Your weight loss can come from fat loss, muscle loss or water loss and this is mainly determined by the amount of calorie deficit you create, the kind of exercise you do and what you feed your body.
Being in calorie deficit means consuming less than the total energy requirement of your body. If my body’s calorie requirement is say 2600Kcal and I eat only 2400Kcal, then I have created a deficit of 200Kcal. To find out your body’s calorie requirement you can read this article on Calorie Math.
We know that of all the types of weight loss, only fat loss is desirable and long term (Ref: Different types of weight loss and what you should aim for). Here we will explore how much calorie deficit would lead to maximum fat loss with minimum muscle mass loss:
A study in 2005 provides an answer to how much your body can compensate for lost calories from the fat stored in the body (Ref1). It states that our body can utilise about 290+/-25 KJ per kg of body fat-weight per day from its fat reserves. This translates to a calorie deficit of 69Kcal per kg of body’s fat weight (or 31Kcal per pound of body’s fat weight).
So, if you are say 90kgs with body fat of 25%, then you can get by in creating a daily calorie deficit of 1552Kcal [i.e. 69 x (90 x 0.25)], which would be compensated for by your fat stores. This would give you a fat loss of about 1.4kgs (or 3lbs of fat loss; *See End Note) per week. If you create an even more calorie deficit then a majority of that energy will come from break down of your muscles.
The more body fat you have, the faster you can lose it!:
With 25% body fat, our friend above could lose 1.4kgs of fat in a week. Compare that to the body fat loss that someone with lower body fat will experience:
Consider again someone who is 90kg but with only 10% body fat, then the calorie deficit that his body will allow him before it starts losing muscles is 620Kcal [i.e. 69 x (90 x 0.1)] per day. This would give a fat loss of 0.56kgs per week (or about 1.2 pounds of fat loss; See End Note). So, if you have low body fat, you will only be able to get away with a small amount of body fat loss per week.
It is for this reason that pro body builders before a show keep their fat loss goals to be no more than a pound a week. If they create a higher calorie deficit than their 1 pound of fat loss per week, they would enter a scenario where they would be sacrificing a lot of their hard earned muscles, which of course is unacceptable to them.
All these theoretical results were also confirmed by other empirical studies that noted the relationship between fat loss and fat gain basis initial body fat levels: ‘Those with initially higher body fat when underfed will lose a higher proportion of fat mass compared to those with lower body fat’. (Ref2,3)
Its not all so simple though!
Weight loss is never linear, i.e. you cannot expect this rate of fat loss from week1 to say week10. When the body is deficit in calories it undergoes complex physiological changes, the exact mechanism of which is still a mystery to researchers. Our bodies do not work with hard-coded numbers. The ratio of weight loss from fat, from muscles or from water (i.e. glycogen stores) varies from one person to another. This is from variation in people’s metabolic rates, insulin sensitivity, etc which are a result of their genes, hormonal health and digestive capacity.
Also, it is true that you will lose both- muscle and water (from loss of glycogen), when you create a calorie deficit. The point of this article is to find out what is your approximate theoretical calorie deficit limit beyond which your body relies more heavily on your muscles to get the energy to sustain its activities. (*See End Note)
These theoretical calculations although not exact, give you a good starting point for your fat loss efforts. Start your diet on the basis of these calculations and then make note of your weekly progress, and slowly adjust your diet based on how your body responds to it.
Continue Reading: What exercises you need to do to maximise fat loss and reduce muscle loss? (Coming soon!)
What to eat in your diet to lose fat while maintaining muscle mass? (Coming soon!)
PS: When targeting fat loss, it is a good idea to invest in a good body fat measuring machine or go by the inch loss from your arms, waist and thighs as an alternate. By weighing only your weight, you would be in the dark about what kind of weight loss your body experiences.
*End Note: A pound of fat has 3500Kcal (Ref4). However, when it comes to weight loss our body does not follow an ideal scenario and lose ONLY fat. The weight lost would comprise of fat, muscle as well as water. A mass of fat contains fluid and proteins in addition to its triglyceride content. To say that a 3500Kcal deficit provides a pound of fat loss is an oversimplification but this is as close as it comes to determine theoretical fat loss without getting into body chemistry analysis.
Ref1: 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 Epub 2004 Dec 8.
Ref2: Hall KD. Body fat and fat free mass inter-relationships: Forbes’s theory revised. The British Journal of Nutrition 2007, 97:1059-1063
Ref3: Forbes GB. Body fat content influences the body composition response to nutrition and exercise. Ann N Y Acad Sci. 2000:904, 359-65
Ref4: McArdle WD. Exercise Physiology: Energy, nutrition and human performance. 4th edition. Williams and Wilkins; Baltimore: 1996
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