According to an exam revision service, 7% of youngsters do not know which food group cheese belongs to.
It also found almost one-third of children do not know fruit and vegetables provide essential vitamins and fibre.
Based on 1.6 million answers to quizzes undertaken by children aged seven to 16, the figures show one in four is unaware carbohydrates provide energy for the body. Almost one-fifth (18%) were flummoxed when they were asked to identify sources of protein such as meat or fish.
Meanwhile, 29% do not know cardiovascular exercise strengthens hearts and just over one-half did not know drinking alcohol damages the liver.
Education Quizzes said too many children are fundamentally clueless about basic health and nutrition facts. But many were comparatively savvy about food hygiene, a spokesman said, with 92% knowing separate chopping boards for raw and cooked foods could prevent the spread of bacteria. And 84% knew good food hygiene prevents food poisoning.
Colin King, co-founder of Education Quizzes, said: "These findings should serve as an extra incentive for parents looking to kick start a healthy regime as their children head back to school.
"Despite health and food appearing on the PSHE (personal, social, health and economic) and science syllabuses, clearly we're failing. Not only are we seeing more inactive children, but too many are also fundamentally clueless when it comes to the basic facts about health.
"It's also interesting to compare the results around food hygiene where young people seem comparatively savvy. My guess is it comes down to scaremongering from adverts for cleaning products that have a big impact on kids, perhaps suggesting there's room for a TV awareness campaign. If shock tactics gets more kids to eat their greens, then we should consider it."