I've lost weight and also muscle tone. my arms fatigue easily. how would i know the difference between sarcopenia from inactivity and cancer cachexia?

See a doctor: The symptoms you described warrant a more detailed history, physical examination and may be some tests. It would be prudent to consult your doctor for an evaluation. In mean time try doing some exercise, even just walking and increase the pace gradually over days and weeks. If the weakness is due to inactivity, it will improve with time. For good health - Have a diet rich in fresh vegetables, fruits, whole grains, milk and milk products, nuts, beans, legumes, lentils and small amounts of lean meats. Avoid saturated fats. Drink enough water daily, so that your urine is mostly colorless. Exercise at least 150 minutes/week and increase the intensity of exercise gradually. Do not use tobacco, alcohol, weed or street drugs in any form. Practice safe sex.