Muscle Retaining Factor #1: Severity of Calorie Deficit. By Tom Venuto ( a trusted friend )

One of the major factors mentioned in the research was the severity of the calorie deficit:
“Very aggressive calorie deficits and very low calorie diets tend to erode lean body mass to a great degree than more conservative deficits.”
I’ve spoken out against crash diets and starvation diets for years – my mantra has always been FEED THE MUSCLE, and BURN THE FAT, don’t starve the fat.
As you’ll see shortly with the subsequent muscle loss factors, you can reduce the chances of muscle loss while in a deficit with well-designed weight training programs and adequate protein intake. However, the risk for muscle loss is higher across the board with very low calorie diets, especially in already lean or semi-lean people.
The problem is that (1) you must have a calorie deficit to lose fat, but (2) too severe of a deficit increases risk of muscle loss. This leads us to the importance of choosing the correct calorie deficit.
The standard way to calculate a calorie reduction for fat loss is to use a 500-1000 calorie per day deficit (below maintenance) for losing one or two pounds per week, respectively.

We can improve our calorie deficit guidelines a lot by using a sliding percentage scale of conservative, moderate and aggressive deficits, which is influenced strongly based on your starting body fat percentage.
This gives you a safer and more personalized approach than an absolute deficit like 1000 calories. In relative terms, a 1000 calorie deficit could be near starvation, or it could be a perfectly reasonable reduction for an overweight person.
For example, if you’re a large, somewhat heavy and very active male with a 3400 calorie per day maintenance level, then a 1000 calorie deficit means a daily caloric intake of 2400 calories per day, a 30% deficit (aggressive, but well within reason).
If you’re a petite, lightly active female with a caloric maintenance level of 1900 calories per day, then a 1000 calorie deficit means a caloric intake of 900 calories per day, a 53% deficit (semi starvation, potentially unhealthy and catabolic). As Einstein would say, that's relativity for you.
The fix is simple: Instead of using generic deficits like minus 1000 calories, use a percentage. I’ve designed such a sliding scale. This new approach accounts for your goals, your desired rate of rate of weight loss and your starting body fat percentage:
15-20% below maintenance calories = conservative deficit
20-25% below maintenance calories = moderate deficit
25-30% below maintenance calories = aggressive deficit
31-40% below maintenance calories = very aggressive deficit (risky)
50%+ below maintenance calories = semi starvation/starvation (potentially counterproductive or even unhealthy)
Should You Be Aggressive or Conservative When Choosing a Calorie Deficit?
Usually, we’d suggest starting with a conservative deficit of around 15-20% below maintenance. Based on the research, however, we see that there can be a big difference between lean and overweight people in how many calories to cut.
If you have very high body fat to begin with, the typical rules of thumb for calorie deficits may underestimate the deficit you need to lose a pound. It may also be too conservative, and you can probably use a more aggressive deficit safely without as much worry about muscle loss or metabolic slowdown.
If you’re already lean, like an “off-season” physique athlete trying to get ready for competition, it’s different: You need to be very cautious with large calorie deficits. You'd be better off keeping the deficit small and starting your cutting phase earlier to allow for a slow, but safe rate of fat loss, with maximum retention of muscle tissue.
When deciding whether you should be more aggressive or more conservative with your deficit (15% vs 30% or even higher), I'd suggest four considerations:
1. Take into account your starting body fat. If you have high starting body fat, then you’re at less risk of losing lean tissue because you have a large storage depot of energy (body fat). If you have a low starting body fat (ie, bodybuilder or lean person trying to get even leaner), then you’re at higher risk of losing lean tissue with an aggressive calorie deficit, especially with a high level of training volume and intensity.

2. Take into account your actual body composition results. If you’re losing fat and maintaining all your lean body mass on an aggressive deficit (30% or possibly even slightly higher), and you’re not experiencing undue hunger, then why not stay with it? (You'll get maximum fat loss that way). If you're on a very conservative calorie deficit of 15% and the fat loss is painfully slow, then why not increase it a bit? (you'll speed up fat loss that way)
3. Use both sides of the energy balance equation. Could you leave your calories at a 15-20% deficit via reduction in food intake, but increase your activity to obtain the overall 30% deficit? Ie, does it fit your lifestyle and schedule to take a "higher energy flux" approach and simply get that extra calorie burn by exercising more rather than eating less? If so, and especially if your activity level is currently low, then consider raising your calories burned rather than cutting your calories consumed (that’s the Burn the Fat, Feed the Muscle approach).
4. Take into account your need to reach a deadline. When you think about deadlines, ask yourself whether you will accept higher risk (of muscle loss, etc.) and the discomfort of more potential hunger, in order to reach a body fat goal on a specific target date. (however, if you're rushed to make a deadline this time, make sure it doesn't happen again. Learn from this first experience, plan better next time, stay leaner and start sooner so you don't have to crash diet to meet your deadline).
What about medical weight loss and protein fasts?
Anyone who has ever been on a medically-supervised liquid weight loss diet or a self-administered protein sparing modified fast (PSMF) diet, may see a contradiction in our deficit guidelines and warnings about muscle loss. They might point out that their own medical doctor put them on 50% of their maintenance calories or even less (often only 800-900 calories per day - technically, a “muscle-burning” crash diet, right?) If it's bad for you to cut calories so much, then why would a doctor recommend it?
Well, keep in mind the context. In the cases of obesity, a risk to benefit analysis has to be done. The risks of staying morbidly overweight far exceed the potential downsides of an extremely low calorie diet, and the doctor knows that the weight will come off fast with a large calorie deficit. In some cases, it's saved as a last resort. In other cases, a doctor will make a judgment call on whether to start someone on a very low calorie liquid protein or PSMF diet right away. In all cases, the patient is under the doctor's supervision and ensures adequate nutritional intake using supplements.
What about the high risk of muscle loss? Well, again consider the patient, the urgency of the need to lose weight and the starting body fat. As the research indicates, the highly obese person is less likely to lose muscle with an aggressive deficit. Some of the weight lost is going to be lean tissue; almost no doubt about it, but it will be less than a lean person and even if LBM is lost, again the benefits outweigh the risks.
One more thing: In very low calorie diets for obesity, there is often only light exercise involved as some patients are not very mobile. One thing we’ve discovered is that very low calorie diets combined with very high volume training is potentially a much more catabolic and metabolism-damaging situation than low calorie diets alone or high volume training alone. In other words, you can get away with high training volume if you eat a lot of food to support the training (think of athletes in training camp, etc). You can also get away with (in some cases, ie, obesity) very low calorie diets, if the training volume is conservative. But you usually can’t get away with both.
EXTREME DIET PROGRAM WARNING: In the non-medical sector, you sometimes get pitched for very strict diet programs, often recommended even by fitness writers, trainers or bodybuilding and figure coaches you trust. These call for extreme calorie cutting, nothing but protein shakes (or mostly protein shakes, nothing but fish and meat, or other such madness). Unlike medical weight loss, this is generally a really bad idea, because the athlete, bodybuilder or figure competitor is already lean.
Combined with a high cardio and weight training volume, this approach can spell total disaster for your lean body mass. Quick and extreme fat loss claims sell, but as the research shows, the lean person is far more likely to lose muscle with an aggressive calorie deficit. Even though high protein intake and weight training spares muscle, lean people (category A and especially category B), need to be much more conservative with calorie cutting.
A lean person wanting to get even leaner should start far enough out from their goal target date so they can lose at a rate of 1.0 to 1.5 lbs per week to maximize retention of lean muscle. In this scenario, the risks of large calorie deficits and fast weight losses outweigh any benefits. At the very least, this approach needs to be called out for what it really is: Crash dieting and impatience.