Scientists agree that a balanced quality diet is key to good health, and most governments in past years have urged citizens to adopt a daily diet of five portions of fruit and veg, and three each of dairy products and starch, while cutting back fats, sugar and alcohol.
Watching what you eat, experts say, does help prevent illnesses such as diabetes, hypercholesterolaemia, which leads to cardio-vascular disease, or osteoporosis.
But researchers are in disagreement over illnesses not directly related to nutrition, such as cancer, AIDS or neurodegenerative diseases, though again all recommend a balanced diet.
Among the thousands of studies on hand, one European investigation concluded that eating fruit and vegetable fibre might limit the risk of colorectal cancer.
Fruit was tipped as a possible protection against cancer of the lung, and to ward off prostate cancer, the study recommended five cups a day of catechin-rich green tea, catechins are polyphenolic antioxidant plant metabolites.
Industrial or processed fats, already known to be harmful for the cardiovascular system, could double the risk of breast cancer while soybeans reduced the risk threefold.
But soybeans, which are rich in anti-oxydants that help cells survive, could increase the risk of infertility.
Lycopene, the bright red anti-oxydant pigment found in tomatoes and other red fruits, also was found by some researchers to reduce the risk of cancer, but the US Food and Drug Administration (FDA) says there is no solid proof for the claim.
Alzheimer's on the other hand could be kept at bay by pomegranate juice, as well as the polyphenols in a glass of red wine.
But while fish are hailed for their omega-3, a family of unsaturated fatty acids, fish are also rich in mercury and toxic PCBs, or polychlorinated biphenyls.
"There are no good or bad foods," said France's Afssa food security agency.
But eating five fruit and veggies a day and little animal fat "is plain common sense", said oncologist Dominique Belpomme.
Experts at France's national food and cancer research centre, NACRe, said a varied diet of mainly fruit and vegetables with not too much alcohol would help prevent cancers of the mouth, pharynx, oesophagus, lung, stomach, pancreas, colon-rectum and bladder.
After leafing through 7,000 studies from across the globe, the World Cancer Research Fund (WCRF) recommends avoiding food and drinks high in sugar, salt or fats and low in fibres, and instead eating fruit, vegetables, wholegrain cereals and pulses.
Eating well is also key to treating HIV-positive patients as malnutrition weakens the immune system, lowering resistance to secondary effects.
But good food cannot replace antiretrovirals despite claims to the contrary by South Africa's controversial health minister, "Dr Beetroot", on fighting AIDS with garlic, lemons and veggies.