by Monica Mollica Brinkzone
A pervasive dieting mantra is that a cumulative reduction of caloric intake of 3,500 will result in a weight loss of 1 pound. This dieting rule popularly states “because 3,500 calories equals about 1 pound (0.45 kilogram) of fat, if you cut 500 calories from your diet each day, or burn 500 calories extra per day from exercising (or a combination thereof) you’d lose about 1 pound a week (500 calories x 7 days = 3,500 calories) and 52 pounds in a year.
This simple weight loss rule continues to be cited on weight loss websites as well as authoritative nutrition textbooks [1, 2], scientific articles [3-6], and expert guidelines. It is even common among health care professionals to believe in the 3,500 calorie rule , and the Patient Page on “Healthy weight loss” at The Journal of the American Medical Association website states in the first sentence on “What you need to know about weight loss” that… “A total of 3,500 calories equals 1 pound of body weight. This means if you decrease (or increase) your intake by 500 calories daily, you will lose (or gain) 1 pound per week (500 calories per day × 7 days = 3,500 calories).” 
However, is this really true? Most people who have ever been on a diet are probably skeptical. And rightly so. In this article I will highlight what’s wrong with the 3,500 calorie rule and present a more accurate calculator that you can use to more precisely predict how much weight loss you can except in real life from a given daily calorie reduction.
What’s wrong with the “3,500 calorie per pound weight loss” rule?
The 3,500 calorie rule is based on a calculation – first published 1952 – that assumes exclusive loss of adipose tissue consisting of 87% fat.[10, 11]
However, in real life, during weight loss lean body mass is lost together with body fat [12-14] and the composition of weight loss (i.e. the relative proportion of lean and fat) is influenced by several factors, such as exercise [15-17], protein intake [15, 18-20], as well as testosterone [21-23] and thyroid hormone status.[24, 25] And making it even more inaccurate, the original “3,500 calorie per pound weight loss” rule is popularly misrepresented by replacing “weight” with “fat” loss.
The main scientific criticism of 3,500 calorie rule is that in real life, there is self-limiting nature of weight loss imposed by a constant caloric deficit. As you lose weight, your body will progressively burn less calories as your weight drops (more on that below), so that over time – even if you adhere to your caloric restriction – the imposed calorie deficit will have a smaller impact and your rate of weight loss will therefore slow down (unless you restrict your calories more or exercise more).[14, 27-33] To clarify, if you lower intake by 500 kcal on diet day one, this closely approximates an energy deficit of -500 kcal, but the magnitude of this deficit and associated weight loss will decrease markedly over time.
Why? Because when you are in a state of negative energy balance – i.e. when you eat less calories than you burn, or burn more calories than you eat – you will lose not only fat but also calorie consuming lean tissue (primarily muscle mass) [34, 35], metabolic adaptations [31, 36-44], and a reduced calorie cost of moving a lighter body.[27, 32, 39, 45] Because metabolism (i.e. caloric expenditure) declines with loss of body mass [46, 47], your calorie requirements will be reduced after weight loss (unless you exercise more). The reductions can range from 170 to 250 kcal/d for a 10% weight loss and from 325 to 480 for a 20% weight loss.[48, 49]
Thus, although sometimes viewed in isolation, energy intake and expenditure in fact exist in a dynamic interplay: calorie intake influences calorie expenditure (and vice versa).
How to improve predictions of expected weight loss with a diet?
The self-limiting nature of weight loss is best captured by so called thermodynamically-based dynamic models [32, 33] and not by the 3,500-kcal rule. Dynamic weight loss models take into consideration baseline body composition, age, height, gender, and degree of caloric restriction, and result in a curvilinear pattern of weight loss over time rather than the linear pattern predicted by the 3500-kcal rule.
Although dynamic weight loss calculations describing the curvilinear weight loss pattern are far more complex than the simple 3,500 rule, there is a great dynamic energy balance calculator – available for free on the Pennington Biomedical Research Center (PBRC) website – that does the math crunching for you:
Note that this calculator uses the SI unit kcal (kilocalorie), which is the correct way of expressing energy. In daily parlance and lay media, the term “calorie” is used as a synonym for kcal.
Table 1 illustrates how grossly the 3,500 calorie rule predicts expected weight loss compared to the new dynamic weight loss simulator that takes into account the self-limiting nature of a given caloric deficit.
Table 1: Weight loss prediction using the 3,500 calorie rule vs. the dynamic weight loss simulator.
When running the dynamic weight loss simulator you will see that the 3,500 calorie rule only is valid (at best) for the first month after having started a diet. However, it should be remembered that the “early” rapid weight loss phase may represent a period during which the energy content of weight change is low (because of loss of primarily glycogen and water mass) and thus does not follow the classic “3,500 calorie rule”. This further invalidates it.
Does a dieting 500 calorie restriction per day apply to everybody?
Let’s put aside the 3,500 calorie rule for a moment and ask the question:
“Is a 500 calorie deficit per day a good guideline for everybody who want to lose weight”?
No! Because a man in general is heavier and has more lean mass than a women, and thus has a greater total caloric expenditure for any given physical activity level, 500 calories of his total will be a lower percentage caloric deficit and 500 calories of a woman’s total, as illustrated in table 2.
Table 2: Fixed (e.g. 500 calories) vs. relative (e.g. 20%) caloric reduction during a diet.
Notes about table 2:
There are several equations for calculating resting metabolic rate. While the Harris-Benedict equation is most widely used, several studies show that the Mifflin-St Jeor equation provides a more accurate estimate of actual resting metabolic rate in the largest percentage of non-obese and obese individuals.[52-55] Here is a good free Mifflin-St Jeor calculator (under the advanced options):
PAL of 1.6 is typical of people with seated work and requirement to move around but little or no strenuous leisure exercise.[56, 57]
While the differences in this example – 18% vs. 23% and 550 calories vs. 442 calories – are not that large, when comparing with a heavier man with a woman, there will be more marked differences in the fixed vs. relative caloric deficit.
Therefore, instead of imposing a fixed calorie restriction for everybody, it is better to use a percentage of each individual person’s baseline caloric intake (which usually reflects that person’s caloric expenditure if he/she is in body weight/composition steady-state, i.e. is not fluctuating in body weight).
A good rule of thumb for dieting is a 20% calorie deficit. This can be achieved by reducing calories, increasing exercise, or a combination thereof. There is a lot to say about diet vs. exercise induced weight loss, so I will save that for another article. Also, diet composition tremendously impacts the relative loss of body fat vs. lean mass during a diet, as well as improvements in cardio-metabolic risk factors, which is also a topic for another discussion.
Don’t get fooled by the “3,500 calorie per pound weight loss” rule. When you lose weight on a diet, your energy expenditure gradually declines with the loss of body mass and metabolic adaptations, which means that over time the calorie deficit gets smaller (even if you adhere to your diet and keep your caloric deficit constant). Therefore, unlike the linear weight loss pattern depicted by the 3,500 calorie rule, the actual weight loss follows a smooth curve and then plateaus at the new energy requirement level.
Use the freely available dynamic weight loss calculator to get a realistic weight loss expectation based on your individual caloric reduction and/or increased exercise calorie burning.
Talking about calories, don’t miss Will’s great video addressing the common question:
“Do You NEED To Count Calories?”
– See more at: http://www.brinkzone.com/weight-loss/is-the-dieting-rule-3500-calories-per-pound-weight-loss-correct/#sthash.3CfU2wLg.dpuf