A doctor takes a single class in nutrition to graduate from a medical school in the USA and so they are the least knowledgeable professionals you will encounter. As we age our bodies process nutrient less well and so it is not unusual to need supplements like zinc, magnesium, potassium, vitamins C, D, E, and CoQ10. My first blood serum test for Vitamin D put be in the low 20% and it takes 5000 IU a day to get me into the 40% range. The 5000 IU is much higher than is recommended as the studies look at levels needed to prevent bone fractures and ignore how Vitamin D is used by the body's organs and how low levels are correlated to increased heart disease and cancers and to the complication from people infected with Covid. Same applies to Vitamin C where the focus was on preventing scurvy and ignoring all the other areas where it is critical for human health. Iron is another factor that can be too low or too high and not part of a regular blood workup.
Food allergies are worth exploring as well. For years I suffered from nasal congestion and asthma and noticed that when I traveled out of town my symptoms would clear up completely. Took a long while for me to realize the problem was eating food containing wheat. No wheat and no nasal congestion and no asthma and no need for corticosteroid to treat the asthma or decongestants to minimize nasal congestion. No GP or specialist even mentioned food allergy as a problem and only focused on treating the symptoms.
There are doctors that have done additional education and are licensed as "functional medicine" practitioners. These doctors are far more aware of the complexity of the human body and how nutrition and various allergies or lack of key nutrients can affect their patients' health. With all the factory food the micro nutrients in food have diminished while the toxic content from herbicides and pesticides and antibiotics has increase tremendously in the past 40 years. We have all been unwitting volunteers in the mass experiments by the chemical companies.