In books and articles on fat loss it is common to see arbitrary recommendations for daily calorie intake or deficit, such as 1,200 calories per day for women and 1,500 calories per day for men, or a deficit of 500 to 1000 calories per day to lose 1 to 2 pounds of fat per week. The problem with arbitrary calorie intakes is obvious – not everybody has the same daily calorie expenditure so the resulting deficit will vary significantly between people. Apparently the problem with arbitrary deficits is not so obvious – many personal trainers and health professionals routinely recommend a daily calorie deficit of 500 to 1000 calories for everybody – a range that is too low for some and too high for others.
A few months back I read about a paper from the March 2005 Journal of Theoretical Biology in an article by Lyle Mcdonald. The paper by Alpert et al, which examined data from various sources including the Minnesota Starvation Experiment, concluded the rate at which the body can get energy from it’s fat stores is about 31.4 calories per pound per day.
“A limit on the maximum energy transfer rate from the human fat store in hypophagia is deduced from experimental data of underfed subjects maintaining moderate activity levels and is found to have a value of (290 ± 25) kJ/kg d. A dietary restriction which exceeds the limited capability of the fat store to compensate for the energy deficiency results in an immediate decrease in the fat free mass (FFM). In cases of a less severe dietary deficiency, the FFM will not be depleted.”
290 kilojoules = 69.31 kilocalories and 1 kilogram = 2.2 pounds, so 290 kJ/kg = 31.4 kcals/lb
In other words, the authors claim the maximal daily calorie deficit for fat loss is approximately 31.4 cals per pound of fat, give or take about three calories, and if your daily calorie deficit exceeds this the difference is going to come from other tissues, including your hard-earned muscle. Keep in mind the specific foods and macronutrient ratios consumed by the subjects were far from optimal for fat loss, and that “moderate activity levels” is not the same as regular, high intensity strength training. The maximum rate is most likely higher for someone eating adequate protein and not overdoing carbohydrate intake and strength training would also contributes to maintenance of lean body mass when calorie intake is below maintenance level.
Since the maximal deficit would change in proportion to your fat stores, as fat is lost the calorie deficit would need to be decreased. However, even if you were losing over three pounds of fat per week this would only reduce the maximal deficit by about 90 calories each week, so it would be unnecessary to re-adjust daily as long as the deficit accounted for the reduction in body fat.
For example, a 200 pound man at 15% body fat would have 30 pounds of fat, enough to provide about 940 calories of energy over the course of a day. Assuming he reduced his calories intake to 940 below maintenance for a day, by the end of the day he would have lost about a quarter pound of fat, which would require the deficit to be reduced by about eight calories the next day.
Nobody can estimate their body composition or daily calorie expenditure, or measure and record their food intake accurately enough for eight calories to make a difference. Additionally, metabolic rate may decrease slightly over time while on a below-maintenance calorie intake due to reduced thermal effect of food and hormonal changes. Rounding down the daily calorie deficit to 30 calories per pound of fat and re-adjusting the deficit every other week based on changes in weight should be more than adequate to maintain a near-maximal rate of fat loss with little or no loss of lean tissue. Body composition should be re-measured monthly to ensure only fat is being lost and calorie intake adjusted accordingly. Very lean individuals may want to re-measure body composition more frequently.
Calculating Daily Calorie Deficit For Maximum Fat Loss
If the above formula is used to determine the daily calorie deficit, assuming body fat measurements and daily calorie expenditure estimates are reasonably accurate, the following formula can be used to estimate the number of days required to lose a certain amount of fat. This would not include refeed or “cheat” days.
FS = starting body fat in pounds
FE = ending body fat in pounds
[(FS - FE) x 3,500] / [(FS + FE) x 15] = Approximate days of calorie restriction required to reach FE when daily calorie deficit equals current bodyfat level x 30.
The above is far from perfect, but provides a rough estimate that can be used for planning a diet or establishing time frames for short and long term fat loss goals. Also consider this is based on a theoretical maximum rate of fat loss. Substituting 13 for 15 in the formula may provide a more realistic time frame for most people.
As I mentioned in Basic Guidelines for Fat Loss, severely obese individuals may have enough body fat to provide more energy than they expend per day. Regardless of the amount of energy obtainable from the fat stores, daily calorie intake must be high enough to at least allow for adequate protein and fat intake and for as much carbohydrate as the individual requires to function adequately – some people handle lower carb intakes better than others. Daily calorie intakes for fat loss for those very over-fat or obese should be calculated based on macronutrient requirements rather than amount of body fat.
While steady-state activities are generally highly overrated for fat loss for the majority of people, the obese are an exception. For most people who are only moderately over-fat or leaner, an increase in activity is not necessary to achieve the calorie deficit required for maximum possible fat loss – a reduced calorie intake can accomplish this while still providing adequate nutrition. If someone has enough fat to provide more energy than they expend per day, however, their daily calorie deficit will fall far short of their potential maximum for fat loss unless activity level is increased significantly. In addition to a program of high intensity strength training, one or two hours per day of walking or a shorter period of a low-force, low-impact activity performed at moderate intensity may increase the rate of fat loss significantly for those who are very over-fat or obese.
Alpert SS. A limit on the energy transfer rate from the human fat store in hypophagia. J Theor Biol. 2005 Mar 7;233(1):1-13.