If a person doesn't want to do a complete (water) fast, but prefers to eat, then yes, you're right, foods that provide a large amount of energy relative to the amount of energy needed to digest them are the best choice. But foods which are high in energy, but difficult to digest (such as meat) are best avoided during illness. Fruits such apples are recommended because they are so light & easy to digest, and also because they are high in pectin, a soluble fibre that helps to carry toxins out through the bowel. Apples are especially beneficial for that reason, but other sweet fruits, such as grapes, melon etc, are also good.
But if there is no appetite for food (which is common with many illnesses), then it's usually best if no energy is used for digestion, to maximise the energy available for healing. The body stores energy in the form of glycogen in the liver & muscles (which will last for about 3 days), and also in body fat (which lasts up to 40-60 days, or more for an overweight person), so there is no need to eat for energy during a short detox period. This assumes that the person can also have complete bedrest at this time. Certainly if that isn't possible, and they have to carry on working etc, then light food or juices should be taken, but it's still best not to overload the body with heavy food.
But if there is no appetite for food (which is common with many illnesses), then it's usually best if no energy is used for digestion, to maximise the energy available for healing. The body stores energy in the form of glycogen in the liver & muscles (which will last for about 3 days), and also in body fat (which lasts up to 40-60 days, or more for an overweight person), so there is no need to eat for energy during a short detox period. This assumes that the person can also have complete bedrest at this time. Certainly if that isn't possible, and they have to carry on working etc, then light food or juices should be taken, but it's still best not to overload the body with heavy food.