Eat little carbohydrate (less than 60 grammes of carbohydrate (not counting fibre) per day) for as long as you can (up to about a couple of weeks) before the exam. This will reduce your insulin level and raise your glucagon level. Glucagon is a diuretic and vasodilator, and insulin is an antidiuretic and vasoconstrictor, so a result will be to lower your blood pressure, and you may shed a few pounds, mostly of water. (I lost 4 kg (9 pounds) in a week when I took up this diet.) And there will be no traces of diuretic drugs or beta-blockers in your blood.
Another effect will be to lower your low density lipoproteins and raise your high-density lipoproteins, lowering your overall cholesterol. The effect should be to make you look a lot better on risk factors for heart attack and stroke.
You should be able to achieve this by cutting out cereals, bread, batter, pastry, rice, pasta, polenta, oatmeal, cous-cous, potatoes, root vegetables in general, sugar, candy, non-diet lolly-water (soda-pop), fruit juice, and very sweet fruits. Eat plenty of green leafy vegetables, celery, mushrooms, cauliflower, and broccoli, and as much meat, poultry, fish, eggs, cheese, and tofu as you want. Don’t worry about how much fat you are eating, but don’t go overboard.
I would suggest that you not eat less than 30 g of available carbohydrate per day. If you did that you might have ketones in your blood and urine. This is harmless in itself, but it is most often found as a sign of diabetics on the verge of insulin coma. It will not be taken as a good sign. You might like to buy a package of ketosticks from a drugstore and test your own urine a few days before your exam: if there are ketones present, edge your carbohydrate up gradually until your urine is back to normal.
Drink plenty of water, but stay off the grog. Coffee will do you no harm, but don’t have more than seven or eight cups in the 24 hours before the test or it may look as though you take stimulants.
If you have a few weeks, eat plenty of red meat and perhaps liver, take vitamin B12 (or eat foods rich in it), and take plenty of aerobic exercise. This will help get your haemoglobin up and make you look fitter.