| Word | Description |
| Xanthine | 2,6 -dioxypurine which is formed from adenine and from guanine which are present in all nucleic acids. Caffeine (coffee and ta) is 1,3,7- trimethyl xanthine; theophylline (tea) is the 1,3-dimethyl derivative; theobromine (cocoa) is the 3,7-dimethyl dersv |
| Xanthine Oxidase | Refers to an enzyme which is present in milk and in liver, specific for the two purines xanthine and hypoxanthine (which it oxidises to uric acid), and will also oxidise a range of aldehydes to the corresponding acids. It is identical with Schardinger's |
| Xanthophylls | The collective term for hydroxylated carotendoids or carotenols. |
| Xanthoproteic Test | For proteins (actually for the benzene nucleus of tyrosine and tryptophan which occur in nearly all proteins). Yellow colour on boiling with nitric acid, turns orange on adding ammonia. |
| Xenobiotic | Substances foreign to the body, including drugs and some food additives. |
| Xerophthalmia | Found in advanced vitamin A deficiency. Epithelium of the cornea and conjunctiva of the eye deteriorates due to impairment of the tear gland, causeing dryness then ulceration. |
| Xylitol | Five - carbon sugaralcohol which is corresponding to the sugar xylulose. As sweet as sucrose, less prone to cause dental decary and used in some; sugar-free' products like chewing gum. |
| Xyloketose | Xylulose |
| Xylose | Pentose sugar occuring in plant tissues as complex polysaccharide; 40% sweetness of sucrose. |
| Xylulose | Five-carbon sugar-alcohol which is derived from the pentose sugar xyloxe. |
