A word, up front, about what this is not intended to be: it is not intended to be a guide to weight loss or exercise. There are approximately ninety billion of those, many of them bogus, and a surprising number of which nonetheless work each about as well as the other. What this is meant to be is a 101 on how the body handles fueling itself and making more of itself; some schools of thought may well emerge as objectively more or less sensible than others with such knowledge, but it’s not meant to be a series of arrows pointing to any approach whatsoever. If the machine is an understandable machine with more-or-less comprehensible moving parts and processes rather than a mysterious and temperamental black box, evaluating approaches is vastly easier. I would also like to note that however much I wind up making snarky asides about nutrition as a scientific field and how profoundly political and murky it is, I have a great deal of sympathy for nutritionists. They are working in a byzantinely multivariable field in which it is nearly to completely impossible to properly isolate and control for almost all of those variables; they’re not so much stuck looking for needles in a haystack as they are stuck with evaluating to which degree each and every straw in the haystack is a load-bearing straw. Not easy, and it’s no surprise we get so many frustrating cases of completely honest and dedicated researchers coming to opposite conclusions based on the same data sets- because those multiple variables can turn the data sets into Rorschach blots. Yes, the politics are disgusting, but it’s no surprise.
So caveated, on with it.
Fuel- the raw materials
When considering any given food, the units of relevance are the three macronutrients: fat, protein, and carbohydrates. There any many other things in food, including vitamins, minerals, electrolytes, and fiber, but the substances that are broken down to either provide energy to run the body and the raw materials to repair it and produce more of it- food’s reason for existence and what defines it as food- are the fat, protein, and carbohydrates.
Fats are made of long, stable hydrocarbon chains. They vary in how many double bonds and little kinks in the chain there are, and this affects how stable in heat, light, and oxygen the fat is. “Saturated” fats have a hydrogen for every two available bonds a carbon atom has that’s not being taken up by the carbons upstream and downstream of it; “unsaturated” fats have fewer hydrogens and more double bonds between carbons. “Polyunsaturated” defines fats with lots of these double bonds. Since having a hydrogen attached to every free bond is a more stable configuration, saturated fats are those most stable in heat and light; this is why you can fry something in lard, a saturated fat, but flaxseed oil, a polyunsaturated fat, must be used raw or else it breaks down and turns rancid. This is also why you can put lard in the pantry while flaxseed oil must be kept refrigerated in an opaque bottle*. You’ll often see the word “lipid” used when discussing fats; all fats are lipids, but not all lipids are fats. Lipids are a broad class of compounds that may be briefly described as hydrophobic hydrocarbon chains; they include fats, waxes, sterols, and the various fat-soluble vitamins. Fats themselves are part of a more distinct subgroup of lipids called triglycerides. All of this biochemical terminology makes reading nutrition textbooks, and your blood chemistry results, very exciting.
Contrary to the overall public health message actually digested by the public (“fat is bad for you”), fats are highly necessary to maintaining good health; cell walls are made of lipids, and thus getting adequate fat in the diet is fairly crucial in order to be able to maintain and repair yourself. Without enough of it, hair will be dull and thin, nails will be brittle, skin will be dry and flaky, and wounds will heal slowly. All that said, unless you’re a vegan or on a food-sensitivity diet with a lengthy list of don’t's, getting enough fat in the diet to maintain health is generally trivial. (You need at least 5% of your calories from fat to survive. At least 25% to thrive.) While fat is largely something the human body can produce itself (and the most frequent Western woe is that it does so with all too much enthusiasm), there’s a small subgroup of fats it requires, cannot manufacture, and must ingest: the essential fatty acids. These are used for a broad variety of cell-maintenance tasks rather than simply being broken down and used for fuel, and while how much of them is required for solid health is still very unclear; that they ARE necessary is undisputed. Omega-6 fatty acids are found in the modern developed-world diet in abundance; omega-3 are rather rarer and problematically tend to come in unstable forms. Omega-3 is why anyone would bother supplementing with fish or flax oil**.
Biologically speaking, fats are the energy storage medium of choice for animals and to a lesser degree for plants. Hydrocarbon chains can store a great deal of potential energy densely. As such, each gram of fat is worth about nine calories to a human.
Proteins are chains of amino acids folded into globules. While fat and carbohydrates are mostly two different ways of arranging carbon and hydrogen atoms into varyingly fast and potent forms of directly burned fuel, protein is the macronutrient that goes most directly into building more body rather than being burned for fuel. The amount of it you need depends on whether you’re growing, trying to recover from injury, have a rigorous normal activity level that involves having to make far more repairs than you would if you were sedentary (this process is broadly known as “becoming fit”), or even aging and getting less efficient in how the body uses its resources. “Protein” as a term represents a much more diverse class of molecules than “fat” does, but as all of them are put to roughly similar purpose by the body- being chopped into constituent amino acids to produce the proteins that comprise the body’s native toolkit- treating all proteins as roughly equivalent when it comes to nutrition works out anyway.
The body can manufacture most amino acids on its own, but there are eight that are essential and must be ingested. Fortunately, the essential amino acids are rather more common than omega-3 fatty acids; it’s generally trivial to get a full complement of amino acids, again unless you are a vegan, and even then you need to be a rather picky vegan in order to be missing out on one or more of the essentials. Many of the most common traditional food combinations- rice and beans or rice and tofu, for example- represent combining two sources of plant protein lacking one or more essentials into a single, complete protein sources. Most animal proteins are complete, so any carnivore and most vegetarians can generally cease to worry about essential amino acids no matter what supplement companies are trying to tell you.
Proteins break down to about four calories per gram.
Carbohydrates, as the name suggests, compounds that consist of nothing but carbon, hydrogen, and oxygen. For the purposes of the subject, carbohydrates come in two forms: saccharides, which are relatively small and simple carbon-and-hydrogen structures, and polysaccharides, which are rather larger and more complex carbon-and-hydrogen structures. Polysaccharides do include glycogen, a substance we will discuss later, but for the purposes of figuring out what’s in food carbohydrates are either saccharides- sugars- or polysaccharides- starches.
No matter what any supplement company tells you, there is no such thing as an essential carbohydrate. Unlike fats and proteins, ingested carbohydrates exist from the body’s point of view as a pure energy-and-carbon-storage medium, with no esoteric side purpose. The body can produce the carbohydrate compounds it specifically needs from any carbohydrate or fat, provided it has the tools to take the molecule apart***. From a biological point of view, starches are the preferred long-term energy storage medium of plants; a potato, domesticated to produce and store vastly more starch than its wild cousins, is the vegetable equivalent of old-style massively fat domestic pigs.
Carbohydrates, like proteins, break down to about four calories per gram.
Speaking of calories, they are purely the shorthand for how much energy is contained in food; they aren’t a discrete item of their own, merely a mathematical shorthand for energetic potential that is being stored, or energy directly used. The “calorie” used in food is not equivalent to the “calorie” you may remember from your chemistry or physics laboratory days- a food calorie is actually a kilocalorie. Making this even more fun to keep track of is that the laboratory calorie as a unit of energy is now archaic, replaced by the joule****.
Fuel- the refined version
The body has its own energy currencies and storage mediums, which are far less varied than their sources. The most basic unit of cellular energy currency, the one that is actually burned to directly cause any cellular event to happen, is adenosine triphosphate, or ATP. The body is only toting around about three ounces of ATP at any given time, however; the vast, vast majority of its fuel is stored in either a short-term sugar storage form (glucose), a medium-term polysaccharide form (glycogen), or a long-term fat form (fat). The vast majority of metabolism is about transforming food into glucose and then storing the excess in one form or another.
Glucose is the most direct energy form for the body, the one that it splits and burns in order to fuel the creation of ATP. Any given metabolic process to create energy involves glucose, and everything else is about breaking down some more complex carbon and hydrogen structure into glucose to then be used in this fashion. It is pure fuel, ready for immediate use. It’s also slightly toxic; a healthy metabolism keeps the supply of circulating glucose limited and stores as much as it can in order to mitigate this effect. The damage caused by unregulated glucose can be seen in the long-term damage to nerves, vision, and blood vessels seen in people who have had diabetes for decades, particularly if it wasn’t well controlled.
Glycogen is glucose packaged for easy-access storage, which lives in the muscles and in a larger store in the liver. Glycogen is what the body expects to be using during non-resting times, and represents the reservoir of ready energy available for athletics, and almost all such exercise is fueled by burning off the current stores of glycogen. If we were to use a combustion engine analogy, glucose is the fuel moving through the engine, and glycogen is the fuel in the tank. When a marathon runner “hits the wall”, this is the moment when his body has run itself entirely out of glycogen. (Marathon runners, thanks to the extremity of their energy requirements, are actually an interesting example of the consequences of metabolic pathways I’ll get into more later.)
Fat is long-term storage, the Federal Oil Reserve of the body. Energy the body doesn’t expect to need any time soon goes here to help the body ride out times when a steady supply of food just isn’t coming in, or to cope with very high extra energy demands such as pregnancy, extreme cold- or running out of glycogen. If the glycogen is the fuel in the tank, fat is the fuel in the gas can you hope you don’t wind up needing at all. Fatty/adipose tissue also stores a few other things besides energy, like the fat-soluble vitamins (which can cause serious trouble in the case of chronic overdose)- and some storage and manufacture of the steroid hormones, which is as of yet not terribly well understood. Suffice to say the tendency of some chronically overweight mean to grow “man-breasts” is due to more than just fat- extra adipose tissue seems to make for extra estrogen which, nastily, tends to itself encourage extra adipose tissue.
Fuel- refining the raw materials
The intuitive thing for humans is to think of fats as what winds up as fatty tissue, sugars and starches as what wind up as blood sugar and glycogen, and the proteins and enzymes***** as things that are immediately pressed into service more or less in their original capacity. Digestion, however, is a relentless process of breakdown; our digestive enzymes exist to turn everything into basic units that the body then uses in the assembly of other compounds depending on its immediate needs. Fats become glucose or glycogen and carbohydrates become fat as metabolic priorities dictate; to the body, it’s all just a question of how best to store or use the carbons and in what configuration.
Insulin is the major determinant in what happens to the food once digestion has finished unpacking it and transforming it into amino acids and glucose. When blood glucose rises, insulin production steps up to move the excess glucose into cells for storage to prevent blood glucose from rising to damaging levels, as well as simply to prevent waste******.
The key thing to remember about insulin is that it’s not simply about moving glucose from the bloodstream into cells, it’s essentially the master control storage hormone. It moves amino acids into cells as well as glucose for assembly into various needed proteins, and it also stimulates adipose- body fat- cells to turn glucose into into triglycerides- the long-term storage fat format. Insulin also inhibits the metabolism of triglycerides back into glycerol and fatty acids for use as energy, which is what we generally mean when referring to “burning fat”- if insulin is present in significant amounts, fat will not be used for energy at all. If insulin is there and energy is needed, it will come preferentially from turning sugars into glucose and burning that.
Without insulin, glucose is not taken up by most of the body’s cells at all. The cells in the liver and the brain can take in glucose without insulin, but the rest of the body requires its signal. For an untreated diabetic, glucose levels continually rise while body cells, faced with an energy drought, turn to fat and the protein in the muscles. The byproducts, ketone bodies, eventually build up along with the glucose to the point of making blood acidotic. Insulin normally controls ketones as well, which is why type 1 diabetes can cause ketoacidosis but simple fasting or a very low carbohydrate diet can’t.
Since insulin production responds linearly to glucose released into the bloodstream as digestion finishes, the rate at which this happens and energy is produced depends largely on what is going on with digestion. Very simple sugars hit the bloodstream as glucose almost instantly; technically speaking, for glucose and sucrose, you don’t even need your stomach or intestines- they will begin entering your bloodstream starting at the mucous membranes in your mouth, which is why glucose gels may be used to revive someone in a hypoglycemic crisis without them being conscious and able to swallow. The enzymes that break down the simplest sugars are present in your saliva, and of course glucose needs no processing at all. More complex starches that require a more advanced digestive toolkit take longer to unpack and hit the bloodstream, and the addition of fiber and other macronutrients such as fat and protein also stretch out the period of digestion and the size, speed, and spread of the blood glucose rise. A spoonful of sugar goes straight to the bloodstream; a bowl of steel-cut oatmeal with butter (but no added sweetener) stirred in takes so long to sort out between the fat, soluble and insoluble fibers, and various complex starches that the release of glucose to the bloodstream will be slow but also last for hours. If you’ve heard of the term glycemic index, it’s a figure obtained by feeding something with exactly fifty grams of available carbohydrates to ten test subjects and measuring their blood glucose response over a two-hour period; high glycemic index describes a carbohydrate source with a sudden, steep rise in blood sugar and low describes a much slower and more modest response.
The blood sugar = insulin = storage effect also causes the steep energy peaks and valleys associated with too many simple carbohydrates; insulin will store everything. Therefore, if you eat a meal that was sizeable but also predominantly simple carbohydrates, the high insulin response will diligently pack everything away- leaving relatively little left hanging around to provide readily available energy. This is the metabolic equivalent of going on a large grocery shopping trip, making one sandwich out of the haul to eat now, and then throwing the rest of the food into the freezer. Hunger returns quickly because, when the body goes looking for ready sources of energy, not much is there.
Glucagon is insulin’s partner; its release signals the liver to liberate the glycogen stores and turn them into free blood glucose. It also stimulates a certain amount of insulin release, since most tissues other than the brain and the liver itself will need the insulin in order to make the glucose available to the cells. It also encourages the production of glucose from non-glycogen stores- fat stored as triglycerides into glycerol, then into glucose, and the same from proteins. After the marathoner hits the wall, glucagon is what directs his body to start cannibalizing itself- fat stores where they can be found, and muscle mass- for more glucose to keep going.
Cortisol is widely known as a “stress hormone” because, while it maintains a constant low-level presence in normal individuals, physical and psychological stressors inspire the secretion of a lot more of it. Together with glucagon and adrenaline, a cortisol dump into your system (JESUS CHRIST IT’S A LION, GET IN THE CAR) will have the opposite effect of insulin and will induce the body to break open the stores, stop the transport of resources into fat cells, and liberate as much energy as glucose as it can. The system is actually fairly sophisticated- the marathoner we keep mentioning is soaking in cortisol, and while the entire signal chain is not understood, his body is peeling off protein to turn into glucose from his nonexercising muscle mass, blocking energy uptake by the nonexercising mass just as it is with the fat cells, and feeding the result to his legs. Part of the way cortisol makes energy less available to tissues that aren’t very busy attempting to save their owner from lion attack is by down-regulating the sensitivity of their insulin receptors, which can eventually cause major problems for someone living with chronic stress and will exacerbate any form of diabetes as a result. People who need massive doses of cortisol in order to keep other diseases in check not only suffer from a muscle wasting effect, they also may wind up with type II diabetes purely as a consequence of the hormone, referred to as “steroid diabetes”. Cortisol does a large number of other things as well, including encouraging leaching of calcium from bone for use by the nervous system, but its antagonistic relationship with insulin is the main relevant subject of concern.
Human growth hormone, if insulin is the storage hormone, is the “building” hormone. Growth hormone encourages the building of muscle mass, mineralization of bone, increased protein synthesis, liberation of fat stores for the energy to fuel these various activities, tickling the immune system encouragingly, and telling the liver to make glucose rather than storing it. If it sounds like a lovely hormone*******, athletes agree and this is why it’s up there next to the androgens in terms of substances banned by various sporting agencies. Aside from its obvious high presence during times when children are actively growing, human growth hormone is stimulated by intense exercise, deep sleep, and low blood sugar/fasting. As it is a resource that could cause profligate use of resources, HGH is tightly regulated by the body; it goes away once blood sugar rises, and cortisol will also chase it away, among other things. (Those other things mostly being a great deal of biochemistry not really relevant to the topic at hand.)
If I continued to list hormones that affect food in some way, I’d keep going for quite awhile; suffice to say that these are the major players that affect how food is sorted, stored, and used. Since we’re already seven footnotes and close to four thousand words in, I’ll close out here. In the second half to this monster I plan to cover in much more detail exactly how energy is used, as well as how the various popular forms of dieting actually work on a metabolic level.
*Speaking of snarky asides on nutritional science and public health, the focus on saturated and unsaturated fats as direct proxies for the “healthiness” of a fat has had some interesting consequences, especially as people try to make their food healthier by cooking with unsaturated or polyunsaturated fats. The products of a fat breaking down and oxidizing under heat too extreme for it can have some unfortunate interactions with the food being fried, such as acrylamide creation from frying starch with an oxidizing fat. Make your french fries in lard, tallow, or peanut oil, not soybean, olive, or canola oil. Poor McDonald’s can’t seem to win on this one no matter which way they jump.
**No, seriously, never cook with these.
***Lactose would be an example of a sugar that some but not others have the capacity to make use of. While enzymes to pick apart varying kinds of complex carbohydrate are something of a hot evolutionary commodity among bacteria, mammals that wish to exploit an unusual food source usually borrow a bacteria to keep in the gut for the purpose. Carbohydrates not digestible by humans are usually referred to as “fiber”.
****Chemistry is dense and frustrating at times. We should probably be glad of this, as the temperament required to learn it well is nearly incompatible with the temperament required for do-it-yourself political violence.
*****Again no matter what anyone selling or advocating anything tells you, for the most part active enzymes that we consume don’t wind up doing a thing except being chopped into amino acids like other proteins. There is no such thing as an enzyme deficiency unless your only possible food source is milk and the genetic lottery didn’t give you lactase persistence into adulthood.
******Actually, I’m oversimplifying by quite a bit here. Insulin does respond to blood glucose levels, but also to the ingestion of carbohydrates and proteins, and it rises in an anticipatory fashion at normal mealtimes and in response to sweet flavors, even artificial sweeteners. However, subsequent blood sugar responds very differently to an insulin spike induced purely by protein than it does to carbohydrates or carbohydrates plus protein, and we are now screaming into “here there be dragons” territory in terms of what is currently well understood, so it’s just as useful in practical terms to think of insulin as the reaction to glucose and back away slowly.
*******If you want to know why you don’t want to just take a bath in this stuff, do a Google image search for “acromegaly”. Then, for fun, always take a close look at athletes in sports in which anabolism-enhancing drugs are a powerful temptation.