Here is a nice explaination of how MSG foments obeisity.
More evidence links MSG with obesity
Monosodium glutamate (MSG) is a flavour enhancer used liberally in processed foods and Asian cuisine. While MSG is a permitted ingredient in the food supply, there have in the past been questions asked about its potential role in weight gain and obesity.
In 2008, I wrote a post  about research linking MSG with increased body weight. This association was found even when ‘confounders’ such as food intake and activity levels were taken into account.
A similar piece of research surfaced recently – published in the American Journal of Clinical Nutrition . The relationship between MSG intakes and body mass index (BMI) was assessed in more than 10,000 Chinese adults over an average of 5½ years.
The results showed that higher MSG levels were associated with higher BMI (and greater risk of being overweight). Again, the association remained even after factors such as food intake and activity levels were taken into consideration.
If you believe that weight is only determined by caloric balance, then these results will not make sense. The authors of the study don’t make that mistake, though, and go on to discuss the potential mechanisms behind their findings.
In particular, they focus on the impact glutamate can have on a part of the brain known as the hypothalamus. Although small in size, the hypothalamus has a regulatory role in key processes including metabolism and hunger.
Recently, one of my posts  focused on the role that the hormone leptin has in body weight regulation through its impact on the hypothalamus. Leptin suppresses appetite and stimulates the metabolism. However, if it doesn’t do its job properly (‘leptin resistance’), this does not bode well for maintenance of a healthy body weight.
As the authors of the MSG paper point out, glutamate has the ability to damage the hypothalamus, and has the ability to cause leptin resistance. This mechanism alone, and its potential impact on metabolism, could explain how MSG might predispose to obesity, even when food intake and activity levels are taken into account.
If you want to avoid MSG, ensure your diet is made ostensibly from natural, unprocessed foods. And avoid soy sauce and other condiments likely to be laced with MSG including gravy mixes and concentrates.