Calorie Partitioning: Part 1

Note: This is an excerpt from The Ultimate Diet 2.0.

At a very fundamental level, the problem that natural bodybuilders and athletes have is one of partitioning; that is, where the calories go when you eat more of them or come from when you eat less of them. In an ideal universe, every calorie you ate would go to muscle tissue, with none going into fat cells; you’d gain 100% muscle and no fat. In that same ideal universe, every calorie used during dieting would come from fat stores; you’d lose 100% fat and no muscle. Unfortunately, we don’t live in an ideal universe.

As I mentioned early in this book, some hapless individuals will lose as much as one pound of muscle for every 2-3 pounds of fat that they lose when they diet. Typically, those same individuals will put on about the same amount of fat and muscle when they overfeed. Thus is the balance of the universe maintained. More genetically advantaged individuals tend to put more calories into muscle (meaning less into fat) when they overeat and pull more calories out of fat cells (and less out of muscle) when they diet. They stay naturally lean and have few problems dieting. Once again, you aren’t one of them or you wouldn’t be reading this booklet in the first place.

When talking about partitioning of calories, researchers refer to something called the P-ratio. Essentially, it represents the amount of protein that is either gained (or lost) during over (or under) feeding. So a low P-ratio when dieting would mean you used very little protein and a lot of fat. A high P-ratio would mean that you used a lot of protein and very little fat. It looks like, for the most part, P-ratio is more or less the same for a given individual; as I mentioned above, they will gain about same amount of muscle when they overfeed as they lose when they diet. This is yet another example of the body’s attempts to maintain itself at a ‘normal’ level.

So what controls P-ratio. As depressing as this is, the majority of of the P-ratio is out of our control; it’s mostly genetic. We can control, maybe 15-20% of it with how we eat or train. Supraphysiological amounts of certain compounds (supplements) and, of course, drugs, can also affect the P-ratio.

So what are the main determinants of calorie partitioning? Obviously, hormones are crucially important. High testosterone levels tend to have positive paritioning effects (more muscle, less fat) while chronically high levels of cortisol have the opposite effect (less muscle, more fat). Thyroid and nervous system activity affect not only metabolic rate but also fat burning; optimized thyroid and nervous system levels mean better fat burning, which means less muscle loss when you diet. It also means less fat gain when you overeat. Unfortunately, levels of these hormones are basically ‘set’ by our genetics; the only way to change them significantly is with supplements or drugs. Beyond that, there’s not a whole lot we can do to control them.

Another factor controlling P-ratio is insulin sensitivity which refers to how well or how poorly a given tissue responds to the hormone insulin. Now, insulin is a storage hormone, affecting nutrient storage in tissues such as liver, muscle and fat cells. In that same ideal world, we’d have high insulin sensitivity in skeletal muscle (as this would tend to drive more calories into muscle) and poor insulin sensitivity in fat cells (making it harder to store calories there). This is especially true when you’re trying to gain muscle.

In contrast, when you diet, it’s actually better to be insulin resistant (note that two of the most effective diet drugs, GH and clenbuterol/ephedrine cause insulin resistance). By limiting the muscle’s use of glucose for fuel, you not only spare glucose for use by the brain, but you increase the muscles use of fatty acids for fuel.

In addition to hormonal advantages, it’s likely that the genetic elite have high skeletal muscle insulin sensitivity. They store tremendous amounts of calories in muscle which leaves less to go to fat cells. Their bodies also don’t have to release as much insulin in response to food intake.

In contrast, individuals with poor skeletal muscle insulin sensitivity tend to overproduce insulin, don’t store calories in muscle well (part of why they have trouble getting a pump, poor glycogen storage in muscle cells) and tend to spill calories over to fat cells more effectively.

So what controls insulin sensitivity? As always, a host of factors. One is simply genetic, folks can vary 10 fold in their sensitivity to insulin. Another is diet. Diets high in carbohydrates (especially highly refined carbohydrates), saturated fats and low in fiber tend to impair insulin sensitivity. Diets with lowered carbohydrates (or less refined sources), healthier fats (fish oils, monounsaturated fats like olive oil) and higher fiber intakes invariably improve insulin sensitivity.

Another major factor is activity which influences insulin sensitivity in a number of ways. The first is that muscular contraction itself improves insulin sensitivity, facilitating glucose uptake into the cell. Glycogen depletion (remember this, it’s important) improves insulin sensitivity as well.

So what else controls the P-ratio. As it turns out, the primary predictor of P-ratio during over- and under-feeding is bodyfat percentage. The more bodyfat you carry, the more fat you tend to lose when you diet (meaning less muscle) and the leaner you are, the less fat you tend to lose (meaning more muscle). The same goes in reverse: naturally lean (but NOT folks who have dieted to lean) individuals tend to gain more muscle and less fat when they overfeed and fatter individuals tend to gain more fat and less muscle when they overfeed.

The question is why, why is bodyfat percentage having such a profound impact on P-ratio. Well, there are a few easy answers. One is that bodyfat and insulin sensitivity tend to correlate: the fatter you get, the more insulin resistant you tend to get and the leaner you are the more insulin sensitive you tend to be.

A second is that, the fatter you are, the more fatty acids you have available for fuel. In general, when fatty acids are available in large amounts, they get used. This spares both glucose and protein. By extension, the leaner you get, the more problems you tend to have; as it gets harder to mobilize fatty acids, the body has less to use. This increases the reliance on amino acids (protein) for fuel. The original Ultimate Diet advocated medium chain triglycerides (a special type of fatty acid that is used more easily for fuel than standard fats) and this can be a good strategy under certain circumstances. The newly available DAG oil ( might have some use in this regards too.

But that’s not all and it turns out that bodyfat percentage is controlling metabolism to a much greater degree than just by providing fatty acids. Research over the past 10 years or so has identified bodyfat stores as an endocrine tissue in its own right, secreting various hormones and protein that have major effects on other tissues. Perhaps the most important, and certainly the one most talked about is, leptin but that’s far from the only one. Tumor necrosis factor-alpha, the various interleukins, adiponectin and other compounds released from fat cells are sending signals to other tissues in the body which affect metabolism.

Without getting into all of the nitpicky details (many of which haven’t been worked out yet), I just want to talk a little about leptin (if you read my last book, this will all be familiar ground).

Leptin: The Short Course

Leptin is a protein released primarily from fat cells although other tissues such as muscle also contribute slightly. Leptin levels primarily correlate with bodyfat percentage, the more fat you have the more leptin you tend to have (note: different depots of fat, visceral versus subcutaneous, show different relationships with leptin). At any given bodyfat percentage, women typically produce 2-3 times as much leptin as men.

In addition to being related to the amount of bodyfat you have, leptin levels are also related to how much you’re eating. For example, in response to dieting, leptin levels may drop by 50% within a week (or less) although you obviously haven’t lost 50% of your bodyfat. After that initial rapid drop, there is a slower decrease in leptin related to the loss of bodyfat that is occurring. In response to overfeeding, leptin tends to rebound equally quickly. In contrast to what you might think, it looks like leptin production by fat cells is mainly determined by glucose availability (you’d think it was fat intake). So whenever you start pulling glucose out of the fat cell (dieting), leptin levels go down, when you drive glucose into fat cells, it goes up.

Basically, leptin represents two different factors: how much bodfyat you’re carrying, and how much you’re eating. That is, it acts as a signal to the rest of your body about your energy stores. I’ll come back to this in a second.

Like most hormones in the body, leptin has effects on most tissues in the body and leptin receptors have been found all over the place, in the liver, skeletal muscle, in immune cells; you name it and there are probably leptin receptors there. There are also leptin receptors in the brain but I’ll come back to that below. For now, let’s look at a few of the effects that leptin has on other tissues in the body.

In the liver, leptin tends to reduce insulin secretion from the beta-cells. In skeletal muscle, leptin promotes fat burning and tends to spare glucose (and therefore amino acid use). In fat cells, leptin may promote fat oxidation as well as making the fat cell somewhat insulin resistant. Leptin also affects immune cell function, decreasing leptin impairs the body’s ability to mount an immune response. Now you know at least part of the reason you tend to get sick more when you diet. On and on it goes.

Leptin and the Brain

Now, I want you to think back to the first couple of chapters of this book, where I talked about the evolutionary reasons it’s so hard to lose bodyfat. To your body, becoming too lean is a very real threat to your survival. From a physiological standpoint, that means that your body needs a way to ‘know’ how much energy you have stored.

As you may have guessed (or knew from my last booklet), leptin is one of the primary signals (along with many others including ghrelin, insulin, protein YY and god knows what else will turn up) that signals the brain about how much energy you have stored and how much you’re eating.

All of these hormones send an integrated signal to a part of the brain called the hypothalamus that ‘tell’ it what’s going on elsewhere in your body. This causes changes in various neurochemicals such as NPY, CRH, POMC, alpha-MSH and others, to occur. This has a variety of effects (mostly bad) on metabolic rate, hormone levels and nutrient partitioning.

So metabolic rate goes down, levels of thyroid stimulating hormone, leutinizing hormone and follicle stimulating hormone (TSH, LH and FSH respectively) go down meaning lowered levels of thyroid and testosterone, levels of growth hormone releasing hormone (GHRH) go down meaning GH output can be impaired, sympathetic nervous system activity goes down, cortisol levels go up as does hunger and appetite, etc., etc. What you end up seeing is an all purposes systems crash when you try to take bodyfat below a certain level.

I want to point out that falling leptin has a much larger impact on the body’s metabolism than raising leptin does (unless you’re raising it back to normal). That is, the body fights against dieting to a far greater degree than it does overfeeding. This is why, generally speaking, it’s a lot easier to get fat than it is to get lean. Of course, there are exceptions, folks who seem to resist obesity (or weight gain altogether). Research will probably find that they are extremely sensitive to the effects of leptin, so when calories go up, they simply burn off the excess calories without getting fat.

Most of us aren’t that lucky. Rather, like insulin sensitivity discussed above, researchers will probably find that leptin sensitivity is a huge factor influencing how changes in caloric intake affect metabolism. Someone with good leptin sensitivity will tend to stay naturally lean and have an easy time dieting; folks with worse leptin sensitivity (leptin resistance) won’t.

Now, as I pointed out in my last book, injectable leptin is a pipe-dream at this point, an effective dose costing nearly $1000/day (not to mention requiring twice daily injections). Using bromocriptine or other dopamine agonists seemed to fix at least part of the problems by sending a false signal to the brain, making it think leptin levels were normal. Recent studies that have given injectable leptin to dieters show that that fall in leptin is one of the primary signals in initiating the adaptation to dieting. However, unlike in rats, injecting leptin into humans doesn’t fix all of the problems.

This is because, in humans, there is more of an integrated response to both over and underfeeding. To make this easier to understand, let’s look at some of the major things that occur when you cut calories. To understand this better, I want to take a snapshot of what happens when you either reduce or increase calories.

Continued in Calorie Partitioning: Part 2.

Bodyweight Regulation: Leptin Part 6

In Bodyweight Regulation: Leptin Part 5, I explained that, while injectable leptin would be a true boon for dieters, it appears unlikely that it will ever reach commercial or clinical use.

This leaves us with other approaches (e.g. nutritional, supplements, training) to attempt to manipulate either leptin levels or signaling.

There are basically three places where dieters might impact leptin levels and/or activity in terms of fighting off the adaptations to dieting.

1. Production at the fat cell

2. Signaling in the brain

3. Transport into the brain

Leptin production in the fat cell
I talked a little bit about #1 in a previous post, when I talked about refeeds. At this point, and this topic is discussed to some degree in nearly every book I’ve written at this point, interjecting high carbohydrate, high calorie refeeds of varying lengths (anywhere from 5 hours to 3 days) is (currently) the best way to raise leptin while dieting.

One of the interesting (and often missed points) is that, as dieters get leaner (and leptin drops more and more), refeeds need to become larger and/or more frequent. That is, rather than necessarily dieting harder as they get leaner, some people are actually doing better by ‘breaking their diet’ (with specific high-carb refeeds) more frequently.

I’d note again that leptin production is related primarily to carbohydrate intake in the short-term, high-fat refeeds aren’t the best way to raise leptin levels. I’d also note that single ‘cheat’ meals won’t impact on leptin levels significantly as leptin doesn’t really change on a meal to meal basis.

Tangent: I’d note that, in this regards, some of the work being done with intermittent fasting and every other day refeeds has relevance here as some data suggests that leptin may be maintained better with that approach to dieting. But until I get Martin Berkhan in here from LeanGains for an interview and dig into it more, I’m not going to talk much about IF’ing as a dietary strategy other than to say: there’s some compelling shit going on here.

An additional strategy, talked about in some detail in my Guide to Flexible Dieting is the idea of full diet breaks, periods of 10-14 days in-between periods of active dieting where calories are brought back to maintenance (and carb intakes brought back to at least moderate levels).

Not only does this provide a psychological break from the grind of continuous dieting, it helps to ‘reset’ some of the metabolic adaptations that occur with dieting. Leptin levels will come up, thyroid conversion in the liver is improved, etc. Assuming dieters have no strict time constraints, I strongly feel that inserting full diet breaks every so often (how often depends on body fat levels) is important for long-term success. Again, for both physiological and psychological reasons.

There are at least two other regulators of leptin levels here, both zinc and Vitamin E intake appears to regulate leptin production and I have suggested supplementation of both in the past to try to help raise leptin. How much (if any) impact this actually has I can’t say.

Leptin action in the brain
Although it seems a bit out of order, I want to jump next to leptin activity in the brain. This is part of the area that gets generally referred to as ‘leptin sensitivity’ in the literature and is, unfortunately, poorly studied and even more poorly characterized.

What causes it, what (if anything) can be done about it is a huge question mark although finding ways to improve leptin sensitivity would probably also have huge benefits. Similar to improving insulin sensitivity, increasing leptin sensitivity would mean that the same level of hormone sends a larger signal. A supplement or drug that increased leptin sensitivity would be expected to do some very nice things.

I would mention that there is indirect evidence that regular exercise improves leptin sensitivity. I say indirect because measuring leptin sensitivity in humans is very difficult. Improved leptin sensitivity is being inferred from the fact that endurance athletes often have leptin levels below what you’d expect given their body fat level; this suggests increased sensitivity. Again, it’s hard to measure in humans.

It does appear that increasing levels of leptin induce resistance to itself (I’ll spare you the mechanism) so it’s conceivable that reducing leptin levels (e.g. with a diet) could transiently reduce leptin resistance/improve leptin sensitivity. How much of an effect or how long this would take is currently unknown.

If this were the case, would provide more support for cyclical dieting approaches such as my Ultimate Diet 2.0. During dieting periods, leptin levels would go down (but sensitivity would go up); during periods of deliberate overfeeding, improved leptin sensitivity (until such time as it went down again) could possibly be taken advantage of.

A similar logic could be applied to weight gain, eventually chronic overfeeding/weight gain might potentially induce leptin resistance; inserting periods of dieting to deliberately lower leptin might offset this.

While I’m on the topic, I should mention that leptin resistance can occur at other tissues such as skeletal muscle (I haven’t talked much about leptin’s actions there).In animals at least, both exercise and fish oils increase skeletal muscle leptin sensitivity.

Leptin transport into the brain
The final topic I want to talk about is that of leptin transport into the brain, something else I haven’t really talked about in this series. But it’s thought that leptin transport issues at the blood brain barrier may be part of the overall ‘leptin resistance syndrome’ and impaired leptin transport into the brain may be part of the problem. It’s thought that leptin transport into the brain can become saturated, that is, once leptin gets above a certain level in the bloodstream, no more can be transported into the brain.

But leptin transport into the brain is also actively regulated by the blood brain barrier, by a variety of things, let’s look at a few:

High blood triglycerides tend to reduce leptin transport and it’s interesting to note that, despite being high in fat, low-carbohydrate diets often reduce blood TG levels; is enhanced leptin transport part of the often observed appetite blunting effect that is often seen (along with other potential mechanisms of course)?

In a similar vein, high-carbohydrate diets, especially combined with low levels of activity often raise blood triglyceride levels, probably hindering leptin transport into the brain.

Both insulin and epinephrine increase leptin transport into the brain. Tying in with my comments above, this might be another reason that high-carbohydrate refeeds ‘work’ after a period of dieting; between (potentially) increased leptin sensitivity in the brain and insulin increasing leptin transport, there is a brief period where leptin signalling should be increased.

The supplements ephedrine and synephrine would be expected to increase leptin transport, ephedrine by raising epinephrine levels and synephrine by directly binding to beta-receptors.

And, of course exercise raises levels of epinephrine and, at least transiently should increase leptin transport into the brain. In that vein, quite a bit of research suggests that the body better regulates food intake when exercise is performed, increased leptin transport (and signalling) might be part of the mechanism.

And while I can’t find the paper now, I seem to recall a rat study suggesting that long-term (4 months if my memory isn’t failing me) fish oil supplementation could increase leptin transport into the brain. But it would likely take a very very long time to occur in humans.

And, at least for the time being that’s pretty much all I have to say about leptin. Next time, I’ll take a quick look at some of the other hormones involved in this system before (finally) moving onto some psychological issues that play a role in dieting.

Read Bodyweight Regulation Wrap-Up

Bodyweight Regulation: Leptin Part 5

Summarizing what I’ve discussed so far:

1. Human bodyweight appears to be biologically regulated, that is it makes some attempt (that can be overcome by environment, of course) to maintain body fat within some range or level.

2. The system regulating body fat is assymetrical, for most people it defends against fat loss much more strongly than against weight gain.

3. For proper regulation, the body needs a way of ‘knowing’ two things: how much fat you’re carrying and how much you’re eating; a variety of hormones play a role here.

4. At least in terms of indicating the amount of body fat is present, the hormones leptin and insulin appear to play a major role. Leptin scales with subcutaneous body fat levels (higher in women), insulin scales with visceral fat levels (usually higher in men); there is some indication of a gender difference in response to the different hormones.

Leptin and insulin also both change with changing food intake; leptin levels can drop significantly within a few days of dieting even with no change in body fat levels. Insulin changes meal to meal.

5. When people reduce calories and lose fat, leptin levels drop, and this appears to be a major part of the overall adaptations to dieting in terms of metabolic rate, hunger, etc.

While leptin certainly isn’t the only hormone involved it appears to be one of the major ones not only having direct effects but also impacting how well or how poorly other hormones (such as CCK) work in the brain.

6. While studies have found that raising leptin in overweight individuals typically does little (for reasons related to either leptin resistance or insufficiency in the brain), preventing leptin from dropping during a diet (or raising it) appears to reverse many of adaptations that occur.

Point 6 raises a question that someone actually brought up in the comments: why can’t I find leptin for sale?

And the answer is that it has never (and I suspect will never) been made available outside of research. When I originally wrote my Bromocriptine booklet, an effective dose of leptin came in around $1000 PER DAY. The last time I looked (about a month ago), it’s down to about $500 per day. That’s assuming a chemical company would sell it to you.

That’s not a typo mind you, leptin makes growth hormone look cheap.

For various reasons, it simply hasn’t been developed for human use outside of research applications. Why? I can’t say for sure. I suspect it’s because drug companies primarily want weight loss drugs that cause weight loss and leptin doesn’t do that.

They don’t seem to want drugs that simply make dieting work better. I’d note that the average dieter isn’t looking for that type of compound either. Drugs that generate weight loss without the person having to change their behavior patterns are the real goal.

There is also the issue of leptin being a peptide hormone, meaning it would have to be injected. Injectable drugs are a bitch practically and there’s been a huge push to develop diabetic solutions not involving injectable insulin for that reason; the odds of the typical person injecting leptin twice daily while dieting are slim.

Bodybuilders would, of course, but that small percentage of people trying to get to 5% body fat are not the target market of the drug companies.

End result: nobody is developing leptin for commercial use so far as I can tell and I doubt this will change.

But for dieters and especially the very lean, injectable leptin would be a godsend fixing a majority of the problems that occur with dieting. Unfortunately, it’s a pipe dream at this point.

Where does that leave us?

Read Bodyweight Regulation Part 6

Bodyweight Regulation: Leptin Part 4

Don’t worry, slowly, I’m getting to the point.

So when you are in an energy deficit and/or losing body fat, leptin levels drop.

Although I haven’t talked much about the role of exercise here I’d only note that whether or not the deficit comes from caloric restriction or exercise per se doesn’t appear to have much of an effect on how much leptin drops.

Basically, the body appears to be sensing ‘energy availability’ (defined as energy intake minus expenditure) and adjusting things based on that. I’d, of course, note that exercise still plays plenty of other crucial roles (including psychological, which I am getting back to slowly but surely) in terms of dieting and fat loss.

In any case, what happens now?

Well, a bunch of stuff. Leptin interacts with various part of the brain but the hypothalamus (where the setpoint is primarily thought to be regulated) appears to be the key aspect. In conjunction with the other hormones I haven’t talked much about yet, when leptin drops a bunch of other neurochemicals change. These all have complicated names like Neuropeptide Y (NPY), Agouti Related Peptide (AgRP), Pro-opiomelanocortin (POMC) and Cocaine Activated Receptor Transcript (CART). The names are not that important practically. When these hormones change, they cause other changes further downstream that affect all aspects of metabolism.

There are other regulators as well, in my little Bromocriptine booklet, I pointed out that brain dopamine levels go down when leptin goes down and this appears to play a role in the overall metabolic adaptation to dieting. The whole idea in that booklet was to use a dopamine agonist to ‘trick’ the brain into thinking it was fed, it worked for about half of the people who tried it; I’m still trying to determine what the cause of the variance was.

Lowered dopamine has a secondary effect that low leptin makes animals (mice and rats at least) more likely to addict to drugs when you starve them (there are other mechanisms at work here, of course): they need something to drive the dopamine/reward system. There is also evidence that obese individuals have impaired dopamine signalling in the brain.

In any case, POMC/AGRP/NPY/CART have further downstream effects and regulate things like metabolic rate (which drops when you diet), appetite/hunger (which go up when you diet), activity levels (you tend to get lethargic, burning less calories in daily activity), hormone levels (including thyroid via TRH/TSH and reproductive hormones via LH/FSH), etc. Testosterone and thyroid generally go down as does nervous system output, cortisol goes up. You get the idea.

Please note again that the extent of these changes depends to a great degree on the extent of the diet and the body fat level of the individual: someone dropping from 35% to 30% body fat might see only small changes (or almost none at all) in these parameters, someone who is getting leaner at the 15% range is seeing bigger problems and someone at 5% body fat (e.g. a natural male bodybuilder) is undergoing massive adaptation.

This is a big part of why dieting gets so much harder as people get leaner, muscle loss accelerates, hormones are crashing, etc. My Ultimate Diet 2.0 goes into much more detail on this topic.

Basically, the body undergoes an overall adapatation that attempts to slow fat/weight loss (via reductions in metabolic rate and activity) and seek out food, these adaptations become stronger the leaner the individual gets (you’ll see that this has implications for how to fix it). I’d note that there is more to the overall adaptation to dieting than just the central effects in the brain; for example, impaired conversion of T4 to T3 in the liver is a well known effect of dieting.

Of course, various hormones have other peripheral effects in terms of energy balance and fat loss; for example leptin directly stimulates fat oxidation in skeletal muscle and a known adaptation to fat loss is a decrease in fat oxidation.

There is also that post-starvation hyperphagia I talked about in an earlier post, whereby signals from fat cells drive hunger to extreme levels when food is made available. Which, I’d note is pretty much always in modern society.

Note again (this ties in with my comments above) that the original observation of post-starvation hyperphagia was made in males who were kept on 50% maintenance calories for 6 months, ultimately reaching a body fat percentage of ~5% (that is, the lower limits of human body fat levels). Someone going from 35% to 30% isn’t going to experience nearly that effect and there’s going to be a continuum of responses from fatter to leaner that’s going to occur.

Finally (ok, probably not finally), leptin also impacts on how well or how poorly other appetite hormones in the body send their signals to the brain (that’s in addition to those other hormones sending a signal to the hypothalamus). For example, cholecystokinin (CCK) is a hormone released from the gut primarily in response to protein or fat intake; it’s involved in making you feel full after a meal. As is turns out, in rats at least, CCK doesn’t work as well when leptin is low.

Hardcore dieters (e.g. contest bodybuilders and figure/fitness competitors) are well aware of this: when they start getting very lean, even if they do everything ‘right’ at a given meal (i.e. lots of lean protein, moderate fat, fiber, moderate amounts of low GI carbs), they simply don’t stay full very long. Because all of the short-term fullness signals just aren’t working as well.

That’s because leptin is essentially setting the overall ‘tone’ of the brain in terms of how it responds to other signals. The various hormones that determine when you get hungry or full aren’t working as well when leptin is lowered from dieting and fat loss. Leptin certainly isn’t the only hormone involved in all of this; but it’s definitely one of the most important ones.

Finally, next time, what to do about all of this (short of not dieting and just staying fat and happy).

Read Bodyweight Regulation Part 5

Bodyweight Regulation: Leptin Part 3

Ok, so now that you know what leptin is and a little bit about what regulates leptin levels, I want to look at what leptin ‘does’ in the body. The short answer is a whole lot of things.

Here’s the long answer:

Like most hormones in the body, leptin has effects nearly everywhere in the body. In skeletal muscle, it’s involved in promoting fat oxidation, it impacts on fat cell metabolism directly, liver metabolism, is involved in immune system function (which may be why dieters get sick when they get very lean) and more recent research is implicating effects on brain function, neurogenesis, breathing and a whole host of other stuff.

Of some interest, leptin levels are crucially involved in both puberty and fertility, it’s been known for decades that a certain level of body fat was required for puberty to hit and achieving critical levels of leptin appears to play a role in allowing puberty to begin.

The handful of folks who don’t produce leptin never hit puberty, for example and it’s thought that some of the reason children may be hitting puberty sooner is because increasing childhood obesity is causing them to hit that critical level sooner.

In a similar vein, leptin is a key factor in regulating fertility, essentially it ‘tells’ the body and brain that it’s well fed enough to spend calories on things like reproduction and making babies. This at least partly explains why dieters are very low levels of body fat lose both sex drive and the ability to function.

Loss of menstrual cycle is a well known effect of dieting and intensive training and while it was always thought to be related to body fat levels per se, it appears that energy availability (which, remember, leptin tells the body about) is a bigger factor. Essentially, when the body ‘senses’ that energy availability is insufficient, it shuts down what are essentially ‘extra activities’ such as reproduction.

In this vein, the most recent ideas about what leptin ‘does’ in the body are that it acts as an adipometer, a measurement of energy stores that tells the brain whether there are sufficient calories available to spend them on things like making bone, maintaining immune function, etc. Essentially the same concept I’m describing here.

My point being that leptin does a lot of stuff in the body, but that’s not mainly what I want to talk about here. Rather, in keeping with the theme of this blog series, I want to talk about leptin’s potential roles in bodyweight/bodyfat regulation.

When it was originally discovered, leptin was originally conceived as an ‘anti-obesity’ hormone, it was thought that leptin should act to prevent weight gain. This led one researcher to quip (and I’m paraphrasing here) that “If leptin is meant to act as an anti-obesity hormone, it has to go down in history as the most ineffective hormone in the human body” or something roughly to that effect.

As I mentioned in previous blog posts, obese individuals invariably have high levels of leptin, raising levels in those folks does little to generate weight loss and because of that failure, everyone sort of moved on in terms of using leptin as a treatment for weight loss.

The problem is that early ideas about leptin were conceptually incorrect; rather than acting as an ‘anti-obesity’ hormone per se, leptin appears to act as more of an ‘anti-starvation’ hormone. That is, leptin doesn’t act to prevent weight gain, it acts to keep you from starving to death.

This reconceptualization would go a long way towards explaining the apparent assymmetry in the bodyweight regulation system I discussed previously: the body doesn’t defend against weight gain very well, it defends tenaciously against weight loss.

Various research found that the drop in leptin was a key aspect triggering (or at least mediating) the effects of starvation (dieting is just starvation on a smaller scale) in humans. In that vein, several studies had individuals diet before replacing leptin to pre-diet levels. This raised metabolic rate, normalized thyroid and increased fat loss. For example.

Basically while trying to raise leptin in overweight individuals is pretty much a bust, preventing leptin from dropping on a diet (or raising it back to normal levels after weight has been lost) is where the real action is.

In this vein, recent work has found that females suffering from amenorrhea (a loss of menstrual cycle) respond to replacement levels of leptin with improvements in reproductive function, bone health, thyroid and overall hormonal axes, etc. Without weight gain.

So now you know basically what leptin ‘does’ in the body at least conceptually: it signals the brain about energy stores (both body fat levels and energy intake) and appears to act primarily as an anti-starvation hormone. Next time I’ll look at mechanistically some of what it does (e.g. impact on appetite, etc) and then about how to go about dealing with this on a diet.

Read Bodyweight Regulation Part 4