Question from Brentwood, Essex, United Kingdom:

My seven year old son occasionally suffers from fatigue and irritability, but as soon as he eats something he becomes full of energy and happy again. (He has been like this for as long as we can remember.) He is slim, fit and eats a very balanced diet (occasional diet soda but mainly drinks semi-skimmed milk and water). A friend of ours witnessed these symptoms and said it was very similar to how her son acted prior to being diagnosed with diabetes. We are very conscious of these symptoms because he has a twin brother who lives an identical lifestyle but never has this problem. He does not seem to suffer any other symptoms (e.g., thirst, urinating frequently, sweating at night or weight loss). On the contrary, his brother sweats more, drinks more and urinates more than him and has always done so.

We do not believe he has diabetes but since it was brought to our attention we are monitoring him and wish to be better informed. Is there a simple test that can be purchased over the counter?