THURSDAY, May 26th, 2011 (www.drsharma.ca) — In previous posts this week, I discussed the risk and potential benefits of bariatric surgery and explained why for someone with severe obesity and significant comorbidities, current evidence comes down heavily on the benefit side, whereas for someone with obesity but no complications, the risk/benefit ratio may not be all that positive.
In today’s post, I would like to look at why bariatric surgery works and hopefully dispel some common misconceptions about what bariatric surgery actually entails.
However, to fully understand why bariatric surgery should even be considered an option, we need to first understand why it is so difficult to lose weight and keep it off.
Readers will recall last week’s discussion on how any weight loss results in a ‘hypometabolic’ and orexogenic state – in short, weight loss drastically reduces the number of calories burnt while increasing hunger and appetite.
This is exactly what makes keeping weight off so difficult – as metabolism slows down and appetite increases, keeping weight off becomes a daily battle – a battle that lasts forever (the more weight you lose, the greater the struggle). This is why only a dedicated few, for whom weight management becomes nothing short of a daily obsession, manage to keep substantial amounts of weight off.
Everyone else, eventually gives in – most people can simply not endure constant restrictions or hunger forever.
Remember we are not talking about simply expecting someone who weighed 300 lbs to lose 50 lbs and from now on live on the same amount of food that a never-obese 250 lb person would normally eat.
No! To sustain the 50 lbs wieght loss, the formerly 300 lb person would need to perhaps survive on the amount of food that a never-obese 200 lb person would normally eat (or less!).
So expecting someone, who normally would have eaten 2500-3000 KCal a day (or more) to, from now on, survive on 1500 KCal a day or less, is a pretty hard sell – especially, as this person, thanks to the orexogenic response to weight loss, would be constantly hungry and thinking of food.
To make this kind of weight loss possible (even in the short-term), virtually all popular diets resort to certain ‘tricks’ to reduce hunger and increase satiety.
Increasing protein intake while drastically reducing carbs is one common variation (e.g. Atkins diet) – this approach takes advantage of both the satiating effect of protein and the anorexic effect of ketosis. This strategy, of course works fine as long as you can stick with it – but adding even a few more carbs or reducing the amount of protein immediately brings back the hunger and you fall off. Very few individuals manage to walk the fine line between hunger management and falling off – many simply get bored.
Another popular trick is to bulk up the food with lots of fruit, vegetables, or simply adding fibre supplements with plenty of fluids. The idea here is that these foods will expand in the stomach and hopefully fill it enough to create a sense of fullness despite eating fewer calories. This may well work for some people, but remember, the stomach is the size of a small football – it takes a lot of food to fill it.
Also, eating large quantities of fruits, vegetables, legumes, high-quality protein and complex carbs, rather unfortunately, given today’s nutritional landscape, is not only impractical, inconvenient, and expensive – it also requires a substantial time commitment and other changes in lifestyle (e.g. regular shopping for fresh ingredients and home cooking).
The third trick is to simply avoid high glycemic index (HGI) foods (especially refined sugars and other carbs), which (at least theoretically) reduces the ‘eat-crash-and-crave’ and ‘antilipolytic” response to the hyperinsulinemic surge that comes from ingesting readily digestible carbs. However, evidence that this is a viable long-term weight-loss or maintenance strategy, is rather limited.
So the bottom line is that sustaining weight loss with dietary restrictions alone requires both subtantial dedication and some clever and sometimes drastic modifications of your dietary intake to make it sustainable.
For the sake of brevity, I do not want to go into a discussion about the rather important role of exercise in all of this, as I am trying to get to the issue of how bariatric surgery works.
But before I get to the surgery bit, here is one last important piece of information regarding how these dietary strategies affect ingestive behaviour.
Most of the above dietary strategies focus on the homeostatic system, i.e. hunger and satiety. Only the Atkins diet, which also allows chocolate and other high-fat goodies as long as they are low in carbs, also caters to the hedonic system, i.e. appetite and reward – which is perhaps why some people find it somewhat easier to stick with.
So here in short is the problem that every obese person, trying to maintain a significant amount of weight faces: losing weight activates both the hemeostatic system (more hunger – less satiety) and the hedonic system (greater appetite, especially for highly palatable energy dense foods that are especially ‘rewarding’) – two systems that will eventually wear down even the most determined dieter.
This is where bariatric surgery can provide help.
In principle, there are two mechanisms by which bariatric surgery can work:
a) Reducing the size of the stomach or otherwise slowing the passage of food, thereby eliciting a stronger and longer-lasting feeling of satiety.
b) Bypassing a significant proportion of the gut to create maldigestion, which means that a proportion of the eaten calories will not be digested and absorbed.
These two principles are referred to as ‘restrictive’ and ‘maldigestive’ surgery, respectively.
Although this sounds simple enough (and variations of both principles have been around for over 50 years), the actual biology of how these operations really work is only now being understood.
Thus, contrary to popular belief, restrictive surgery (formerly referred to as ’stomach-stapling’ or ‘vertical gastric banding’ (VGB) and its modern cousin, the adjustable gastric band (AGB)), does not work by simply making it difficult to eat.
If this was the case, you would see the same results from simply wiring your jaw.
The real reason restrictive surgery works is because it sends powerful neuronal and hormonal signals to the brain to create an early and strong sense of satiation, thereby reducing the need to eat large portions.
In other words, restrictive surgery tricks the brain into thinking that you have eaten a 12 ounce steak, when all you have eaten is 4 ounces. Suddenly, those tiny serving sizes shown to you by the dietitian is really all you need to feel completely full and satiated – portion control is no longer a problem. Wiring your jaw does not produce that sense of fullness but putting a band around the upper part of your stomach does – this is why banding works while jaw wiring does not.
A variation of this approach is the increasingly popular vertical sleeve gastrectomy (VSG), which essentially reduces the size of the stomach to that of a small banana. Once again, this operation works because, it no longer takes a 12 ounce steak to feel full.
In addition, VSG also removes a large part of the stomach that produces the hunger hormone ghrelin. This is why many patients with VSG no longer feel as hungry as before (some patients literally say that this is the first time in their life that they have never felt hungry).
Thus, VSG has two modes of action: it significantly reduces hunger while increasing satiation with smaller portions. Suddenly, eating less is no longer that difficult – imagine losing weight without being hungry and not having to eat huge portions to feel full anymore.
The mode of action of the ‘gold-standard’ Gastric Bypass (GB) is even more complicated. Not only is the size of the stomach reduced (greater satiety), the remaining detached stomach secretes less ghrelin (less hunger) but the food also bypasses part of the gut, which affects food digestion and absorbtion (maldigestion).
But even this ‘triple whammy’ is not the full story. It turns out that the way in which this operation redirects food by bypassing the duodenum also has a profound effect on the secretion of gut hormones like GLP-1, which control insulin secretion and other metabolic responses.
In fact, experiments with devices that simply prevent this part of the gut from coming in contact with food (as in the endoluminal sleeve or ‘duodenal condom’), lead to an almost instant improvement in type 2 diabetes, even without any appreciable weight loss (although in the long-term, improvement in glycemic control generally tends to be proprtional to the degree weight lost). This mechanism of action is commonly referred to as the ‘foregut hypthesis’.
In addition, it may also be that this operation, by allowing more undigested food to rapidly enter the large intestine, leads to a release of gut hormones like PYY-36, which delivers a potent satiation signal to the hypothalamus. This effect is referred to as the ‘hindgut hypothesis’.
Thus, each of these bariatric operations, by different means, tricks the ’starving’ brain into thinking that it is still getting all the calories it needs thereby ‘accommodating’ the hypometabolic response by allowing the ingestion (or absorbtion) of fewer calories, while at the same time ‘overriding’ the orexegenic response to weight loss.
This is what allows patients to survive on as little as 1400 KCal per day without feeling hungry – a feat that takes an almost inhuman amount of willpower to do otherwise.
So why do some people fail with surgery?
The short answer would be because surgery, primarily affects the homeostatic system (hunger and satiety) of ingestive behaviour and not so much the hedonic system (appetite and reward).
In other words, bariatric surgery deals well with the issue of ‘being hungry all the time’ and ‘never feeling full’ (especially after weight loss) but not so well with the issue of emotional eating or food-addiction.
The latter is not always true, because not being hungry and feeling full (not to say ’stuffed’) also affects the hedonic system – but only indirectly – certainly not enough to fully stop emotional eating (see tomorrow’s post for more on this issue).
And of course, any form of surgery can be ’sabotaged’ by not following the recommended diet – you can always still drink your calories or graze all day and and gain all the weight back – no bariatric surgery will stop that.
Fortunately, ’self-sabotage’ is the exception and not the rule and needs to be dealt with in a very different manner – remember, the surgery is on your gut and not your brain.
So what exactly do patients have to do to ensure success of their surgery? Why is surgery anything but a quick or simple fix? And, what are the potential long-term complications of surgery?
More on this in coming posts.