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what is obesity?

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What is Obesity?

The disease of obesity is very complex. We know it is not a simple case of calories consumes versus calories burnt. This is far too simple. If this was the case then we would be able to treat all patients with diet and exercise. However most of my patients have tried this often multiple times. One of the frustrating things that patients that tell me is that they go on diets but then when they stop the weight goes back on, often with extra weight.  All this means that there is more to obesity than just diet.

I am a firm believer in obesity being a disease that is driven by gut hormones.  Probably ones that are closely related to insulin or insulin related peptides. These hormones are produced in all of us and regulate appetite, fullness and weight gain. There are large amounts of these hormones produced by the gastro intestinal tract. These are produced to a number of stimuli including food hitting the stomach and food hitting various parts of the gut including the small bowel. When people produce higher quantities of these hormones the body responds by hunger, increased need for food and ultimately weight gain. To disrupt these hormonal signals surgery is sometimes the only way to help. Ultimately a drug that blocks some of these hormones may be developed but we must remember we don’t entirely understand what these hormones are or where they work so a drug will not be easy to develop and may be many years away.

It is these hormones that push people towards eating the wrong foods, primarily foods high in carbohydrates. Carbohydrates are a necessary part of our diet and include sugars and starches. These are important for energy production however if we eat too much the excess is converted to fat.  One of the things that most people I speak to snack on or binge on, is foods high in carbohydrates. These carbohydrates are then converted to fat and the cycle of obesity begins.  It is this addiction to carbohydrate that I believe is the primary cause of obesity in our society.

Although I believe that most people who are obese can’t entirely control their disease, there are ways to help with maintaining a healthy weight. I don’t believe that anybody should not have a regular exercise routine or a healthy diet, but it is soul destroying when you do all the right things and nothing much happens with your weight or health. It is even more soul destroying when your weight increases after a strict diet or exercise programme ends.

So knowing that obesity is a disease we can now understand why it is so hard for some people to lose weight and maintain a healthy weight. Surgery is not an easy thing to accept if you are obese but sometimes it is the only option for sustained weight loss and health benefits.

BMI Calculator




Normal Range 18.5 – 25
Overweight 25 – 30
Obese 30 - 35
Severely Obese 35 – 40
Morbidly Obese 40+


The Physiology of Obesity
Why are People Obese?
Why Diet And Exercise Is So Important
The Physiology of Obesity
Why are People Obese?
Why Diet And Exercise Is So Important

What are these hormones?

The truth is we only understand a small part in the way these substances work but we do know of some research that has confirmed that surgery has an effect on the blood concentrations of certain hormones produced by the gut.


The above link is an interesting piece of research that looked at gut hormone levels pre and post meal. It looked at ghrelin, CCK, GLP-1 and Peptide Y. All of these are thought to be important in regulation appetite, fullness and weight gain.

Hormones such as ghrelin, CCK, Peptide YY (PYY) and GLP 1 and 2 are very important drivers of fullness and satiety.  I will examine each one and then we can explain the mechanism of action of some of the operations that we use for obesity.

Peptide YY
GLP 1 and 2
GLP 1 agonists
Pancreatic Peptide (PY)

A complex hormone that is produced primarily in the gastric fundus or top of the stomach (remember this for later when we talk about sleeve gastrectomy). The pancreas, small bowel and even some parts of the brain also produce it. Ghrelin has been called the hunger hormone by some.

Ghrelin certainly has an effect on appetite but also has some effect on insulin secretion and therefore the ability to convert carbohydrate to fat. This is really important in weight gain and deposition of fat around the abdomen, or the so-called bad fat.

Simplistically we can think of ghrelin release as a signal to the individual that they require food. Once food is consumed the levels drop and we fell full. It also controls how we store the food we eat. I have linked to an interesting article that explains Ghrelin in more detail.


This article gives a good overview of the hormonal changes with obesity surgery.

Peptide YY
This hormone is released by the last part of the small bowel or ileum and the colon. It is likely released in response to food hitting the small bowel. This is the first part of the digestive tract beyond the stomach. When PYY is released you feel full, the stomach stops contracting and the colon absorbs more water. It also has a role in sugar or carbohydrate metabolism. A diet low in carbohydrate and high in protein can lead to higher levels of PYY, this may be part of the reason a paleo diet is effective (thepaleodiet.com).
GLP 1 and 2

Produced by the small bowel. This hormone is currently the subject of intensive investigation in regard to the treatment of diabetes. Higher blood levels increase the sensitivity to insulin. This is very important in type 2 diabetes which in some cases is an increased resistance to insulin. The other thing it is good at doing is making you feel full. It increases satiety or the feeling of fullness.

GLP 1 agonists

These drugs are used to stimulate the receptors that produce GLP 1. They include Victoza and Byetta. These drugs can help to treat type 2 diabetes by increasing the sensitivity of the body to insulin. Theoretically they will also increase the feeling of fullness. More study is needed to fully understand there effect on obesity.

Pancreatic Peptide (PY)

Poorly understood but closely related to PYY with similar effects. This is perhaps the hormone that induces some weight loss with gastric banding.

What can we take away from this?

So we now understand that that feeling of hunger we get before meals is a tightly controlled effect of a variety of hormones produced by the gut. The mechanism of action is poorly understood for most of these hormones. What we can do is study patients who have had obesity surgery and see what the effect is on these hormones.

Let's take a look at hormonal effects of the various operations.

Gastric Band

My thoughts are that bands are purely restrictive. They limit food that can be eaten and work purely on calories in, however there is some data that pressure on the vagus nerve may induce release of PY. This may lead to satiety and fullness. This probably is important in the early period after banding but I believe it is not a permanent change.

I have seen band patients have permanent changes to satiety and without doubt these patient have some hormonal change however it is common to see this effect wane in most people. What often happens is that people will begin to rely on restriction, which is very dangerous as the esophagus can only have so much pressure put on it before it begins to dilate. Esophageal dilatation is a difficult problem and is a common reason for band removal.

Sleeve Gastrectomy and Roux en Y gastric Bypass.

Removing the gastric fundus reduces circulating levels of ghrelin. This leads to less appetite and changes in the desire to eat certain foods including carbohydrates. Perhaps this is why people often say that sugar tastes too sweet after a sleeve operation. Studies have shown that there is a 60% permanent reduction in ghrelin production after a sleeve. 

The same reduction in ghrelin can be seen after bypass but the levels tend to return after 12 -18 months. Remember in RYGB we leave the fundus behind. Perhaps this is why weight loss can be resumed after removing the stomach in patients who regain weight after a RYGB? There is no definitive evidence of this at present but the research continues.

The release of PYY and GLP 1 is also higher after sleeve and RYGB. This is likely because the stomach empties into the small bowel bowel quicker and induces release of these hormones. This also has an effect on insulin release and insulin resistance meaning that even without weight loss there is an immediate improvement in type 2 diabetes. RYGB tends to increase the levels of these hormones to a higher level and perhaps this is why RYGB has a greater effect on the diabetic patient. It is not uncommon to see patients leave hospital without the need for insulin or other diabetic medication.

For further information on RYGB and Sleeve gastrectomy see the section on operations for obesity.


Victorian Obesity Surgery Centre
5 Burgundy Street
Heidelberg VIC 3084

03 9450 6800     View Map


The Gardens Medical Centre
Level 6, 470 Wodonga Place
Albury NSW 2640

03 9450 6800     View Map