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I was wondering how could one calculate the proportion of fat in daily calorie intake to achieve particular Body Fat Percentage and BMI goals.

Let's take my own example, here are some details:

  • Age / Gen : 19 / M
  • Body Type : Skinny Fat
  • Height : 5'9"
  • Weight: 130lbs
  • BMI: 19.8
  • Body Fat %: 18%
  • Weight Goal: Gain 13lbs
  • Body Fat Goal : 10%
  • I'm not sure what you mean by calculate the proportion of fat in daily calorie intake. Can you clarify? – DeeV Jun 18 at 13:27
  • @DeeV The daily calorie intake comprises of calories from protein, fat and carbs. Of this how much % should fat be. – Aether Jun 18 at 14:19
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Considerable debate surrounds what proportion of the energetic macronutrients might represent the ‘ideal’, but let us assume that you are observing a high-protein diet, providing 2.0 grams of protein per kilogram of body mass per day, and let us assume, also, that you would like to maintain a slight energy deficit in order to metabolise fat quickly whilst facilitating hypertrophy.

First, you must approximate your Total Daily Energy Expenditure (TDEE), which is calculated by estimating your Basal Metabolic Rate (BMR), then multiplying that figure by a factor known as the Physical Activity Level (PAL). Various formulae have been proposed, perhaps the most well-known of which is the Harris-Benedict equation, which has undergone a number of revisions since its original publication in 1918. The 1990 revision by Mifflin and St. Jeor is as follows:

 Men:    BMR = 10 × mass (kg) + 6.25 × height (cm) - 5 × age (years) + 5
 Women:  BMR = 10 × mass (kg) + 6.25 × height (cm) - 5 × age (years) - 161

These equations do not account for differences in body composition, however, and are therefore best used only when body fat is not known. When body fat is known, it is preferable to use the Katch-McArdle equation, which accounts for the fact that BMR is primarily a function of fat-free mass, and is largely independent of height, age, and sex once fat mass is discounted. The equation is as follows:

 BMR = 370 + 21.6 × mass (kg) × ( 1 - fat% ÷ 100 )

Thus, your BMR would be estimated by the Katch-McArdle formula to be 1,414 Cal (5,918 kJ). And this must then be multiplied by a factor which represents your physical activity level, as follows:

 Activity level        Description                                             PAL
 
 Sedentary             Clerical worker/person doing little or no exercise      1.40-1.69
 Moderately active     Construction worker/person running 1 hour per day       1.70-1.99
 Vigorously active     Agricultural worker/person swimming 2 hours per day     2.00-2.40
 Extremely active      Competitive cyclist/multi-sport athlete                 > 2.40

Let us assume that you are most aptly described as ‘moderately active’, and that your PAL is 1.7. TDEE is thereby calculated as follows:

 TDEE = BMR × PAL

Thus, your Total Daily Energy Expenditure is approximately 2,400 Cal (~10,000 kJ), and applying a deficit of 20%, for example, your Daily Energy Intake (DEI) should be 1,920 Cal (8,000 kJ). The relative proportions of the energetic macronutrients—protein, carbohydrate, and fat—that should be contained in your diet can hereby be calculated from this figure. Their energy content is summarised as follows:

 Nutrient              Energy density (Cal/g)

 Fat                   9
 Protein               4
 Carbohydrate          4

At a body mass of 130 lbs (59 kilograms), your protein intake should be around 118 grams per day, which therefore equates to around 25% of your DEI. That is:

 118 (g) × 4 (Cal/g)  ÷ 1,920 (Cal) = 0.2458 ≃ 25%

It is generally recommended that fat not exceed 30% of DEI. So you can calculate the final quantities based upon proportions of protein, carbohydrate, and fat as 25 : 45 : 30, respectively.

 Nutrient         Proportion (%TDEE)     Energy (Cal)           Amount (g/day)

 Protein          25                     0.25 × 1,920 = 472     118
 Carbohydrate     45                     0.45 × 1,920 = 864     864 ÷ 4 (Cal/g) = 216
 Fat              30                     0.30 × 1,920 = 576     576 ÷ 9 (Cal/g) = 64

So based upon these proportions, and the assumptions that I have made at the beginning of this post, you would design your diet around a daily intake of 118 grams of protein, 216 grams of carbohydrate, and 64 grams of fat. Of course, you could tweak these variables according to your needs and priorities. It has been found, for example, that greater protein intake tends to result in more rapid fat loss, given the same caloric deficit, and the evidence suggests that for healthy individuals, higher protein intake (of up to 2.4 grams per kilogram of body mass per day) is safe. It should be noted, however, that lowering carbohydrate intake may limit both exercise performance and hypertrophy.

I hope that helps.

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