Intermittent fasting can be a sustainable, safe, and easy-to-follow way to lose weight and improve your health when practiced together with a nutritious diet and a healthy lifestyle. However, for some people, intermittent fasting is not advisable. While in certain conditions, it's necessary to consult a doctor before starting an intermittent fasting routine.
Under the age of 18 years (children and teenagers). Fasting is not recommended in periods of rapid growth since, during this period, nutrient needs increase. In children, restrictive eating might lead to an eating disorder in adulthood. Moreover, some scientists and health professionals state that it is simply unethical to suggest dieting for this group instead of healthy food choices as they are in the process of learning various food habits and overall eating habits.
People suffering from eating disorders, underweight BMI, or disordered eating (bulimia nervosa and anorexia nervosa). Adults with eating disorders or disordered eating reported significantly more dietary restriction behaviors, including eating fewer meals per day and a higher frequency of fasting with consuming small and low-calorie meals. Intermittent fasting might trigger an increase in such behavior or fall back to old disordered habits.
People with kidney disorders. Longer fasts can potentially increase the risk of kidney damage in those that already have kidney disease. This is most likely because prolonged fasting can lead to volume depletion, which can affect kidney function. Poor health outcomes, such as kidney damage, worsened function, acute kidney failure, and increased heart disease risk, appeared in people suffering from stage 3 or higher kidney disease when fasting.
People suffering from hypothyroidism and thyroid abnormalities. Intermittent fasting is allowed, but people must consult a doctor about possible adjustments of medications for these diseases during intermittent fasting.
People with diabetes. Major adjustments in eating patterns could lead to swings in blood sugar levels. If certain medications or additional insulin is used, eating much less than normal or fasting may cause a significant drop in blood sugar, also called hypoglycemia. If increased hunger appears after the fasting window is over, there is also a possibility of consuming more food than normal, which could possibly cause high blood sugar, also called hyperglycemia. Moreover, fasting for a longer period might cause diabetic ketoacidosis, which occurs when the body does not have enough insulin to move blood sugar into the cells to use for energy. The liver responds by producing too many ketones, which build up in the body and damage the kidneys or can even cause brain swelling.
Breastfeeding/pregnant women. During the fasting window, there is a lack of nutrients needed for the body to produce an adequate amount of breast milk. This is why breast milk production can be suppressed while fasting. Moreover, it is suggested by the research that fasting does not necessarily impact the macronutrients in breast milk but significantly affects some micronutrients, such as potassium, iron, iodine, and vitamin B12. When it comes to fasting during pregnancy, this practice might lead to lower birth weight of a child and a significant negative impact on general fetal growth as well as birth indices.