Referring again to the list on page CLVI in Secret Teachings of All Ages, alchemists intimate Digestion as the fifth step in the alchemical process by which they confect their Philosophers’ Stone. In the twelve-stepped process, Digestion is attributed to the fifth sign of the zodiac, Leo. In the human body Leo governs the heart and spinal column. However, it is the Serpent Power, Kundalini, inside the spinal column, but rooted in the Saturn Chakra, that is eventually sublimated through this process. The result is the opening of the Heart center, the Gold. The Sun, Gold, rules Leo and Digestion. None of this unfolds, however, until the physical body is ready. The body's cellular structure must be sufficiently prepared to withstand the intense influx of light-power. Alchemists consider the optimal health of the physical body the gross work. The Alchemical Woman, sub-consciousnesss, affects this through the proper digestion of food. The Strength Tarot Key, presented in the Rider and B.O.T.A. versions, offers a distinctive, pictorial explanation. There the Alchemical Woman embraces the Red Lion (the passionate, lower, animal body, and the raw will-sex urge) in a loving and caring manner, as if she were actually molding a finer vehicle from clay. The physical structure is only as strong as the materials with which we give her to work. We tender careful balances of fire, water, air and earth. Strength, Leo, and therefore Digestion, are assigned North-Above on the Qabalistic Cube. The position joins the top face assigned to Mercury with the North face attributed to Mars. This hints that the elemental foodstuffs we choose through our conscious selection (Mercury) are given over to the re-channeled reproductive power of the Mars center as new energy to rebuild and maintain our physical bodies. Foodstuffs are converted into cellular energy via Digestion. Body cells built from minerals in vegetables are ones that hold the most light-power and resist rapid deterioration. Cells manufactured from fatty, heavy or useless foods burn up quickly from the Fires of Life, resulting in sickness and premature physical death.
In this regard may I suggest reading "From Dust Thou Art" in the Articles section at the top menu bar.
The step Digestion ought not be considered a mere phase of the alchemical process. Proper digestion of sensible food is a prudent measure throughout the life, even if we are not interested in alchemy. On the Tree of Life, the Tarot Key Strength occupies the nineteenth path connecting Geburah, the sphere of Mars, to Chesed, the sphere of Jupiter.
The path is called שכל סוד הפעולות הרוחניות, Saykel Sod ha-Pehulloth ha-Ruachnioth, 'the Intelligence of the Secret of All Spiritual Acts,' and assigned letter Teth, the Serpent. The implications are best explained in a fifteenth century alchemical drawing from the British Museum Library, reproduced in the paperback edition of Titus Burckhardt’s Alchemy. The picture facing page 129 illustrates a toad holding up a crescent moon. Cradled within the curve of the crescent rests a solar disk held in a serpent's mouth. The serpent is entwined in the shape of a figure eight, knotted at its center, above which poses an eagle in flight. Twenty-two fatty droplets appear in the background. All this illustrates the Saturn force, pictured as a toad, raised up through the combined actions of Sun and Moon. This is the Serpent Power, Kundalini, modified at the Digestion stage and symbolized by the path of Teth and Tarot Key Strength, turned into the power of the White Eagle. An eagle is the bird of Jupiter (Chesed). What was once self-destructive will-force is now a unifying, healing beneficence, giving the alchemist a subtle potency to be used in other steps of the Work. Subconsciousness performs all this through self-conscious suggestions with attention to diet. Twenty-two background droplets of corpuscular 'fatty dew' represent the potencies behind the Hebrew alphabet within the 'oily fatness' of the First Matter.
The inner perception of Prima Materia for human consciousness is the direct result of a steady rise of increasing amounts of Serpent Power, Kundalini. A healthy body at the cellular level attracts light. This is the core purpose for the study of Alchemy.