There are none. Exercise is a poor way of losing weight. You have to use large muscle groups for extended periods of time to achieve any significant weight loss. There are no muscles in the brain and there is little extra use of energy when playing chess (Subscription required). See also this:
Quantitatively, metabolic and circulatory changes associated with [brain] activations are also distinctive. These changes are very small relative to the ongoing hemodynamic and metabolic activity of the brain. Attempts to measure whole brain changes in blood flow and metabolism during intense mental activity have failed to demonstrate any change (2). This finding is not entirely surprising considering both the accuracy of the methods and the small size of the observed changes. For example, local changes in blood flow measured with PET during most cognitive tasks are often 5% or less
The most effective way of losing weight is to permanently change your diet to one which consists mainly of highly nutritious food (meat, eggs and dairy) and consists of little (preferably none) of low nutrition foods which are the ones high in carbohydrates.
Brain consumes a lot of glucose during such tournaments
The brain functions best on ketones which the body will manufacture from fat when glucose levels are low. High levels of glucose are very bad for the body and it will take immediate steps to reduce levels when this happens.