I suppose it could be both. But as it appears, eventually there is a bit of hormonal basis. For example, high fat is associated with a set of alterations in endocrine balance, most common being hyperinsulinemia which is a kind of counter-regulatory measure. High levels of cortisol/corticosterone is associated with insulin resistance.

Further, i firmly believe that we really have to look at type of fuel than energy balance it self. It is exemplified by the fact that not all types of fatty acids lead to insulin resistance. Also, equal amount of glucose and fructose will have different impact.

I wont be wrong in saying that a little extra or more of starch is not dangerous as compared to same amount of sucrose or fructose.

Energy balance first and foremost. Perhaps a smaller role for hormonal status that, when combined with overeating, results in the greatest weight gain. Hypothyroidism and Cushings are not as common as the general public think, yet I hear a lot of overweight individuals attributing their weight-gain to these conditions. A big problem is GP's using vague diagnostic terms like "hormonal imbalances" to explain why the world has a weight problem.

Thanks for the great responses guys. I sit on the side of the fence that says overeating is the cause of the obesity and any "hormonal imbalance" is secondary to the overeating in the first instance. So for me obesity will always remain an energy balance equation. No human being can deny the laws of thermodynamics no matter what their homonal status is. So if the same goes out as comes in you'll be fine.

There is much more to obesity - and many other conditions - than mere energy balance.

The real problem is that we eat foods which we are completely inappropriate for us as a species. We in industrialised countries eat a diet which is based on carbohydrates. But no other mammal on the planet does - including human cultures which still live a traditional lifestyle. (see http://www.second-opinions.co.uk/should-all-animals-eat-a-high-fat-low-carb-diet.html ).

To that, add the fact that only dietary carbohydrates cause obesity (see http://www.second-opinions.co.uk/carbs-weight-gain.html), and over a century of evidence that dietary fats are slimming, (http://www.curediabeteswithdiet.org/healthy-diet-weight-gain.html) and the real picture is a lot clearer.

So, yes we eat too much in calorie terms, but we only do so because our 'healthy' diet forces us to eat an unnatural - fattening - diet, for which we are not genetically adapted.

Being interested in physiology, I hate to admit it but I think the most important factor in obesity is probably psychological. The regulatory hormones play a role in controlled lab settings, but these are completely overidden by emotions when people are presented with highly palatable foods (I get hunger when I smell greggs!). I also agree with some of the previous points made regardign macronutrients where they differ in their contributions (protein will stimulate muscle protein synthesis and therefore not contribute to as much of a gain in fat mass if excessively consumed).

@Barry Goves - thanks for a great response, I agree with you to a certain degree in that the high availability of CHO's have made it much easier to consume a hyper-caloric diet and in most instances we have not evolved to eat the diets we do. I hate to use anecdotes but check out the link to the story about the "Twinki Professor" http://www.cnn.co.uk/2010/HEALTH/11/08/twinkie.diet.professor/index.html

He ate a calorie restricted diet composed mostly of twinkies - he lost weight and measures of his metabolic health improved. I also used to diet to make weight and I found that the macro-nutrient compostion of my diet didn't matter for making weight - only restricting calories allowed me to make weight. I also agree on the idea of fat not being the bad guy.

@JavierGonzalez - thanks for the great input. I guess the one point I would make (and you'll probably know more about this than me) is the link between the hormones and the appetite control centres in our brains and how these link in with the hedonic response to things like smelling Greggs (makes me very hungry too). So in that sense I totally agree on you with the psychology and these interactions between nutrition and apetite regulating hormones can influence the psychology of eating behaviour and alter the drive to eat and I think that exercise and diet compostion can have its role to play. In this context I very much agree on what you say about the dietery protein for instance Rachel Batterman showed that a high protein diet in rodents induced satiety and promoted weight loss and this was mediated by the regulation PYY http://www.sciencedirect.com/science/article/pii/S1550413106002713

So perhpaps there is slightly more to it than simple energy balance :-)

http://aje.oxfordjournals.org/content/176/suppl_7/S44.long

Check out the above link to this meta-analysis on low carb vs low fat diets. Low carb diets slightly better on metabolic risk factors but weight loss the same. Food for thought :-)

@David Hamilton, completely agree and higher protein diet also recently shown to reduce spontaneous energy intake in humans http://ajcn.nutrition.org/content/97/1/86.full?sid=94e987de-0ad8-4fa8-8266-21da87f8ed0e

Fantastic abstract - backs up Rachel Batterman's animal work nicely. Big sample sizes too. If you have the pdf could you email it to me? Can't access it from here.

Not really, David. The authors are seemingly unaware of basic facts, although I make that claim based on the abstract as I do not have full access and on similar studies I have read in full.

Firstly, a diet where 45% of calories are from carbs is not a low-carb diet. A true low-carb diet would have no more than about 15% of calories from carbs.

Secondly, with a low-carb diet, ad libitum intake, which is the norm, weight will only be lost such that the subject only reduces weight down to their normal weight. It is impossible to go underweight. This is one of the reasons why a low-carb diet is healthier that a low-fat diet, which is invariably calorie controlled, meaning that there is no limit to weight loss.

For that reason, once a person eating a low-carb diet approaches her normal weight, weight loss will slow down and once normal weight is reached, it will cease. On low-fat diet the dieter can lose weight until she is well underweight. These two facts, together, mean that no matter how much more quickly a person loses weight on a low-carb diet the dieter on a low-fat diet will always catch up eventually. This, of course, allows the researcher to claim that the two diet are of equal value.

I am gratified that this study confirmed previous ones in finding that the low-carb diet was healthier in lipid profiles. If they had used a true low-carb, high-fat diet, however, those figures would have been even better.