore the short-term ups and downs on the scale.




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