What does a white coating on your tongue mean?

Every tongue is supposed to have a coat. The tradition says the stomach’s qi makes it, rising the way steam rises from a pot, and a healthy one is thin, white and even, so that the pink of the tongue shows through. A white coat, then, is not the question. The question is how thick it is, whether it is wet or dry, and whether it is greasy, because each of those says something different about what is sitting in the stomach.

Thick and white reads as cold or damp settled there: the pot with too much in it and too little fire. Thick, white and greasy, like a film of lard, is dampness proper, and it usually comes with heaviness, a full chest and a sweet or sticky taste in the mouth. Thickest at the back, the morning after a large supper, is a meal that has not moved. And thin, white and dry points the other way: not too much, but fluids that have run low. The colour stays white in all four; everything else changes.

The coat has two characters in the books. The Shanghan Lun, c. 200 CE, writes it 胎; later books write 苔, moss, which is the word used today, and Ye Tianshi uses both. His reading of the white coat, taken down by his students in the eighteenth century, sorts white coats by thickness and moisture exactly as a physician still does.

The greasy white coat has its own line, and it names the taste that so often goes with it.

The sweet taste goes back to the Neijing, which traces it, and the full middle with it, to the richest food on the table.

Five pictures. Thickness, moisture and what comes with them decide between them.

• Warm and cooked, above all for the thick white coats. Iced drinks, raw salads and fruit from the fridge are what the tradition blames first for cold and damp in the stomach.
• Less sweet, less dairy, less fried, and nothing heavy late in the evening. The greasy white coat is, in the tradition’s account, largely made at supper.
• Seven parts full, 七分饱, and ten minutes of walking afterwards, 饭后百步走. The coat thickest at the back is almost always the size and hour of the last meal.
• For the thin, dry coat, the opposite advice: more to drink, warm rather than iced, soup in the evening, and some moisture in the air of a heated room.
• Look before you scrape. Scraping takes the coat off but not what made it, and a coat that is thick again by morning is being made every night.
• Look on three mornings running, before brushing. A coat that is the same on all three is yours; one that changes daily is mostly last night’s dinner.

None of that is a promise, and we make none. For most white coats it is simply the order in which to try things.

• Job’s tears 薏苡仁 and poria 茯苓, together in a morning congee, for the thick, greasy coat: the two bland foods the tradition gives dampness.
• Tangerine peel 陈皮, a piece in hot water after meals, for the thick coats: it unsticks the middle.
• Ginger 生姜, fresh, in the cooking, for the white, wet coat on a pale tongue: it warms the stomach.
• Hawthorn, 山楂, as a sour tea after a meal that was too big, for the coat thickest at the back.
• Mulberry leaf 桑叶, steeped, or a pear stewed soft, for the thin, dry coat of a dry autumn.

Three finger-widths above the wrist crease, in the groove between the two tendons on the inside of the forearm, is Neiguan, Pericardium 6, 内关: classically the point for the chest, the heart and the stomach, and the one the travel wristbands press. Thumb pressure, a minute each side, when the thick coat comes with a full chest or a queasy stomach. It is not a needle and it is not a claim about outcomes.