Is obesity all in the mind?

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I’ve written before about the fact that many people in the area of obesity research have the opinion that obesity is simply the result of eating more calories than the body burns through metabolism and activity. My position, and that of a growing number of researchers, is that there’s more to obesity than calorie balance, and that body weight and fat mass is regulated by a complex system that involves an array of hormones and feedback mechanisms.

In recent months I’ve become interested in the potential role of a hormone by the name of leptin in obesity. I actually ‘got my brain round’ leptin by reader the blog of Dr Stephen Guyenet, who researches the ‘neurobiology of body fat regulation’. You can read his blog here.

Leptin’s main site of action appears to be the brain (hence the title of this post). Specifically, leptin acts on a part of the brain called the hypothalamus. It’s only the size of an almond, but the hypothalamus controls key bodily functions including hunger, thirst and sleep.

Leptin acts on the hypothalamus and does two main things:

• It speeds the metabolic rate

• It reduces hunger

Leptin sounds like a good thing to have around if we’re looking to maintain a healthy weight. Where can we get some? Leptin is actually secreted by fat cells (chemicals like leptin that originate in fat cells are termed ‘adipokines’). As we put on weight and fat, leptin levels generally rise. The action of leptin on the brain results in us eating less and gives a boost to our metabolism. These effects bring weight down again. As fats stores diminish, leptin levels come down and so we eat a bit more and burn a bit less.

Many researchers believe that leptin plays a key role in the regulation of the body’s weight and fat stores. Some have theorised that as long as leptin does its job properly, there’s no need to consciously control weight. Those individuals who eat what they like and maintain a steady weight probably have, among other things, a well-functioning leptin system.

But, and it is a big but, there is evidence that leptin doesn’t always do its job properly.

One reason for this can be inflammation in the hypothalamus. As a result, leptin doesn’t register so well here, and as a result the body is allowed to be ‘fatter’ than it would ordinarily be. Another reason why leptin may not do its job is because it can fail to get into the brain properly. The general circulation and brain as separated by a type of ‘filter’ known as the ‘blood-brain barrier’. Transport of leptin across the blood-brain barrier is inhibited by blood fats known as triglycerides. Triglycerides are formed in the liver and the major stimulus for their secretion is the consumption of carbohydrate. Eating carbs that cause spikes in blood sugar also stimulate inflammation.

Regular readers know that I am, generally, a fan of carbohydrate-controlled eating. Such diets can help to lower levels of insulin (the chief fat storage hormone) and tend to sate the appetite more effectively than diet low in fat and consciously restricted in calories. It seems another way these diets might help individuals lose weight is by allowing leptin to do its job.

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