When you crave a certain food, is this the body's way of telling you what nutrients you need? Or is it simply what appeals to you at the time?

I agree with Kim on this — cravings aren't usually caused by a lack of nutrients, but I would still argue that this can also be the case!

As Kim's done such a brilliant job in explaining what usually causes cravings, I'll let you read her answer and will, instead, explain why I believe that cravings can sometimes be your body's way of telling you what nutrients you need.

Cravings And Mineral Or Nutrient Deficiencies

I'm anaemic, so I need much more iron than most people. If I've forgotten to take my iron tablets for a while, I often having cravings I can't explain, such as spinach.

I don't dislike spinach by any means, but it seems a bit odd to crave a bag of it for dinner. If I indulge these cravings, I don't tend to have anything with the spinach, so the only thing I can possibly be craving here is the nutrients in spinach. Spinach is particularly high in iron and vitamin C (the vitamin that helps your body to absorb iron).

Food-group Cravings

I also find that if I've been eating too many carbs, I start to crave high-protein foods such as fish, eggs, nuts, and meat. My body runs better on protein and I always feel better if my diet is protein-heavy, but I can't tell whether it's my body or my thoughts that use cravings to "remind" me what to eat!

A craving is a strong desire to eat a certain particular food.  It is much more than hunger and comfort eating.

Professor Marion Hetherington says ' Food most likely to be craved is high energy, high calorie and high fat food.  They are treat foods that are usually restricted.  People don't crave broccoli or cabbage.  It is more likely to be sweets or chocolates.'

The craving does not come from your stomach but your brain, which makes it much more complex. 

You normally crave food for 2 reasons:

1. You need a boost of energy
2. You are miserable

Energy

People crave high calorie foods, high in carbohydrate, fat and sugar because they give them a fast burst of energy.  It would be more beneficial to eat something like nuts or yoghurts which would keep you fuller longer.

Mood

Nutritionist, Liz Tucker says 'We have a primitive response, as humans, we live to experience pleasure, through sex and relationships and also through food and drink.  If you are miserable or in a crisis in one area of your life, you aren't getting pleasure from it and turn to food to give you that happiness kick.' 

It is a common myth that you crave food to fill a nutritional deficiency.

"It is very rare that people in the UK have deficiencies that lead to cravings," according to Professor Hetherington. "Most cravings are categorised in terms of pleasure and reward."

There has been an experiment where young people have been put on a nutrient full liquid diet and they have craved foods with texture, like steak and pasta.  This indicates that the texture of food in your mouth may also add to the craving.  However, ultimately it is your brain that requires the food, not your belly.

So no more blaming the fact you are low in salt, when you sit and eat a family-sized packet of crisps.

To answer your question, it is what you fancy at the time. To stop yourself from bingeing, follow a healthy diet with foods that will make you feel fuller for longer, and you should be able to control these cravings.