In one sentence: as fast as your protein requirements let you.
First, there are stupid ways to do this. Some of those ways (juice fasting, relying on designer shakes, etc) are stupid at best and dangerous at worst. I am interested in the scientifically and anecdotally verified method of losing fat as fast as possible, doing so in as safe a manner as possible.
There are a couple of concepts one should know when it comes to this topic. First and foremost: We want to lose fat mass, not lean mass.
Lean mass is basically everything else that isn’t fat. If you weigh 80kg and have 20kg fat mass, then you have 60kg lean mass. That includes water, and thus becomes a very fickle measure due to changes in the body’s water balance. I won’t go into water balance now, but just to give the layperson a sense of scale, some athletes routinely drop 10kg of water for contest weigh-ins. Even an average 60kg person can lose 2kg of water on a low carbohydrate diet.
Do note that losing water isn’t too big a concern as long as negative dehydration effects are avoided. In this case, one can easily “put back the water” after dieting is done. What we are more interested in are our muscles. [1] With that goal in mind, we should then ask ourselves how do we prevent muscle loss; to elicit a protein sparing effect.
Protein Turnover
The body basically has a certain level of protein turnover determined by various factors. Labcoats would be familiar with net nitrogen balance. What this says, is that the body requires a certain amount of protein every day for regular function. More accurately, there is a demand for various nitrogenous compounds, which are derived from protein. The higher the demand for such compounds, the greater the amount of protein being “processed”.
Various factors affect our need for protein. Of most relevance to our topic today, would be an energy deficit. Essentially, the greater an energy deficit we are in, the higher our protein requirements.
So the scenario is that when one goes into an energy deficit, protein requirements go up. If protein intake from the diet does not increase, the body will find other sources of protein. It gets this from lean tissue; you’re using your muscles to meet your protein requirements. [2 - note to labcoats]
Why bother with sparing muscles? I Just Wanna Lose WEIGHT
First, the way such a diet is set up (to prevent muscle loss) is arguably the safer way to commence a rapid fat loss diet.
Second, question again why you want to lose fat. Be it for aesthetic reasons, health reasons or otherwise, retaining lean mass is usually the healthier and ultimately more fulfilling option. [3]
However, I will admit that sometimes, weight is all that matters. The best example would be athletes needing to make weight for a certain weight class. I will leave it to the reader to think of other possible scenarios.
So What Does One Need to Do?
Eat enough protein and lift heavy weights.
Again, the reason for the first point is to ensure that our body gets enough protein from our diet to meet requirements, therefore sparing muscle tissue.
How much is enough? For the average untrained individual, a value of 1g of protein per pound bodyweight every day is a good place to start (That’s 180g of protein for a 180lbs individual). For more experienced trainees, a value up to 2g/lbs Lean Body Mass is the value that is used.
The second point is a wholly complicated topic in it’s own right, but a simple explanation can get the point across.
Essentially, training a muscle “tells it that it is needed” and swings the balance in favour of keeping it in its current shape. Don’t train the muscle, and it is seen as “unessential”, and prone to being used to meet protein requirements. In fact, a training stimulus is probably our greatest ally in preventing muscle loss, and it does so through a whole bunch of processes which ultimately lead to increased protein synthesis within the muscle itself.
How much training is needed? For those who already do resistance training, strength retention in the 5-8 rep range is your best indicator. For a general routine, view this link. For those who don’t, its a good time to get started.
Back to the Original Question
Now that I’ve explained several concepts, we can finally answer the original question. Note that one’s protein requirements depend of various factors. One important factor is the amount of body fat a person is carrying.
That makes sense, since the more fat there is in the body, the easier it is to derive energy from that fat, and less protein is required. That also explains why the fatter one is, the faster one can lose fat. The reverse also holds true.
That said, using the principles above along with some other specific recommendations, it is possible for relatively lean individuals (15% body fat for males, 24% for females) to lose 1kg of fat per week.
Proceed with Caution
Rapid fat loss approaches aren’t easy, and if you have never paid attention to diet and training, it is unlikely that you can succeed without the relevant knowledge. While I cover a few basics here, they are far from enough. To get the full info, you will have to get ‘The Rapid Fatloss Handbook’ by Lyle McDonald. I can vouch for the effectiveness myself, having routinely dropped 1kg/week without muscle loss.
Even after that, actually sticking to the plan isn’t easy. Low calorie diets are stressful and draining. While it is possible to rapidly lose fat, it may not be (usually is not) the smartest thing to do.
Example Diet
Just to give an example of the macronutritional ratios of such a diet, let’s use a 80kg man with 65kg lean mass, and a 60kg woman with 45kg lean mass, both with training experience.
Male:
1300 Calories
65 x 2 x 2.2 = 286g protein
10g fat
20g carbohydrates
Female:
950 Calories
45 x 2 x 2.2 = 198g protein
10g fat
20g carbohydrates
Very high protein, basically nothing of everything else. You will have to see ‘The Rapid Fat Loss Handbook’ for the actual details.
Final Words
My recommendation to avoid rapid fat loss approaches still holds. Bear in mind though, that nothing has to be black and white, and the principles here can still be used to good effect; adhering to a less extreme version of the examples I give above can give some really good results.
More importantly, ask yourself why you want to lose fat quickly, and then ask yourself if you’re willing to do what it takes to do it safely.
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Notes:
[1] When I say muscles, I really mean everything that is essential to their function. This includes connective tissues and the like.
[2] Yes, I will admit, what I just said bears hardly any scientific merit. Bear with me in the simplistic explanation.
[3] Note: with some very obese people, there is “extra” lean tissue which is not muscle. A combination of water, connective tissue etc may be lost without harm (it may in fact be good).
Another scenario is probably with athletes who have non-functional muscle mass, and would like to reduce that. Eg: Cyclists arguably don’t need big arms and a huge chest. That said, such athletes wouldn’t go about doing that through diet alone, and neither would we be anywhere near the sillyness of some fad diets nowadays.
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