Humans necessitate vital nutrients from 5 broad classes: proteins, fats, carbohydrates, vitamins and minerals. Essential amino acids (protein) are required for cell, especially muscle, construction. Essential fatty acids are required for brain and cell wall construction. Vitamins and minerals are important for many functions.
 Any diet that fails to meet minimum nutritional requirements can make threats to general health (and physical fitness in particular). If a person is not well enough to be active, weight loss and good quality of life will be doubtful.
The National Academy of Sciences and the World Health Organization bring out guiding principle for dietary intakes of all known essential nutrients.
Sometimes dieters will consume excessive amounts of vitamin and mineral supplements. At the same time this is usually harmless, as some nutrients are dangerous. Men (and women who don't menstruate) need to be wary of iron poisoning. Retinol (oil-soluble vitamin A) is toxic in large doses. On the whole most people can get the nutrition they need from foods (there are specific exceptions; vegans often need to supplement vitamin B-12). In any case, a multivitamin taken once a day will be enough for the majority of the population.
A reasonable weight-loss diet is a normal balanced diet; it just comes with smaller portions and perhaps some substitutions (e.g. low-fat milk, or less salad dressing). Extreme diets may lead to malnutrition, and are less likely to be effective at long-standing weight loss anyhow.
| 
