= Solution
\b[A <food> supplies material or usable chemical <energy> that contributes to the organism's maintenance, growth or normal function after ingestion and physiological processing.] It must be usable in the amount and form supplied, without its harmful effects defeating that nutritive role. Mere combustibility, a pleasant taste, or the presence of <carbon> and <nitrogen> is insufficient.
An edible material may need <digestion> before its constituents can be absorbed. A <protein> need not enter the <blood> unchanged to nourish: <hydrolysis> supplies absorbable <amino acids>. Conversely, a substance can burn in a laboratory yet furnish little accessible <energy> to human <metabolism>. The nutritional value of a mixture depends on availability, proportions and the organism consuming it. No requirement says that each separate <food> must contain every constituent of a complete diet.
A useful nineteenth-century division separates nitrogenous or plastic <foods> from non-nitrogenous or respiratory <foods>. The first includes animal and plant <proteins>, which supply <amino acids> for tissue renewal. The second includes <carbohydrates> and <fats>, whose oxidation supplies much of the body's <energy>. But the division is functional rather than absolute: <proteins> can also be oxidized, and <lipids> and <carbohydrates> supply material for cellular structures as well as fuel. The modern account follows <anabolism> and <catabolism> rather than assigning each class one exclusive destination.<Anatomy and Physiology 2e by J. Gordon Betts and others/24.1 Overview of Metabolic Reactions>[Anabolic and Catabolic Reactions: synthesis, breakdown and energy transfer]{ref}
A fuller classification includes <proteins>; <carbohydrates>, including sugars and <starch>; <lipids>, including <fats> and essential fatty components; <water>; and required <mineral> constituents. <Water> provides solvent, transport medium and a participant in many reactions. <Mineral> constituents supply, among other needs, ions for electrical and osmotic functions and material for mineralized tissues. Neither class has to supply combustible <energy> to be indispensable. Meat, milk, grain and legumes are mixtures spanning several classes, rather than chemically pure examples of just one.
In modern classification, <vitamins> and other required micronutrients must also be included. Their specific deficiency roles and present classification should not be attributed wholesale to 1864. <Dietary fibre> is not equivalent to digestible <starch>; it can have important intestinal functions, and fermentable fractions can contribute metabolites through the microbiota. The adequacy of a diet concerns the whole combination of nutrients and the person's requirements, not a single measure of heat released on burning it.<Anatomy and Physiology 2e by J. Gordon Betts and others/24.7 Nutrition and Diet>[Macronutrients; Vitamins; Minerals: energy-yielding and non-energy-yielding nutritional requirements]{ref}
<Oxygen> is essential to ordinary aerobic life, but in the usual dietary sense it is a respiratory input rather than an ingested <food> class. Likewise, a drug that changes appetite or stimulates a physiological response is not thereby a substitute for nutritive material. A convincing experiment on <food> value must distinguish failure to eat, failure to digest or absorb, and a genuine missing dietary constituent; maintaining equal gross <food> mass alone does not control those differences.
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