Recently the Hyderabad based Muslim seminary Jamia Nizamia, started in 1876, issued a ban on Muslims eating prawn, shrimp, and crabs, calling them Makruh Tahrim (abominable)

According to Islam, there are three categories of food : Halal (allowed) , Haram (prohibited, Makruh (strictly to be avoided as abominable)

Most Muslims eat meat, every kind. In fact the religion defines itself by the eating of meat – even though the Holy Prophet was a vegetarian. However, most Muslims have no idea of what they are allowed to eat. The maximum they know is that butchery is divided into two – Muslims eat Halaal and non-Muslims eat Jhatka. (It is another matter that the animals slaughtered in India are neither halaal nor jhatka and make a mockery of both religions) If you have Muslim acquaintances, you could pass this on to them.

There are four categories of food -

1.  Halal - lawful.

A halal slaughter involves a sharp knife. The animal should not see before it is slaughtered; the animal must be well rested and fed before slaughtering, and the slaughtering may not take place in front of other animals.  The jugular vein of the neck should be cut in order to drain all the blood of the live animal and the butcher should invoke Allah's name saying "Bismillah" in order to take the animal's life to meet the lawful need of food. Only vegetarian animals are allowed to be killed. Birds that eat seeds and vegetables are permitted. Birds that eat forbidden items like insects are only permitted if insects are not a major part of their diet. Insects such as Locusts are permitted, all others forbidden. Fruits and Vegetables must be inspected before eating to see that they have no insects. Fish killed by removal from water, or by a blow, are permitted. Shellfish are forbidden. Cheeses coagulated with acid or vegetable enzymes are permitted. Grains are permitted, provided they have not been prepared using animal fats or other forbidden ingredients. Vinegar which is not made from fermenting alcohol is permitted.

2.  Haram - forbidden, unlawful.

Haram is an Arabic term meaning "forbidden".

Acts that are haram are prohibited in the religious texts of the Quran and the Sunnah. If something is considered haram, it remains prohibited no matter how good the intention is, or how honourable the purpose is. 

In Islamic law, dietary prohibitions are said to help with the understanding of divine will.

Muslims are prohibited from consuming flowing blood. Meats that are considered haram, such as pork, dog, cat, monkey, or any other haram animals, can only be considered lawful in emergencies when a person is facing starvation and his life has to be saved through the consumption of this meat. However, these meats are NOT considered a necessity or permissible if his society possesses excess food. All carnivores with fangs such as lions, tigers, wolves, dogs, cats are haram. All birds with talons such as  hawks, falcons, vultures, eagles are haram. Domesticated donkeys are haram. Animals which are commanded to kill such as mice, scorpions, snakes, are haram. In fact all reptiles, amphibians (frogs) and rodents are haram. Any animal that has died before being slaughtered in the Islamic manner, or has not been properly slaughtered, is haram. Animals that are slaughtered in the name of anyone but Allah are prohibited.

Intoxicants, or Khamr, are prohibited in Islam. The Prophet forbade the trading, export, import, gifting of intoxicants, even with non-Muslims. It is not permissible for a Muslim to work in, or own, a place that sells intoxicants. This is not just alcohol but intoxicants, such as tobacco, paan, dokha, and khat. A Muslim is not even allowed to sit at a table where alcohol is being served. Heroin, cocaine, marijuana, and any other substances which cause intoxication, are also forbidden.

Nutmeg, asafoetida, vanilla extract and gelatine are also forbidden, either due to being intoxicants containing alcohol (vanilla extract) or other forbidden items such as pig parts (gelatine). This actually rules out most confectionary, as it contains nutmeg, vanilla extract and gelatine.

Anything made from a human part is haram. (But all commercial biscuits use melted human hair called L Cysteine. And most of the world’s supply comes from the Hindu temple of Tirupati where the hair has been consecrated to the Hindu goddess).

Carnivorous animals, birds of prey and land animals without external ears (i.e. snakes, reptiles, worms, insects etc.)  Since all birds eat insects as the larger part of their diet , this  should technically rule out all of them including chickens. But only Muslims run roadside chicken shops. Foods contaminated with blood or by-products, or any of the above products, is illegal.

3.   Mashbooh, Mushtabahat - questionable or doubtful.

There is a grey area called mushbooh. If one does not know the Halal or Haram status of a particular food or drink it should not be consumed.

4.   Makrooh - inappropriate, distasteful or offensive.

Although makruh actions are less severe than haram, it is recommended to avoid performing them. This will give a Muslim a better chance of reaping Allah's rewards.

Makruh food, determined by the Quran, states that man should only eat pure food, and anything that is impure is regarded as makruh. This includes food that is spoiled or rotten. Into this comes now prawn , shrimp, crabs – all of which are carrion eaters.

So, a Muslim should look out for :

Soup stock made of bones as these are likely to have pig in them, unless specially stated. Any cosmetic (lipstick etc.), or food dye of a pink/red colour as these are usually made from crushed and dried female insects called Cochineal beetles. Lard, which is usually fat from swine  and is used in pastry. Gelatine, which is obtained by boiling the bones, and other waste parts of animals, and forms the basis of most sweets and jelly.

I am not even going into the emulsifiers used in food like Diglycerides and others (E470 to E483) which can be obtained from pork, or non-halal sources, or magnesium stearate which is used in medicine tablets. Even digestives have pepsin: a digestive enzyme made from pig stomachs.