Originally Posted by Lyle
So imagine my surprise when this little theoretical paper (note the journal title) showed up on my Pubcrawler last year (1). Titled "A limit on the energy transfer rate from the human fat store in hypophagia", it examined (from a somewhat simplified and theoretical way) exactly the question I gave above: what is the maximum rate at which the body can derive energy from fat stores to cover a diet induced deficit while sparing lean body mass.
It's a nasty little paper, filled primarily with equations, explanations of those equations and some more equations to boot. Headache inducing to be sure. I'll spare you the details. Based on a somewhat simplified analysis of what data exists (including the seminal Minnesota semi-starvation experiment), they conclude that the maximal rate at which fat stores can provide energy to the body is 290 +- 25 kj/kg which is approximately 31 kcal/lb of fat per day.
So, if you are carrying a mere 10 lbs. of fat, you can sustain a 310 cal/day deficit.
20 lbs. = 620 calories.
30 lbs. = 930 calories
You get the idea and this is not difficult math. Multiply your total fat mass in pounds by 31, that's how much of a caloric deficit that fat mass can support on a daily basis.
One quick note: the above values are for dieting only and one of the simplifying assumptions in the paper was relatively 'normal/moderate' activity levels. The paper mentions specifically that the value above might be varied through pharmaceutical means (which target the rate limiting steps of fat energy transfer) or through high levels of activity. It even mentions bodybuilders specifically as a group that might exceed this value with a lot of training. For now, I'll just focus on the diet end, I'll come back to drugs and exercise afterwards.
So, the basic assertion of the paper is that, so long as the net daily deficit does not exceed what your fat stores can provide, you should spare lean body mass. And based on the small amount of research that they found, this seemed to be generally true (many studies find an initial rapid LBM loss but this is most likely glycogen and water and stuff, not muscle mass). By extension, if your daily caloric deficit exceeds the above, your body will have to mobilize LBM to cover the difference. So let's look at an example.
Say we have a 180 lb male at 15% bodyfat. He has 27 lbs. of fat, and his maintenance calorie intake is 15 cal/lb or 2700 calories. With 27 lbs. of fat, he should be able to sustain a caloric deficit, from diet alone, of 27 lbs. fat * 31 cal/lb = 837 calories/day. So he could reduce his calories to 1863 (ha! 10 cal/lb) and shouldn't lose any LBM at that level of intake. He should get a weekly fat loss of just over 1.5 lbs./week.
If the same 180 lb guy was at 10% bodyfat, only 18 lbs. of fat, he could only sustain a 558 calorie/day deficit (2150 cal/day or 12 cal/lb), he's down to 1 pound per week. By the time he's at 8%, he's down to 14.5 lbs. of fat and a total deficit of 446 calories/day and about 2/3 a pound of fat loss/week. Oh yeah, if he were a fat I need to read the rules post.I need to read the rules post.I need to read the rules post.I need to read the rules post. at 30% bodyfat, that's 54 lbs. of fat, he could sustain a deficit of over 1500 cal/day and lose over 3 pounds per week of pure lard; of course he'd only be eating 1300 cal/day. Again, the above all seem to roughly pass the reality check in terms of what we see in human dieters.
Now, one implication of the above is that, as a diet proceeds and your fat stores shrink, your net deficit has to decrease. Ok, step back, take a breath and read that again. More importantly, note my use of the word 'net' in the first sentence of that paragraph.
Now it's going to get confusing.
At first glance, the above seems to be saying that, as you get leaner, you'll need to raise calories to compensate, so that the deficit isn't as extreme. But that's incorrect; it is saying that fat loss will need to slow (b/c the net deficit you can sustain will be smaller). By 'net' deficit, I mean the difference between your current maintenance requirements and your intake. This is important because, as you diet, your maintenance requirements go down due to the loss of bodymass along with the adaptive component of metabolic rate (due to insulin, leptin, ghrelin, peptide YY, etc). Let's simplify this by looking at the math.
Our 180 lb man at 15% starts his diet. He has 27 lbs. of fat and can sustain a maximum deficit of 27 lbs. * 31 cal/lb = 837 calories. Assuming a maintenance of 15 cal/lb (2700), his starting calorie level will be 2700 cal - 837 calories = 1863 calories/day. He'll be losing around 1.5 lb fat/week.
So now we check in 8 weeks later, he's down 12 lbs., almost purely of fat (we'll ignore any small LBM losses). His new numbers are
168 lbs. with 15 lbs. of fat = 9% bodyfat. Maximal sustainable deficit = 15 * 31 = 465 cal
Assuming his maintenance is still 15 cal/lb (not automatically a safe assumption), his maintenance requirements should now be 2520 calories. But the adaptive part of metabolic rate reduction has probably dropped him a good 10% below that. So let's say his maintenance is 2250 cal/day or so. 2250 cal/day - 465 calories = 1785 calories. So, not much of a reduction from his previous 1863 calorie/day diet. Basically, the drop in his maintenance levels over the course of 8 weeks offsets the fact that he can't sustain as much of a deficit and is now leaner. Of course, his fat loss has also slowed to just under a pound/week.
Now 4 weeks later, he's dropped about 4 more pounds of fat. His new numbers are
164 lbs. with 11 lb of fat = 6.7% bodyfat. Maximal sustainable deficit = 11 * 31 = 341 cal
His maintenance will have dropped further, let's say 14 cal/lb (people's daily activity tends to go down due to the hormonal changes from extreme dieting) and a 15% adaptive reduction which brings him to 1951 calories/day. Reduce by 341 to get 1610 calories/day. He will need to reduce daily calories by a couple of hundred (from 1785/day to 1610/day) to achieve the maximum deficit but his fat loss will be down to 2/3rds pound per week.
Ok, step back for a second: the above calculations aren't meant to be the holy word of god, there are a lot of estimates upon estimates being made, especially my guesses as to the changes in maintenance level and how big of an impact the adaptive component is having. The adaptive component is a big question mark with not enough data for my liking. Tracking morning body temperature gives a rough guideline: for every 1 degree drop in morning temperature below 97.8 degrees, your metabolism is suppressed by about 10%. This was more to illustrate what I meant by 'net deficit' with changes in both fat mass and maintenance requirements.
But, again, the above seems to scale roughly with reality. As people get leaner and leaner, fat loss slows drastically. To keep it moving, they have to either cut calories further or increase activity, both can cause muscle loss. Every bodybuilder who has had to move to 2 hours/day of aerobics to keep the fat coming off knows what I'm talking about. Drugs become more and more attractive as the myriad other systems start to fight back against you as well (on which note: will someone please get the folks working on intranasal leptin to hurry it up).
Drugs and exercise
As mentioned above, the paper I'm basing all of this nonsense on was looking at non-exercising dieting or fasting men, not folks who were training or taking drugs. And it mentioned specifically that both of those could potentially increase the maximum rate of fat mobilization value (above 31 cal/lb) without sacrificing lean body mass. Certainly, once again, this idea passes the reality check. Even the addition of the ephedrine/caffeine stack elevates fat loss while sparing muscle mass. Clenbuterol is more potent, GH is great and DNP is like I need to read the rules post.I need to read the rules post.I need to read the rules post.I need to read the rules post.ing magic.
Clearly exercise also has an impact. Even back in Bodyopus, Dan mentioned that only part of the total deficit (he used 20%) should come from diet, the other part should come from increased activity. This usually means aerobics but some prefer to use high rep/short rest period weight training or interval training and there is some logic to picking one of the latter options. He suggested that men do better with 15% calorie deficit and 5% aerobics and women at 10% apiece. Given the issues women have with lower bodyfat blood flow, and that aerobic activity can overcome some of the limitations that make lower bodyfat so stubborn, that makes a lot of sense. As well, women, because of their lower maintenance requirements, end up eating a tiny amount of food if they cut too much out of their diet. Increasing their net daily deficit via activity allows them to eat more and not starve to death on a daily basis. I've been working on the stubborn bodyfat issue for years now, my next book outlines not only the problem but multiple solutions to that problem.
Conclusion
In this article, I've been able to give dieters a starting point for the maximum sustainable deficit which can come from calorie restriction. To summarize: simply determine how many pounds of fat you're carrying. Then multiply that value by 31 calories. That's how much you can potentially decrease your daily food intake. If you want to try to increase fat loss, any further increase in the deficit should either come from increased activity or compounds that either increase the mobilization or burning of fatty acids for fuel. As well, as you get leaner/lighter, you will need to periodically recalculate your daily calories to take into account your diminishing fat mass and decreased maintenance requirements due to both decreased bodymass and the adaptive component of metabolic rate. An argument can also be made for saving increases in activity for later in the diet when your diet deficit has to be lower.
Please keep in mind, however, all of these theoretical calculations sort of pale to real world results. If you're losing strength in the weight room like crazy, your deficit is too big regardless of what the math works out too, increase them until you stop hemorrhaging strength (and probably muscle). And even if you have to trial and error it a bit, the above should at least give you a starting point.
Reference
1. 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.
