Why Do We Yo-Yo?

Biological Factors

We hear a lot these days about the reasons for obesity. "It's fast food!" "It's large portions!" "We're not exercising enough!" It's much easier (and more comprehensible) to focus on the things in our environment and our behavior that may contribute to the problem. And, indeed, understanding these factors is vital, as these are the things we actually have control over. However, I feel that before we start to make environmental changes it's important to start with understanding the foundation, which is what is happening in our bodies.

The regulation of energy intake (food) and output (activity, including internal activity) is tremendously complex, intricate, and finely tuned. In a recent issue of Science, Jeffrey Friedman, expert on the biological causes of obesity, points out that we take in close to 10 million calories over the course of a decade, and that for most of us, our weight varies very little. We may take in a little more one day, have an increase in activity on another - and yet, without much thought, most people naturally adjust and weight stays fairly stable. He goes on to write, "this extraordinary level of precision exceeds by several orders of magnitude the ability of nutritionists to count calories and suggests that conscious factors alone are incapable of precisely regulating calorie intake." Additionally, people of the same height and weight can vary tremendously in the number of calories they use, even in the resting state.

In other words, we've got to find other ways of dealing with weight besides merely counting calories. What are some of the mechanisms that keep some people more well-padded than others?

Genetics

To a large extent, the weight we are is genetically determined - probably more accurately, the weight range we can comfortably be in, unless other factors intervene, is genetically determined. Before recent developments in DNA research, we didn't know much about which genes were contributing. We did know by looking at twin, sibling, and adoption studies that genetics plays a dominant role. For example, in general, adoptive children have weights more similar to their biological than their adoptive parents, no matter how different the environments are. Identical twins reared apart can have vastly different environments vis a vis food and activity level, but often end up looking almost eerily similar in most cases, including weight.

Now, however, the actual genes involved in obesity are being identified - and the situation is incredibly complex. Weight is determined by MANY genes - some more important than others. More than 300 genes (so far) have been linked with obesity. Where our fat is deposited, how insulin is regulated, how much of various hormones our bodies secrete, and many, many more. For most of them, we don't understand the mechanisms yet. The next few years will be most interesting in that regard.

Even though we don't yet know the full implications of our genetics in regards to weight, knowing that individuals vary tremendously is important to know. For example, those who are more prone to insulin resistance syndrome, which certainly has a genetic component, may do better on a different diet that those who aren't predisposed to this syndrome.

Hormones, Peptides, and other Biochemicals

The regulation of appetite is complex, and deserves an article of its own. It's important to understand what is known about it, because what we eat and how we eat it can affect appetite on a short-term basis. One example: a major short term satiety regulator is cholecystokinin, or CCK. When we eat fat or protein, CCK is released, slowing down the digestive process and dampening our appetites. Interestingly, soluble fiber can slow down the breakdown of CCK, keeping us feeling satisfied for longer.

It's the longer-term mechanisms, though, that really influence yo-yo dieting. It seems that as we lose fat, a bunch of regulators step in to say, "Enough! We don't want this body to starve, so start eating!!". Here are some prominent examples, currently in the news.

Ghrelin is a hormone that is secreted from at least two places - the stomach and the brain - and is a trigger for the feeling of hunger. Ghrelin levels go up before meals, and down after meals. After weight loss, ghrelin levels don't go down as much after a meal and are quicker to rise. It's turning out that stomach surgery for obesity decreases ghrelin secretion - in fact, this is likely the most important reason surgery works well when it is successful.

Leptin - Leptin is a hormone produced in the fat cells that has the opposite effect from ghrelin. Normally, leptin levels are associated with satiety. However, many obese people become resistant to the effects of leptin, and despite higher levels of the hormone, do not experience its satiating effects.

Insulin - Insulin is one of the "master hormones" that direct a lot of activity in the body. One of the principle ones is the regulation of glucose use. Rising blood sugar triggers insulin release, which causes energy to be stored as fat for later use. Chronically high levels of insulin (hyperinsulinemia) cause more energy to be stored, and less to be released from storage. Then (some theorize) we become hungry for more food, and are more apt to become overweight. (See also insulin resistance.)

Resistin - Yet another hormone, thought to be involved in promoting insulin resistance. (See above.)

There are many other mechanisms that are being discovered at a rapid clip. Neuropeptide Y, melanocortins, agouti-related protein - the list goes on and on. Here's a diagram that depicts some of them.

Conditions Associated with Overweight

Some medical conditions tend to cause weight gain. Among them are hypothyroidism, polycystic ovarian syndrome (PCOS), and Cushings Disease. Successful treatment of the underlying condition should at least partially alleviate the weight problem.

Viruses?

The first time I heard the theory of a virus being responsible for some obesity, I thought it sounded totally whacky. However, as I've looked into it more, I see that there really is some evidence for it. There is a certain adenovirus, variants of which are known to be associated with obesity in mice, chickens, and other animals. It turns out that there is a similar virus which is raising suspicions of causing overweight in humans. Studies have found that 15-20% of some samples of obese humans have antibodies for the virus, whereas almost no lean people do. Furthermore, the prevalence of the virus has increased in recent decades, at the same time that obesity has been on the rise.

Pharmacology

Many medications impact our systems to cause weight gain. Antidepressants, steroids, hormones, and many others change our physiology enough to cause us to gain weight. Sometimes, it's a few pounds of water that comes off when the medication is stopped, while others will impact the appetite system and cause increased hunger.

Interplay Between Genetics and Environment

Recently, more thinking has turned to how our behavior and the environment may interact with genetics to produce more weight gain in some people. Some examples:

- Calorie-restricted dieting may actually cause weight gain in the long run under some circumstances and/or in some individuals - possible driving the natural range of weight up.

- Studies of immigrants to the United States are showing differences between groups. Whereas the move doesn't seem to affect some ethnic groups much at all in terms of their weight, other groups seem to begin to gain almost from the moment they set foot on the tarmac.

- A recent prevalent dietary change is the increase in the use of high fructose corn syrup in soft drinks and other beverages. Some obesity experts feel that large amounts of fructose may short-circuit the body's usual regulatory mechanisms, making some people more susceptible to Type 2 diabetes and obesity. (This is controversial, but I feel it has good science behind it.)

So much work is currently being done in this area that the next few years should yield much more information about the mechanisms by which some bodies seem to have more of a propensity to gain weight than others. Hopefully, we will be able to use this information in our quest for healthier lives. One of my goals is to stay informed on the latest information and pass it along to you.

Related Links

Return to Yo-Yo Dieting Article

Turtleway Home

My Blog (mostly Diet/Nutrition)

Copyright © 2003-2004 by Laura Dolson. All rights reserved. Please submit reprint requests to turtleway@skyhighway.com

The material on this page and Web site is for informational and educational purposes only, and should not substitute for medical advice. Anyone having questions about the application of information appearing here to a specific person or situation should obtain advice from a qualified health care professional.