Your body uses several sources of fuel for energy, and it uses them in a certain order of preference. So, the fuel used to sprint for 30 seconds is not the same fuel used in the second hour of a marathon.
Believe it or not, your body is quite stingy about burning stored bodyfat for fuel. It would rather break down your muscle and lean tissue first. This happens when:
- people drastically reduce their calories, as in most crash diets
- people diet stringently for too long
- people try to “burn calories” through excessive endurance-based training (like long-distance running or a lot of “cardio”)
Metabolic activity and lean body mass
In all these cases, the body first turns to muscle breakdown to make up the difference. People also lose other lean body mass such as bone mass. The body simply eats through its muscle tissue to survive.
Why is this a bad thing? Well, besides the fact that you feel weak and crummy, muscle is a very metabolically active tissue. You need to actually expend energy to keep your muscle healthy. Think of the difference between muscle and fat as the difference between orchids and cacti. An orchid has to be fussed over and carefully maintained, while you can pretty much leave cacti to fend for itself.
So, if you don’t provide your muscles with enough caloric nourishment, and if you don’t encourage them to grow with resistance exercise (aka lifting heavy stuff), then they’ll be like an orchid in Death Valley. But that fat will stay there with only minimal encouragement.
When it comes to maintaining a healthy metabolism, muscle is thus fundamental to keeping the body chugging along. If muscle is reduced, so is metabolic rate. The more muscle you have, the faster your metabolic rate. This is why bodybuilders’ daily food intake would make an elephant jealous: they need lots of food to feed their metabolically active muscle.
More than just “burning calories”
But energy expenditure (aka “calorie burning”) isn’t the whole picture. Let’s look more at “muscle” and “lean body mass” for a moment.
People often use the terms “muscle” and “lean body mass” interchangeably (an error of which I am often guilty as well). LBM is everything that is not fat — bones, water, organs, etc. LBM is highly metabolically active… but “muscle” is only part of that.
People often confuse “burning calories” with “metabolically active”. Metabolic activity also includes the synthesis and expression of things like hormones, cytokines, and other proteins. (Bone, by the way, is also quite a busy bee.)
From an overall “metabolic” standpoint of coordinating and expressing chemical activity, fat is quite active (some folks have even suggested that bodyfat should be considered a distinct part of the endocrine system… maybe even a separate system, rather like the “second brain” of the GI tract is considered distinct from the central nervous system of the brain). There is almost no tissue in the body that is truly metabolically inert. So, fat and muscle are both “metabolically active”… but in the sense of being active participants in shaping the body’s biochemical environment.
Thus, muscle contributes to resting energy expenditure (REE) — some folks figure around 15 calories per pound of muscle — but that’s not the only reason why the muscle/fat or LBM/fat ratio is important. It’s also important because of what LBM is doing, chemically.
The hormones, cytokines and other proteins involved in fat metabolism have different effects than the hormones, cytokines, and other proteins involved in muscle metabolism. I think of it more like air traffic controllers deciding who gets to land.
Here is an older but often-cited article that explains the concept of skeletal muscle metabolism.
Here is a more recent piece that shows that lean and obese people have different responses to the same diets. Lean people’s bodies are better able to handle and process nutrients, and put them towards good purposes. Another study notes:
Severe obesity is associated with insulin resistance in skeletal muscle due to impairments in multiple steps of the insulin signaling pathway. Increased intramuscular lipid content and a reduction in FAO, both conditions linked with insulin resistance, are also evident in the skeletal muscle of severely obese individuals. The deficit in FAO in skeletal muscle with severe obesity may also help explain a propensity towards the development of obesity/severe obesity and weight gain in these individuals as lipid is partitioned towards storage rather than oxidation. (Houmard, Joseph A., et al. “Is There a Metabolic Program in the Skeletal Muscle of Obese Individuals?” Journal of Obesity 2011 doi:10.1155/2011/250496)
Translation: less muscle and more fat equals problems with using and storing nutrients, because of the metabolic actions of these types of tissues. This also holds true for the “skinny-fat”: the folks who are technically a smaller size or lighter weight, but who still have relatively more body fat and less LBM.
Thus, because of the way that hormones and cell signaling works, if you have more body fat and less muscle/LBM:
- you’re less able to handle carbohydrates properly;
- you’re less able to transport and store fat appropriately;
- whatever you eat isn’t able to do its job of keeping you healthy and strong;
- you probably have a lot of systemic inflammation, which also compounds the metabolic problem.
Example: The case of “skinny-fatness” |
 Here's an example of a "skinny-fat" body. In clothes, this person would look relatively slender, and she's probably a "healthy" body weight. But her LBM is low and her body fat relatively high. This means that metabolically, she may in fact be unhealthy. |
“Starvation response”
Most of us have heard about the “starvation response”, but few people know how it actually works.
For one thing, it’s not an instantaneous reaction. You don’t go into “starvation mode” if you have a late lunch. But there are indeed systems that kick in to prevent long-term, drastic calorie restriction. When the body perceives that it has not been fed enough to meet its needs for too long a period, as would be the case in a crash diet lasting more than a couple of days, it compensates in a few ways.
First, the hormones that control appetite, hunger, and satiety kick in.
You feel hungrier as ghrelin levels go up, and you get more interested in eating as leptin levels drop. Levels of neurotransmitters such as dopamine and serotonin (which are involved in satiety and food reward) change.
You start thinking about food more. Your tummy growls. It becomes much harder to resist trigger foods. It seems like the messages are coming from your conscious brain, because they’re in the form of thoughts such as “I could really go for waffles” or “Boy that Taco Bell commercial looks good”.
But in reality, they’re probably coming from deep down in your reptilian hindbrain that’s screaming FAMINE FAMINE SAVE THE SPECIES MUST EAT NOWWWW!!
Studies on prolonged starvation found that people who were even moderately calorically deprived for long periods of time became obsessed with food: they traded cookbooks and recipes, spent inordinate amounts of time planning elaborate meals, etc. They became emotionally unstable and did weird things like hoard unusual objects. What can I say — hormones can make you kind of nutty.
And when you do get to eat, you eat more than normal. Scott Abel has described how bodybuilders and fitness competitors will actually eat to the point of intestinal rupture following a contest.
A friend of mine who competed successfully in bodybuilding shared her experiences of dieting so restrictively. She described to me how once a week she would quietly go insane and set up a little “circuit” in her kitchen, which enabled her to move from “food station” to “food station” continuously in a sustained binge. The foods she chose were straight-up brown sugar, butter, and peanut butter. Hardly rational eating behaviour, especially for an athlete who’d normally prefer salads to burgers.
Second, the body lowers its basal metabolic rate (BMR), or the rate at which it burns calories throughout the day.
Think of the BMR as the “idling speed” of your body. If your BMR drops, then your body burns calories more slowly, which means that once you go “off” the diet and begin eating normally, your body will more readily deposit bodyfat. This is the principle behind yo-yo dieting. But what most people don’t know about the starvation response is the connection between BMR and muscle tissue (as well as other lean body mass, like bone).
One of the classic signs of someone who’s deep in the “starvation response” is body temperature. Chronically low waking temperature signals that the body is “idling” at a “slower speed”.
The bottom line
If you restrict calories consistently and excessively for long periods, you’ll lose lean body mass. LBM is metabolically active. It takes energy to build and maintain it. Growth and sustenance is costly. The less LBM you have, the lower your BMR will be. Combine that with hormonal and cell signaling changes in response to prolonged caloric restriction and you’re looking at a hungry, frail — and most frustratingly, still relatively over-fat — future.
What to do instead
- Cultivate your lean body mass. Weight train and stay active in other pursuits that load the bones and muscles with resistance.
- Don’t overdo the “cardio” and endurance training if you’re trying to lose fat.
- Don’t drop your energy intake (aka calories) too low for too long.