( ( ( The energy ) ( available ( in food ) ) ) ( ( is ( measured ( in ( ( calories , ) ( ( a unit ) ( of heat ) ) ) ) ) ) . ) )
(ROOT (S (NP (NP (DT The) (NN energy)) (ADJP (JJ available) (PP (IN in) (NP (NN food))))) (VP (VBZ is) (VP (VBN measured) (PP (IN in) (NP (NP (NNS calories)) (, ,) (NP (NP (DT a) (NN unit)) (PP (IN of) (NP (NN heat)))))))) (. .)))
The energy available in food is measured in calories , a unit of heat.
(ROOT (S (NP (DT A) (NN calorie)) (VP (VBZ is) (VP (VBN used) (S (VP (TO to) (VP (VB express) (NP (NP (DT the) (NN amount)) (PP (IN of) (NP (NN energy)))) (PP (IN in) (NP (NN food)))))))) (. .)))
A calorie is used to express the amount of energy in food.