Everything in the world is rooted in four essential elements: fire, air, water, and earth. Man and the food that he eats are also rooted in these elements. The essence of one's "life-spirit" is drawn into a person through the food that he eats as is empirically observed. Through this a person's vitality is sustained. But, the fact of the matter is that it is not only through bread alone that a person is sustained, that is, "through all that comes forth from Hashem's mouth," meaning, man is sustained through the "life-spirit" that Hashem draws into the food that is eaten. The essence of this vitality is from the Torah, from where the "life-spirit" is drawn as elaborated inside [both Likutei Moharan and Likutei Halachos].
Therefore, one needs to eat according to the Torah and Halacha, which relates to the concept of, "And Your Torah is within my inner parts." (Tehilim 40:9) No food item has strength to sustain a person unless it is bound up in Torah, which is the source of its vitality. Again, this is a concept of, "All that which comes from Hashem's mouth gives life to man." This corresponds to the brocha that a person must say on every food item he wishes to eat, because the brocha relates to "All that which comes from Hashem's mouth, etc," which relates to the "life-spirit" of holiness. This is the essence of vitality.