Atomic Nerds

Now that we’ve covered what food consists of from the body’s point of view and what it does with it once you’ve swallowed it- gaining, storing, and mobilizing energy- we move on to how the body actually uses it.

The word “metabolism” technically means the process of biologically transforming one kind of molecule into another kind of molecule; catabolic metabolism refers to breaking something up, usually for the purpose of gaining energy, whereas anabolism refers to building something from more basic components. In the context of the human body, we usually refer to the concept “metabolism” to describe as a whole our body’s systems for storing and burning energy as though it were a single variable; a “slow metabolism” is one biased to storage, where a “fast metabolism” would be one more biased to burning energy. “Basal metabolism” is usually used to describe a general “set point” for an individual’s metabolism when not actively busy exercising or otherwise stressing the body’s need for energy. We tend to think of “burning energy” as something we exclusively do when exercising, but basal metabolism actually accounts for the primary bulk of our energy requirements; as mammals, simply regulating our body temperatures to stay at a constant, warm set point is where most of our consumed calories go. Activity* and repairs account for the remainder of the body’s calorie needs; the reason that the bulk of the burning of fat stores a person will go through in a day occurs during deep sleep is that this is when the body is busy repairing and rebuilding from whatever insults it suffered during the day- and, if the day included significant exercise, building it back a little better as a training adaptation.

Basal metabolism will vary a great deal across individuals just as a matter of genetics; some people are built with “thrifty” metabolisms that diligently store energy and don’t let it go readily, others are high-burning, store little, and experience the excess as “nervous energy” if they don’t put it to some use. While everyone seems to have a natural set point, other variables will still influence them powerfully in various directions; age, activity level, hormonal fluctuations whether natural or medically induced, illness or injury, or an experience with starvation**. Any significant push on any of the variables that change the body’s energy needs will reflect in basal metabolism; although we usually don’t think of temperature as a factor because most of us sensibly inhabit temperate climes that don’t significantly push us, extreme cold will create pressure on the metabolism that translates into a significant chunk of the massive calorie needs for arctic explorers and others forced to spend a great deal of time coping with it. Body composition also has a significant effect on basal metabolism; fat is energy-cheap to maintain and a few pounds more or less of it don’t alter the budget much, but muscle is very expensive and more or less muscle mass has a much more dramatic effect per pound on the metabolic rate needed to maintain it, let alone put it to serious work.

Perhaps an ideal “case in point” example of the degree to which one person’s basal metabolism can differ from another’s is swimmer Michael Phelps, who famously subsists on a 12,000 calorie a day diet. (In contrast to the 1500-3000 daily needs of an “average” nonathlete, depending on age, sex, and size.) Phelps probably has a fairly high basal metabolism by genetics, and being a young man gives him another bump up, but the entirety of the rest of those calories are going toward maintaining the gargantuan energy output allowing him to spend five hours a day on intense exercise, and then repair his body from the damage done and turn it into training adaptation. None of those calories, regardless of source, are sticking around as fat- all of them are used to maintain his muscle mass and put it through daily strenuous effort. The fats and carbohydrates are turned into massive stores of glycogen, and then burned during training, rather than being stored in adipose tissue. All of this is normal for an Olympic-level athlete during training-and the high basal metabolism itself, the muscle mass, and the upper limit of glycogen storage in skeletal muscle are all trained responses rather than what the metabolism of an average sedentary adult would do under the same conditions.

Having discussed the gestalt effect of energy demands and output we refer to as metabolism, let’s have a look at how that’s actually accomplished. For the purpose of generating energy, the human body relies on three major systems. For the first two, everything begins with glycolysis, the conversion of a molecule of glucose into pyruvic acid. What then happens to the pyruvic acid depends on whether oxygen is present or not.

Aerobic respiration is our body’s main glucose-burning, energy generation system. (High school flashback trigger: the Krebs cycle!) This is where we get the bulk of our energy, and it is by far the most efficient way to use glucose to produce ATP. When you’re doing anything that doesn’t rapidly require you to slow down or stop and catch your breath- within about two minutes or so- you’re relying mainly or totally on this primary pathway. During aerobic metabolism, one molecule of glucose yields 30-38 ATP. This is why oxygen is so popular with life on Earth. Most people are familiar with exercise as defined by “aerobic” or “anaerobic”, but there are actually two “anaerobic” pathways.

The lactic acid system is what happens to the pyruvic acid in the absence of enough oxygen for aerobic respiration. The pyruvate becomes lactic acid, which becomes lactate and free hydrogen. The lactate circulates to the liver to be reassembled into glucose, the buildup of free hydrogen ions then begins interfering with respiration. This interference is what puts the hard upper limit on lactic acid metabolism and creates the sensation of pain and fatigue. You can run for about a minute and a half this way. The net yield is two molecules of ATP per original molecule of glucose- vastly less efficient than aerobic metabolism, but not dependent on your ability to rapidly supply vast amounts of oxygen to your tissues.

The phosphogenic system is the shortest of the short term for energy generation. Adenosine triphosphate becomes adenosine diphosphate when one of its phosphates is ripped off (producing the energy), and is resynthesized back into adenosine triphosphate by ripping the phosphate off a molecule of creatine phosphate, which can last for as long as the supply of the latter lasts. This is the only energy system that is truly local to a muscle cell, as the supplies of ATP and CP live within the cell itself rather than being delivered by the bloodstream. This system is for short-term, maximum-effort energy bursts: the available supply of ATP and CP in combination lasts no longer than a maximum of about eight to ten seconds. When the ATP runs out, energy drops, the other systems replenish ATP, and the cycle can continue, though it will be always running at a deficit relative to that first big burst.

Any and all exercise can be roughly broken down by the degree to which it leans on these pathways; sprinters and weightlifters rely on a combination of the lactic acid and phosphogenic systems, while a distance runner (or any endurance athelete) is working as aerobically as he or she can. Aerobic exercise can be maintained for far longer, but anaerobic exercise burns the glucose much less efficiently and thus tends to go through a great deal of it in those smaller chunks of time. In either event, after the activity is at an end and the body’s owner peacefully asleep, training adaptations occur in response to how the body was challenged; in terms of activities that were limited by oxygen, building more and further extending capillaries enhances the body’s ability to deliver oxygen to tissues, as well as strengthening the heart and lungs. Activities that were limited by maximum muscle fiber recruitment, lactate threshold, size and capacity of muscle fibers, and neurological efficiency will produce improvements in these areas***.

If you’ve ever heard the term “VO2 Max”, it measures an individual’s capacity to utilize oxygen; it measures the maximum amount of oxygen in milliliters that the individual can use in one minute per kilogram of body weight. Effectively speaking it’s a measurement of an individual’s capacity for maintaining aerobic metabolism; running at a pace of 5 mph is easily sustainable aerobic activity for one person and more anaerobic for another who will be out of breath in a few minutes because the first person has a higher VO2 max than the other. A large number of variables- metabolic ability to rapidly and efficiently handle lactic acid buildup, cardiovascular strength, capillary network, volume of blood plasma and density of red blood cells, and more- go into what raises VO2 max, but the principle “if you want to improve it, train it”- in this case by repeatedly doing things that bring you to running out of oxygen for aerobic respiration- always apply. The explicit purpose of Tabata intervals, for example, is to spend as much time as possible using the anaerobic pathways, using only enough rest to get enough oxygen back to restore them as possibilities before returning to the work****.

Fatigue is what ultimately limits exertion. Most of us are familiar with fatigue due to exceeding our lactate threshold- which boils down to the working muscle cells, and the metabolic processes they use, becoming acidotic, which interferes with muscle contraction and the smooth operation of the ATP factory. This is not, however, the only process that produces a sensation or effect of fatigue; the nervous system will also experience a burnout effect when repeated, intense muscle contractions are demanded, as the supplies needed for motor neurons to deliver constant “CONTRACT HARD NOW” messages to the muscle fibers can exceed the body’s ability to quickly recycle them*****, especially when there are a lot of hydrogen ions already floating around from a lot of anaerobic glucose-burning. These are what will stop a sprinter or a power-lifter; what stops an athlete working at endurance activities that let him get the majority of his energy from aerobic metabolism will ultimately be his supplies of fuel and his body’s ability to use it efficiently.

To illustrate this, let’s go back to our marathon runner. We’re going to assume that this runner is extremely fit and also intelligent about what his limiting factors will really be in a race; he’s spent the previous five days or so going very light on his running and chowing down on tons of carbohydrates in order to top off his supplies of muscle glycogen, and more of the same the morning of his race to make sure his liver is nice and stuffed with yet more glycogen. He’s excited and moving about, and the hormonal response from his adrenals is blunting what otherwise might be a big insulin spike. As he begins running, he has plenty of glycogen for his body to readily convert to glucose, and he’s maintaining a pace that allows aerobic respiration to dominate. As this goes on, cortisol becomes the hormone dominating his metabolism; as far as his body is concerned, it must be experiencing a stressor that for some reason causes running miles and miles to be a good idea. Aside from continuing to burn up his existing stores of glycogen, his body starts burning his fat stores; since there are double the number of metabolic steps that need to be taken to turn fat into glucose than there are to glycogen into glucose, this is necessarily a much slower and less efficient process; fat is like coal compared to glycogen’s refined gasoline as a fuel, and what will limit the runner won’t be the extent of his fat stores but rather his body’s ability to rapidly convert the fat into something useful while still relying on the glycogen as what will allow him to take his next stride; the fat is more a supplement that will stretch the life of his glycogen than itself a primary supply of energy. All of it still boils down to glucose, but his body simply can’t turn fat (or protein, for that matter, which the cortisol is also stripping for glucose generation) into glucose fast enough to make it primary distance running fuel, though it’s good enough for walking.

If our runner hadn’t been so careful to make sure he had as much glycogen as possible stored in his muscles, at some point as the miles stacked up he might run out; in this case he would experience “hitting the wall” as still feeling relatively alert and energetic but his legs simply refusing to continue working. Since he has plenty of muscle glycogen to burn, his limiting factor then becomes liver glycogen, which is what his brain the energy pig is drinking from. If he runs out of liver glycogen, he will experience his wall as an overwhelming loss of motivation or sense of fatigue, loss of alertness, or even hallucination or other clear manifestations of cognitive dysfunction. His muscles have the fuel and the energy efficiency to keep going, but his central nervous system needs its fast, rich glucose supply to keep running them. Aside from breakfast, one thing he can do to stave this off is to make sure his supplies of fluid also represent a supply of very simple sugars- sports drinks rather than simply water******. He needs cool fluids in any case, since overheating or dehydrating will put so much stress on his cardiovascular system that fatigue and forced cessation of activity will arrive rapidly. In theory, as long as he can keep getting glucose to stave off the collapse of his stored energy and fluids to keep his body temperature regulated and his blood pressure up, he can keep going like this until his tissues actively start to break down from the overuse without the chance for a good long sleep and repair cycle- which is exactly what ultramarathoners do. (Our marathoner will be content with 26 miles, a nice pasta dinner, and a really long nap.) All of this, however, requires the runner to have trained well enough to have a very high VO2max to limit the amount of time he spends in anaerobic energy pathways, an efficient metabolism, and muscles and bones conditioned to the long work. A completely untrained individual probably won’t be able to walk 26 miles no matter how much glucose you throw at him, let alone run it.

A sprinter or a weightlifter, in contrast, spends his workouts in a very different metabolic environment. He certainly has enough glycogen to sustain his workout; it’s going to be his body’s ability to tolerate and clear hydrogen ion buildup, the size of his muscle fibers and his nervous system’s conditioning to recruit many muscle fibers per contraction and keep up contraction, and efficiently pump out glucose in a low-oxygen environment that’s going to dictate how much power his body can put over a short period before fatigue stops him in his tracks. High intensity and short time period (it has to be if the work is really high intensity) will also change the hormonal picture for him to favor more growth hormone and less cortisol- which is one reason among many that marathon runners tend to end up looking like this and sprinters like this even though both are training exclusively for running performance rather than any aesthetic purpose.

I promised in the last post that I would at least attempt to translate all of this information into a short guide to how various popular dieting protocols actually work, and that I shall. Fundamentally speaking, all diets intended to alter body composition either focus on calorie content or on controlling hormones.

Low-fat diets are the flagship diets of calorie restriction; when one macronutrient source is worth 9 calories per gram and the others 4, lowering the fat content of any given food item is usually an easy way to lower its calorie content. Low-fat diets *do not* work when overall caloric load is not carefully tracked- if you simply replace the satisfactions of fat with lots of sugar or starch, no matter how little dietary fat you’re eating you can still maintain or gain body fat. Not all calorie-restriction diets are necessarily low-fat- see also Weight Watchers- but you certainly won’t find any fans of NOT carefully watching fat content here.

Low-carb diets are diets of insulin control. There are several variants in this family- Atkins, South Beach, primal/paleo- with different philosophies and different amounts of “allowed” carbs and what kinds of carbs are allowed, but all of them rely on either dramatically lowering the amount of produced insulin (remember, storage hormone) or blunting and smoothing out insulin responses. South Beach, for example, puts its dieters through a period of sharp insulin restriction- a ban on carbohydrates for the first two weeks- and then moves to focusing more on staying low on the glycemic index. Protein and fat both induce a higher satiety response- a feeling of fullness and satisfaction- in normal individuals than carbohydrates do, so calorie restriction tends to happen as a natural consequence. Some of these diets go all the way into advocating staying in a state of ketogenesis- shifting the bulk of the body’s energy needs to burning fat and requiring the brain to run on ketone bodies rather than glucose. (Yes, this can be done. It’s how we survived starvation Back In The Day.) Some people tolerate ketogenic diets much better than others; there seems to be a lot of natural variation in how well or poorly people do with extreme carbohydrate restriction. Perhaps needless to say, 100% of endurance athletes do poorly on ketogenic diets, though some bodybuilders combine ketogenesis with runner-style muscle glycogen loading in a cyclical fashion.

Any diet that produces the desired result for its user- less body fat while keeping a reasonable proportion of muscle mass, sustainability, and energy levels adequate to support activity level- will result in fewer consumed calories and reduced peaks and valleys in blood glucose. Any diet that actually does this is a good one, whatever metabolic lever it relies on, and any that doesn’t is either straight bunkum or really, really incompatible with the needs of the user.

*Believe it or not this includes mental activity. The brain sucks down quite a lot of glucose doing something difficult for it; if you’re struggling through a challenging math test you may as well be running when it comes to how fast you’re going through glucose. Big brains are expensive, which is one major evolutionary reason so relatively few animals bother with them. Given that the glucose needs go down as tasks are learned and mastered, however, you’re much better off with running than math when it comes to planned calorie-burning.

**Extreme calorie restriction counts as a starvation experience regardless of whether it happened because you were stranded in the desert or because you wanted to be in swimsuit shape. This is why crash diets are such a pernicious cycle; each experience trains the metabolism more firmly to store every scrap of energy and only let it go if absolutely forced. Not only does the weight come back, it comes back much faster than it went on.

***Believe it or not strength training is every bit as much about your motor nerves as it is your muscles; you will be stronger performing motions that your nervous system has learned thoroughly. This is also why varying movements- and making sure movements are complex and involve multiple joints- helps in making sure strength transfers from the gym to daily life, and why there is often seemingly poor carryover in strength between two different motions that work the same muscle or muscles.

****The good news about Tabata intervals: an effective workout in only fourteen minutes! The bad news: they will be the most profoundly miserable fourteen minutes in your day unless you manage to run across that lion I kept talking about in the last post. If you’re NOT miserable, you’re probably slacking.

*****I could explain this in more detail, yes, but it would require a really long side digression I think we will all be much happier without.

******Sports drinks will hit your bloodstream even faster than a can of soda, given that doing so is their reason for existence. Sports drinks are not diet drinks.

Posted Food, powerlevelling, Science: we'll fuck you up |