How to Lose Weight Effortlessly
You've probably seen this claim many times: a pound of fat contains 3,500 kilocalories (kcal). A slice of toast is 80 kcal. All you have to do is forego one slice of toast per day-- just a few percent of your total calorie intake-- and you will lose 8.3 lbs of fat per year. Fat loss is so easy!
This reasoning is extremely common both in the popular media and among researchers. Here's an example from the otherwise excellent book Mindless Eating, by researcher Brian Wansink:
...the difference between 1,900 and 2,000 calories is one we cannot detect, nor can we detect the difference between 2,000 and 2,100 calories. But over the course of a year, this mindless margin would either cause us to lose ten pounds or to gain ten pounds. It takes 3,500 extra calories to equal one pound. It doesn't matter if we eat these extra 3,500 calories in one week or gradually over the entire year. They'll add up to one pound.
This is the danger of creeping calories. Just 10 extra calories a day-- one stick of Doublemint gum or three small Jelly Belly jelly beans-- will make you a pound more portly one year from today. Only three Jelly Bellys a day.
According to this reasoning, if I reduced my calorie intake by 80 kcal per day, I'd become skeletal in two years and vanish in a puff of smoke within 10 years*. All from a meager 3 percent reduction in calorie intake!
Thermodynamics of Weight Loss
Here's the first fly in the ointment: energy usage by the body is proportional to body size (particularly lean mass). When a person undereats and his body gets smaller, it burns less fuel. As fuel usage declines over time with weight loss, the gap between calorie intake and expenditure gradually closes, and eventually calorie expenditure matches intake and weight loss stops. The second fly in the ointment is that the brain senses fat loss and tries to further curtail calorie expenditure, closing the calorie gap even sooner than would be expected by the reduction of lean mass.
National Institutes of Health researcher Kevin Hall and his team have created a mathematical model that accurately predicts weight and fat changes in response to changes in calorie intake (1). This model has been validated against the results of weight loss trials (2). Based on this model, his team put together the Body Weight Simulator app that allows us to calculate and graph weight changes over time.
Let's play with it. We'll start with a typical 45-year-old obese woman named Sandy-- 5'5" tall, weighing 194 lbs. Her job is fairly sedentary but she walks to and from work every day (1.80 PAL). Her daily calorie intake is 2,751 kcal (estimated by the simulator).
Now, we put her on Wansink Diet #1, which is 100 kcal less than previously, for a new total of 2,651 kcal/day. According to the calculator, Sandy will lose 5.5 lbs over one year if she maintains this 100 kcal deficit-- far short of the 10 lbs that was promised. Her body fat will drop from 43.1 to 42.2 percent-- less than one percent fat loss.
The effect of this diet becomes even less impressive over time as calorie expenditure approaches intake. Over the next year, Sandy only loses an additional 2.2 pounds. Over the third year, Sandy only loses one additional pound.
After 6 years, Sandy eventually plateaus at 185 lbs, just 9 lbs below where she started. Her body fat percentage has declined from 43.1 to 41.5. She is still obese.
Here's a graph of her weight change over 6 years, from the Body Weight Simulator, illustrating the gradual slowing of weight loss (horizontal axis represents days):
As you might imagine, Wansink Diet #2-- avoiding one stick (10 kcal) of gum per day-- is even less effective. Over the course of 6 years, Sandy loses one pound.
This is one of the key explanations for the weight loss plateau, the scenario where it feels like you're using the same weight loss strategy that was effective before, but weight loss has stalled.
What it Takes to Become Lean
The following graph is adapted from a study that measured the calorie requirements of lean and obese people (3). Most (86%) of the subjects were women, but the findings have been replicated in men as well. On the horizontal axis, we have body mass index (BMI), and on the vertical axis we have the calorie requirement for weight maintenance (i.e., energy expenditure). As a reminder, a BMI of less than 25 is lean, 25-30 is overweight, and greater than 30 is obese.
As you can see, calorie requirements increase with BMI. A typical moderately obese woman (BMI 32.3) requires 453 more kcals per day than a lean woman (BMI 22.6) of similar height. Another way of putting this is that the former burns and eats about 453 more calories per day to maintain weight and body composition. This might sound like a lot, but it only amounts to a ~20 percent difference in intake**.
Lucky for us, our friend Sandy's BMI is 32.3, the same as one of the data points on the graph above. Let's see what happens when we reduce her calorie intake by 453 kcal per day, approximating the calorie intake of a lean woman.
Over the course of 6 years, Sandy loses 41 lbs and plateaus at a weight of 153 lbs, BMI 25.5-- almost in the lean BMI range. Her body fat declines from 43 to 35 percent, better but still too high.
But we've left out a key factor here. In the study referenced above, lean people didn't just eat less. They also moved about 20 percent more (1.6 vs. 1.3 METs). If we increase Sandy's (previously modest) physical activity level by just 20 percent, and assume the same calorie intake as above, she plateaus at a weight of 138 lbs and a BMI of 23.0-- similar to the lean group in the graph above. Her body fat percentage is 32 percent, compared to 29 percent for the lean reference group (admittedly, the lean reference group is not particularly lean by % fat). In other words, if she eats like a lean person, and moves like a lean person, she achieves the body composition of a lean person eventually, but it takes time.
As you can see, delving a bit deeper into the thermodynamics of weight change gives us answers that make more sense than commonly held beliefs, and also explain certain fundamental challenges to weight loss, such as:
- Calorie restriction not producing as much weight loss as expected
- Frustrating weight loss plateaus
An additional problem that these calculations don't give us much insight into is that the brain resists fat loss, making it difficult to consistently restrict calories due to an increase in hunger and thinking about food. Together, these factors go a long way toward explaining why fat loss can be challenging, and why simple calorie restriction strategies usually fall short. Calorie counting works fine for some people, but most people find it unsustainable and may prefer the following strategies instead:
- A food environment that reduces "mindless eating", à la Wansink
- Improved food quality, e.g. lower calorie density and higher protein, to increase satiety per calorie and reduce the damage done during instances of overeating
- Improved diet and lifestyle quality to reduce the degree to which the brain 'defends' against fat loss
- Regular physical activity
- Restorative sleep
* I'd exhaust my ~17 lbs of fat in just over two years, and the remaining lean mass has a much lower calorie density than fat tissue, so I'd wither away rapidly after that.
** This isn't much when we consider that the moderately obese women carry twice as much fat mass as the lean women.