Saturday, June 28, 2008

Energy Regulation 1: Do Calories Count? And Who's Counting?

In the last post discussing acylation stimulation protein, I made several references to the various regulatory mechanisms that control energy intake, utilization and storage. My claim is that if all of these are working correctly, the body will more or less maintain itself in a healthy state, as that is presumably the evolutionary point of all this stuff. "Healthy state" includes not becoming obese. My guess is that unless you break one or more of these mechanisms, you would find it very difficult to store much excess body fat, because the body doesn't want that, and tries very hard to avoid it by influencing behavior and metabolism.

As we'll see, there are lots of possible things to break and ways to break them: genetic defects, drugs, disease. But for most of us, the major influence is probably diet, mainly through it's influence on insulin. Insulin is arguably the "master hormone" in charge of energy balance. As we'll see, insulin not only controls of blood sugar, but also acts as a signal to start or stop eating, and signals the amount of stored energy in the form of body fat. Insulin interacts with many other hormonal and nervous system mechanisms, and screwing up insulin balance also potentially fouls up a lot of other things as well; take a look at all of the problems inherent in "metabolic syndrome", and you'll get the picture.

I don't believe obesity is a disease in itself, but rather the symptom of an underlying metabolic problem. To "cure" obesity, you really need to restore the appropriate balance, so that the regulatory systems can operate properly. For instance, some obese people have a genetic defect that causes them to make little or no leptin, a hormone secreted by fat cells which is involved in control of both appetite and fat storage. Once you know somebody has this problem, it can be treated by giving them leptin to make up for their deficit. Type I diabetics (who are not obese) lack insulin, so they are treated with insulin. But Type II diabetics have too much of both leptin or insulin, and reduced response to both. Treating them with either leptin or insulin would not be expected to succeed in restoring their metabolic balance and thus normal bodyweight, an expectation borne out by experience. If you're going to fix a problem, you'd better have some idea of the root cause.

So this is the first in a series of posts to delve into the broad topic of "energy regulation", including feeding behavior, energy utilization, and energy storage. Considerable scientific progress has been made on these topics in recent years, but the understanding is far from complete. I'm going to try and touch on the high points, and hopefully avoid too many technical details (which honestly, I don't completely understand myself). Part 1 will be mostly a setup to the subsequent discussion. At the end of this post, I'll put some links to scientific publications or textbooks used, so you can delve into the details if desired.

There's been a lot of discussion lately about whether or not "calories count" in weight gain or weight loss. Much of the argument surrounding this point seems to be unfortunately misguided, with people taking absolute positions on either side. The reality is more complicated. The short answer to the first question is "Yes, calories do count", but is qualified by the fact that many hormonal and nervous system mechanisms regulate caloric intake, storage, and output. Roughly speaking, these are influenced by caloric content of food, but greater influence is exerted by the composition of those calories. As we go through this series, we'll see several examples where macronutrient composition plays a much larger role in influencing the biological response than does simple calorie content. In short, as far as metabolic regulation is concerned, the oft-repeated phrase "a calorie is a calorie" does not apply.

Thinking about the question "Who's counting calories" starts us down the path of understanding. After all, what organisms in nature consciously count the calories they eat or expend? That's easy: humans, and humans alone. Clearly an animal like a rat isn't keeping a tally like "I ate 5 extra grams of rat chow this morning, did 20 minutes on the exercise wheel to compensate" etc. Somehow, they "just know" how much to eat and be active, and their body adjusts accordingly. It is often stated that humans become obese due to an overabundance of readily available food. But in their natural environment, animals will not become obese regardless of food abundance UNLESS there is some other biological imperative to do so. Foxes don't get fat when there's lots of rabbits around, they make more baby foxes. Storage of excess body fat is again clearly regulated by other mechanisms. Mice, for instance, will lay on bodyfat as winter approaches in anticipation of hibernation. Further, they will store excess fat largely independent of how much or little they are fed. Bears similarly lay down fat stores for winter hibernation. Yet once they pass a certain age, they lose the ability to store enough fat for the winter, regardless of how much food is consumed. So the amount of input calories would not seem to be the major controlling factor in fat storage or loss.

Many recommendations for diet and health are based on a grossly oversimplified view of how food intake is regulated. The fullness of the stomach is widely thought to be the primary regulator. You eat until the stomach is full, food moves into the intestines, where your body sucks up whatever it can at a fixed rate until the stomach is more or less empty. Then you get hungry and eat again. This supposedly happens about once every four hours, leading to the idea of three meals a day during waking hours. This oversimplification spawns silly ideas like drinking lots of water or eating high-fiber foods to make you feel more full on less calories, or even sillier interventions like bariatric surgery. Just a little thought shows these ideas can't be right. If it were, a rat would happily eat wood chips and water until it felt full, and would ultimately starve to death from a lack of energy nutrients. Clearly the rat "knows" the energy content of possible food items, and thus avoids the wood chip diet. And we'll see later that surgery such as gastric bypass does more than simply shrink stomach capacity: it also causes measurable and significant changes in the levels of hormones associated with appetite and energy regulation.

The oversimplified view is part of the web of flawed thinking underlying diet. Obesity is not simply a result of being gluttonous, and weight-loss is not simply a process of curtailing caloric intake. "Willpower" is unlikely to enter in to the equation, unless your plan for avoiding or reducing obesity requires that you fight against millions of years of evolutionary programming, life-preserving impulses, and mechanisms regulating appetite and metabolism. Rats and bunnies and bears don't need willpower if fed their natural diet; but feed them something outside of their evolutionarily defined diet, and their bodies often go haywire, with obesity as one possible outcome. One presumes the same holds for humans. Similarly, I think it's pretty easy to poke holes in the idea that higher brain functions (e.g. "willpower") have the capability to override behavior which is key for survival of the organism. Next time somebody blabbers at you about having "willpower" to lose or keep off excess fat, ask them if they have the willpower to hold their breath until they pass out. Fighting against hunger is, I think, the same thing: you can do it for awhile, but the body isn't going to let itself die, and will sooner or later induce behavior it thinks is necessary for survival. This will hopefully become more clear as we delve into the regulation of diet and metabolism.

Before diving into some of the biochemical details, it might be useful to think of a simple model system which requires similar regulatory capabilities. The hybrid electric vehicle (HEV) seems to be a good one, and has some nice similarities with the body. An HEV takes fuel (usually gasoline or diesel) from an external source, and stores it in the gas tank. It also can store energy in a battery, and when moving also "stores" kinetic energy (the energy of motion). Energy can be used from these various sources as demanded by the usage of the car. When accelerating, gasoline is burned in an internal combustion engine and/or electricity from the battery is used to power an electric engine. Energy can be converted amongst it's different forms. The internal combustion engine can be used to either accelerate the car (increasing kinetic energy) or charge the battery. Kinetic energy can be converted to stored electrical energy through regenerative braking.

All of this requires some regulation, so that you don't store/use too much energy, possibly causing inefficient use or damage. One mechanism is simply mechanical: the gas tank has a maximum capacity. If you try to put in more gas than it can hold, gasoline spills out all over your shoes. The battery has a maximum capacity as well: charge it too much, and it may explode. The car's "brain" (a computer and related electronics) monitors the various systems as well as the energy requirements based on your usage. Thus, if the battery registers as not full, applying the brakes will generate electricity which charges the battery. If the battery is full, then that energy must be "wasted" as heat, because there's no place else to put it. If power requirements exceed that of the electrical motor or if the battery is empty, then the internal combustion engine needs to be turned on.

The human body has many parallels. Fuel is supplied externally, but we can take in multiple types: carbohydrate, fat, protein, and alcohol (though obviously the latter is not recommended). This fuel is stored in the stomach, much like the gas tank. Rather amazingly, unlike an HEV, the body needs only one power plant for all different fuel types: the mitochondria. The body has "batteries" as well. Fat cells can store fat, muscles and the liver store glycogen (the storage form of sugar), and lean tissue throughout the body contains protein, though this is generally used for energy only in emergency situations. Different fuel types can be interconverted: carbohydrates can be changed to fat, protein to glucose, fats to ketones. Excess energy can be wasted as heat. And all of this is monitored and regulated by a combination of the nervous system and glands, to maintain the body in a healthy state over a wide variety of usage conditions, whether sleeping or avoiding becoming a bear's lunch. As humans are omnivores, the system can also deal with a very wide range of different macronutrients from plant and animal sources.

The differences between people and HEV cars are informative as well. An HEV can't go get it's own fuel. Instead, it reports on the fuel status to the driver via the fuel gauge. Humans of course need to obtain their own fuel. The "fuel gauge" is ultimately appetite, which is driven by a complex system of hormones and several parts of the brain. An HEV also uses fuel for only one thing, which is to generate energy. While energy is one main purpose of food intake in humans, humans are also constantly regenerating new tissue and other functional substances like hormones and enzymes. Food provides the raw material for this as well. As we'll see in a bit, these functions, most importantly including growth in children, are also closely tied in to the same systems which regulate food intake and energy metabolism.

The cycle of food intake and energy usage/storage can be broken into several steps. Each of these tends to have several interacting regulatory mechanisms, both hormonal and nervous. The steps are:
  • Appetite stimulation, which in turn stimulates food-seeking behavior.
  • Initiation of the meal (start putting stuff in your mouth).
  • Termination of the meal (stop putting stuff in your mouth).
  • Movement of food from the stomach to the small intestine for digestion and absorption.
  • Utilization or storage of nutrients.
  • When everything eaten is used up, start again.
If the regulation of any step is disrupted, we have the possibility of non-optimal health, the most outward symptom of which is obesity. For instance, researchers use several strains of rats and mice which have been genetically modified to be predisposed to obesity. The modified genes affect different regulatory systems, with various different outcomes like overeating, underactivity, increased storage of fat over lean tissue, etc. (the "willpower" gene has yet to be identified.) But the main outcome is the same: obesity. When you break a regulatory mechanism, the animal exhibits some combination of behavioral and/or metabolic changes that cause it to become obese. Conversely, if you repair whatever is broken, or compensate for it's effects, the animals generally lose their obesity and normalize metabolism. Why would it be any different in humans?

Subsequent posts will delve into these regulatory mechanisms more deeply, and explore some possible implications for diet and health. Again, much is unknown in this field, so the best we can do is take what is known and apply rational inference; but I think we'll see that some knowledge of how eating and energy storage are controlled provides a powerful explanatory framework for much of what is observed in terms of obesity, weight-loss, and just general health.

Here are some links to the science papers, if you want to get a head start:

11 comments:

GreatnessBlog said...

Thanks.

David Brown said...

Over the past few years I've tried to drum up, with little apparent success, some interest in the unabsorbed calorie phenomenon.

Most weight control experts write as though every calorie that passes between the lips gets absorbed into the bloodstream. If this were true, force feeding experiments would produce far more weight gain than generally reported.

Will you be discussing this aspect of caloric utilization in future posts?

For more discussion regarding unabsorbed calories visit these web pages:

http://www.caloriesperhour.com/forums/forum19/75.html
http://blog.kir.com/archives/2007/10/kolata_on_good.asp
http://blogs.mtengine.com/cally/2006/02/pure_white_and_deadly.html
http://www.bmj.com/cgi/eletters/330/7504/0-g#109662
http://www.medicalnewstoday.com/youropinions.php?opinionid=4166
http://www.fiber35diet.com/_articles/article_f35_04_FlushFormula.aspx
http://bmj.bmjjournals.com/cgi/eletters/328/7455/1558#64638

Dave said...

Hi David. Has anyone measured the unabsorbed calories on the output end? It would be interesting to do this across a variety of diets.

Differences in caloric absorption across different diets would underscore another reason we need some regulation of caloric intake beyond just mechanical filling of the stomach.

Thanks for links. I suspect Blogger will maul the URL's you posted, and for the life of me I can't figure out how to edit other people's comments to fix things like this. Here they are linked up:

http://www.caloriesperhour.com/forums/forum19/75.html
http://blog.kir.com/archives/2007/10/kolata_on_good.asp
http://blogs.mtengine.com/cally/2006/02/pure_white_and_deadly.html
http://www.bmj.com/cgi/eletters/330/7504/0-g#109662
http://www.medicalnewstoday.com/youropinions.php?opinionid=4166
http://www.fiber35diet.com/_articles/article_f35_04_FlushFormula.aspx
http://bmj.bmjjournals.com/cgi/eletters/328/7455/1558#64638

David Brown said...

Thanks for linking up the URLs.

You asked, "Has anyone measured the unabsorbed calories on the output end? It would be interesting to do this across a variety of diets."

As far as I can tell, there's been very little of this sort of research. You can access a study published in the AJCN here:
http://www.ajcn.org/cgi/reprint/11/2/142

Dave said...

Hi David. Very interesting article. If I read it right, the key results are that butter doesn't change bile excretion much, but sunflower seed oil did; serum cholesterol increased on the butter diet, but fell on the sunflower oil diet. Fiber also changed excretion of bile and fatty acids, but I believe that fiber not only physically blocks absorption, but also can bind up some bile and fat. Other studies saw similar results in the consumption of coconut oil (output unchanged, cholesterol increased) and sunflower oil (output increased, cholesterol decreased).

The differences from fatty acid composition are pretty interesting, and it's a little surprising this hasn't been picked up more broadly. Butter and coconut oil contain both a larger proportion of both saturated fatty acids and short and medium chain fatty acids, while vegetable oils are largely mono/polyunsaturated long-chain fatty acids. That leads to some interesting questions, such as whether it is the chain length or saturation that induces the observed changes. Also, it appears that the amount of fatty acids excreted is largely unchanged.

Bile is generally reabsorbed farther down the intestine (in the ileum) than most fat, and supposedly pretty efficiently. Maybe long-chain or polyunsaturated fats just require more bile? Interesting stuff, hopefully some more studies will be done.

Dave said...

Just to follow up on the bile excretion topic. It appears that fatty acid chain length affects secretion of the hormone cholecystokinin (CCK). One of the roles of CCK is to stimulate gall bladder contraction. Fatty acids of 12 carbons or longer may increase CCK secretion, while 10 or shorter have no measured effect. So the increase in bile excretion may occur because more bile is secreted.

Here's another paper on the topic of fatty acid absorption.

David Brown said...

Hi Dave,

I couldn't access the paper on fatty acid absorption. All I get is this: File not available. [S0007114599000124a.pdf].

Dave said...

Try this one.

Anonymous said...

I just wanted to say thank you for posting this blog and I really look forward to your future posts. Finally someone is making some sense. I have been eating low carb and reading various blogs and forums for the last 8 months. Most of it is noise. As someone who has been overweight my whole life but has NOT been a big eater, I have always known there has to be something going on besides calories in, calories out. Low carb has helped to take almost 30 lbs off me but it obviously is not the whole answer either as I've come to realize over this time. But the low carb experts will say you must be cheating or eating too much if low carb isn't working for you. So I continue to search for more pieces of the puzzle. And you are helping with that!

Dave said...

You're most welcome.

It is unfortunate that most diet advice (even when it's partially correct) is usually based on an oversimplified and dogmatic view. Part of the reason I think this occurs is that those who devise the diet try to make it as simple as possible, so it's easier to follow; but then the simplified version becomes entrenched as "truth", and the details sacrificed for practicality get lost completely.

Low-carb diets have two advantages: they're effective because they allow the body to restore something approximating a healthy equilibrium in the regulatory systems, and they're dead easy ("don't eat anything on this list"). Unfortunately, an healthy equilibrium internally may correspond to an external state where we still want to lose a little more fat. I'm guessing that in many cases it requires some extra effort to move the external equilibrium such that it corresponds with our desired external appearance.

David Brown said...

Hi Dave,

I hope your system alerts you to this comment because I believe I've discovered something important regarding the energy balance controversy. It has to do with gut bacteria and the heat they generate while feeding and multiplying.

As far as I can tell, obesity researchers such as George Bray, using a metabolic chamber, can fairly accurately measure total heat generated within the body. However, since they do not subtract out the heat energy generated by gut bacterial activity, which diffuses into the body, they're actually measuring two things; heat generated by the body and heat generated by gut bacteria. For example, "The total available energy of a food may be defined simply as its heat of combustion, minus the heat of combustion of the faecal and urinary residues to which it gives rise." http://www.fao.org/DOCREP/MEETING/004/M2763E/M2763E00.HTM

What the author(s) of this definition seem to have overlooked is the fact that bacterial metabolic activity in the gut takes place outside the body and involves calories the body did not absorb. Since, as mentioned earlier, these calories are dissipated as heat, it looks like the body itself is metabolizing calories to produce that energy.

I don't yet know how large this effect might be. My next move is to contact some microbiology researchers to determine if any of them know where to find data. If the effect is relatively large, it would explain why overfeeding and underfeeding experiments often produce perplexing results. http://books.google.com/books?id=iVqZdRh6ICoC&pg=PA127&lpg=PA127&dq=Vermont+prison+overfeeding+studies+conducted+by+Ethan+Sims&source=bl&ots=hJQgA0wFIQ&sig=ptFHOlcjLpdt6IrvzdkeCdEaFsE&hl=en&ei=hpT2SdryFJTEswOy15hG&sa=X&oi=book_result&ct=result&resnum=2

In the end, we may learn that energy equilibrium may involve a rather large range of caloric intake for some people and a very narrow range for others. It may also turn be that different combinations of macronutrient intake may widen or narrow that range.