Why Your Diet Doesn’t Work: The Imprecise Science of Caloric Balance

“Want to lose one pound of fat? Just cut 500 calories from your diet each day and you’ll shed one pound per week!” We’ve all heard that sensationalized claim before, and if it sounds a bit too good to be true, that’s because it is. This math is based on the premise that it takes exactly 3500 calories to burn one pound of fact, but that information itself is inherently flawed.

Weight maintenance is essentially a function of our basal metabolic rate, or BMR: the amount of calories your body burns daily just to survive. Just like it takes more energy to power larger machinery, it takes more calories to feed all of the cells in larger or heavier people– so they naturally have higher BMRs. If you were to lie in bed for 24 hours without any activity or intake, after fasting for at least 10 hours, your BMR is the number of calories your body would burn.

We can estimate our BMR with equations like the Harris-Benedict and Mifflin St Jeor equations, which take your height, weight, age, and gender into account. If you’ve ever tried to calculate your calorie needs online or via app, they probably use these equations.

The only way to actually measure your BMR, though, is through ‘calorimetry‘ (which literally means ‘calorie measurement’); direct calorimetry requires you to stand in a specialized chamber that measures how much heat your body is producing (impractical for most people), and indirect calorimetry can use respiratory tests to measure how much oxygen you inhale and carbon dioxide you exhale over a set period of time (somewhat more practical and actually available at some fitness and medical centers).

Of course we don’t just lie in bed all day. When we factor in our daily activities, we can find our Total Energy Expenditure (TEE), which means how many calories you burn on a typical day doing your usual activities. This is about 20% higher than your BMR if you are mostly sedentary to up to 90% higher if you are a professional athlete. Most people who are moderately active (1-3 days of intentional physical activity or exercise) burn about 38% more calories than their BMR.

This is where the error happens: It is very difficult to know how many calories you truly burn in a day. According to predictive equations, as a 5’4″, 27-year-old, mildly active, average weight female, my estimated BMR ranges from 1360 calories (Mifflin Jeor) to 1430 calories (Harris-Benedict), and my TEE should be about 1630 – 1720 calories daily.   I actually had indirect calorimetry done at a local gym last year, however, and my results showed that a better estimate for my BMR is 1123 calories and my TEE is close to 1350 calories.

That’s about 20% fewer calories fewer than traditional estimates- meaning that my body needs way fewer calories than the textbooks tell me.

Moving onto the second major issue: the 3500-calorie rule just doesn’t seem to be true.

This number came from a 1950s study by Max Wishnofky called “Calorie Equivalents of Gained or Lost Weight,” which posits that one pound of fat would require about 3500  calories to burn based on the scientific principles of fat. (If you’re interested in the gritty details: 1 pound (454 grams) of fat cells contains about 87% actual fat, and since it takes about 9 calories to burn one gram of fat, than a whole whole pound (454 grams) should burn up using about 3700 calories).  If you took a literal pound of fat and threw it into an incinerator to measure how much energy was required to literally burn it, that might be accurate. However, it doesn’t take our physiology into effect, and for better or for worse, our bodies are extremely adaptive at trying to preserve our energy stores.

Wishnofsky also examined a 1930s study by Strange, McCluggage, and Evans (“Further studies in the dietary correction of obesity”) which essentially starved for weight loss and found that they lost 0.6 pounds per day with a 2100 calorie deficit. How would one have such a severe deficit, you ask? They were put on a 360 calorie diet.

  • The diet: 360 calories made up of lean steak, fish, egg whites, whole milk, orange juice, yeast, minimal vegetables, and salt contributing ~58 grams of protein, 14 grams of carbohydrates, and 8 grams of fat each day. By today’s medical standards this would be a study of intentionally invoking severe malnutrition.
  • The participants: only 13 patients participated, and they were all in a hospital setting. Their weight ranged from 180 pounds to 427 pounds at the start of the study.
  • The outcomes: everyone obviously lost weight – the average was 35 pounds over 59 days. This was VERY inconsistent, though: actual weight loss ranged from 5 pounds over 8 days to 104 pounds over 176 days. 

This very small data pool based on a severe starvation diet showed that people lost about 0.6 pounds with a 2100 calorie deficit each day – making the weight loss ratio 1 pound to every 3500 calories. The 3500 calorie rule is based on these 13 people, severe starved for anywhere from a week to 25 weeks. 

I think this just exposes a truth we all know deep down inside: it’s just not that simple. Reviews of studies indicate that we lose weight more slowly than the rule would predict because our body burns fewer and fewer calories as we lose pounds. If you’ve ever watched a season of The Biggest Loser, you’ve seen this reality in action. Contestants used to lose over 20 pounds per week in the beginning when they were larger and had more excess weight to lose, but by the final weeks, they were following severe diets and working out 10 hours per day only to lose 2 – 3 pounds per week. Over time, their bodies required fewer calorie per day to run, so that calorie deficit rule shifted for them.

The good news is that researchers are now starting to battle this well-known rule to promote more realistic weight loss attempts. Dr. Diana Thomas and colleagues are leading this crusade with a math-driven approach, modeling actual weight loss journeys to create calorie / weight loss curves showing significantly less weight loss than the 3500 calorie rule predicts. Though many weight loss apps still use standard formulas to calculate how many calories you need per day for weight loss, a much more sustainable method would be to try to improve the overall quality of your diet, monitor what you currently eat and try to implement a slight deficit, increase your physical activity, and adjust things your plan as you go.

There is one slight upside to the 3500 rule, which is that it is memorable enough to communicate that caloric deficit is needed to burns weight. The downside is that it dramatically overestimates the rate of weight loss and can inspire dangerous levels of calorie cutting. I would never recommend eating less than 1200 calories; crash diets slow down your metabolism as your body adapts quickly to its new low-energy state, and nearly everyone rebounds back to a poor higher-calorie diet later on. Emphasizing overall nutrition quality, seeking a registered dietitian for ongoing nutrition counseling and support, and focusing on nutrition as a health goal rather than a means to an end can help you actually achieve your goals and maintain them long-term.

 

The overall takeaway:

  1. It is difficult to calculate your accurate BMR.
  2. It is even harder to then calculate your TEE
  3. Even if you can figure out how many calories you need daily, it is difficult to know how steep of a calorie deficit you would need to burn 1 pound of fat as the 3500 calorie rule is unreliable and based on absolutely extremist, insufficient research.
  4. Focus on improving diet quality, achieving a mild but sustainable caloric deficit, and seek assistance from a nutrition professional if you feel lost!

 

 

Resources:

Wishnofsky M.  Caloric equivalents of gained or lost weight. American Journal of Clinical Nutrition. 1958 Sep-Oct; 6(5):542-6.

Strang JM, McCluggage HB, Evans FA. Further studies in the dietary correction of obesity. American Journal of the Medical Sciences. 1930;179(5):687–693.

Thomas DM, Gonzalez MC, Pereira AZ, Redman LM, Heymsfield SB. Time to Correctly Predict the Amount of Weight Loss with Dieting.  Journal of the Academy of Nutrition and Dietetics. 2014;114(6):857-861.

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