One in every 14 schoolchildren do not know that cheese is a diary product, figures suggest.
Education Quizzes said that 7% of youngsters don't know which food group cheese belongs to.
The exam revision service also found that almost a third of children don't know that 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 that one in four are unaware that carbohydrates provide energy for the body.
And almost a fifth (18%) were flummoxed when they were asked to identify sources of protein such as meat or fish.
Meanwhile, 29% don't know that cardiovascular exercise strengthens hearts and just over half didn't know that drinking alcohol damages the liver.
The organisation said that 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 that separate chopping boards for raw and cooked foods could prevent the spread of bacteria. And 84% knew that 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 kids 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 that there's room for a TV awareness campaign. If shock tactics gets more kids to eat their greens, then we should consider it."