When the appetite is decrease it is very commonly helped with a good source of B vitamins and possibly phosphorus. Supporting with Bio B 100 and Super Phosphozyme may do the trick. Low thyroid function can also cause a poor ability to break down and use carbohydrates. So avoiding sweets is important. Just eating a lot of junk food can burn up your B vitamins and not replace them. Same with Coffee, tobacco and alcohol.
There may also be a need to have a doctor look for heavy metal poisoning.
If a person supports with digestive enzymes for the stomach called HCl , this can improve the appetite in many cases.
There are a lot of other causes but seeing a doctor is important to avoid malnutrition and handle the cause.
If it is a broken heart, one can always go out side and take a walk and extrovert ones attention onto the environment and walk until you have seen it all and you are hungry. Getting a puppy is helpful too.