Sour tends to have an affinity for the Liver and Gall Bladder. Sometimes even sugar is included in an herbal formula because the person needs it. For example, salt is harmful for many people and will raise blood pressure in many individuals. These people will benefit from a low salt diet. But a low salt diet can have a devastating effect on people with adrenal insufficiency or Neurally Mediated Hypotension.
Some people require more fat in the diet than others.
Children in particular can develop health and growth problems when overzealous parents limit fat intake too much in their diets. Some people have greater than normal needs for certain vitamins or minerals because of genetics or because of an assault to their systems. A shortage of iodine can cause goiter, but too much can trigger some cases of hyperthyroidism. But you can eat the wrong things at the wrong time and aggrevate an existing condition. The same precautions apply if a certain herbal treatment is applied over a long period of time.