Definative answer from NHS Direct, but not about food poisoning.
Drink fluids
You can avoid becoming dehydrated by drinking lots of fluids. You are more likely to be dehydrated if you are also vomiting.
Take small, frequent sips of water or diluted fruit juice.
Rehydration drinks
If you are worried that you are becoming dehydrated, your doctor or pharmacist may advise you to take rehydration drinks. You can buy sachets of rehydration salts from your pharmacy and add them to water. They provide the correct balance of water, salt and sugar.
Rehydration drinks do not help to cure diarrhoea, but can prevent or treat dehydration.
Eat as soon as you can
The old advice was to not eat anything for a day or two, but now it is recommended that you eat foods high in carbohydrates (bread, pasta, rice or potatoes) and other foods as soon as you feel like it. Salty foods such as soup can help replace salt lost from your system.
If you feel you cannot eat, it shouldn't do you any harm, but make sure you continue drinking fluids and eat as soon as you are able.