To explore the effects of fructose (a naturally occurring sugar in fruit, often refined and added as a sweetener to a variety of processed foods and drinks) and DHA on our health, the researchers split rats they had trained to escape a maze into three groups. The first group was given water spiked with fructose – about the equivalent of drinking one litre of soft drink each per day – and the second group drank the fructose water but was fed a high-DHA diet, while the third drank normal water and had no DHA. When the rats ran through the maze again after six weeks on the diet, the fructose group were twice as slow at finding their way out. The DHA rats and the control rats navigated the maze at about the same speed, suggesting the DHA offset the effects of the fructose. "Food is like a pharmaceutical compound that affects the brain," said co-author Fernando Gomez-Pinilla, a UCLA professor of neurosurgery and of integrative biology and physiology. Co-author Xia Yang said: "DHA changes not just one or two genes; it seems to push the entire gene pattern back to normal, which is remarkable."

In fact, it was not just memory and learning that were affected by nutrition. The high-fructose rats had higher blood glucose, triglycerides (a type of fat found in the blood) and insulin levels than the other two groups, factors that significantly increase the risk of obesity, diabetes and other diseases. The findings are consistent with other research showing fructose is harmful to humans. Similarly, DHA – which occurs naturally in small quantities in our brain cells but which we mostly get through DHA-rich foods such as wild salmon and other fish, walnuts, flaxseed and fruits and vegetables – is protective. "We've known for years that DHA, particularly found in oily fish, is protective against conditions such as depression, heart disease and Alzheimer's disease," said dietitian Melanie McGrice. "This research suggests that it may be DHA's impact on our genes that is protective." It is the effect on the genes' expression that McGrice says is particularly interesting.

"Science is now showing that what we eat has an important impact on our DNA (nutragenetics)," she said. "Eating is no longer just about kilojoules, but the importance of good food to protect us from changes to our genes." Does this mean we can have our fructose and eat it too, so long as we ensure our diet is rich with nuts and oily fish and vegetables? Not quite, says McGrice. "Eating foods rich in DHA is not likely to undo harm caused by other foods, as everyone's genes are different," she said. "Scientists believe that in the future we'll be able to tell who is at risk of certain diseases, and what foods they'll need to eat, or avoid, to avoid developing those diseases. Until we're able to isolate these risks, we can all benefit from eating a diet rich in protective foods such as fish."

FRUCTOSE Although fructose is present in fruit, the fibre and other beneficial nutrients slow absorption and counter its effects. If you stick to the recommended two serves a day, you'll be sweet. Sweet foods such as desserts, cakes, chocolate and other confectionery, and sweetened beverages such as carbonated soft drinks, sports drinks and so on, contain large quantities of added fructose. What is one serve of fruit? One medium apple, banana, orange or pear

2 small apricots, kiwi fruits or plums 1 cup diced or canned fruit (no added sugar) Or only occasionally: 125ml (½ cup) fruit juice (no added sugar) 30g dried fruit (for example, 4 dried apricot halves, 1½ tablespoons of sultanas)

Fruit and fructose: Lower-fructose fruits include blueberries, raspberries, grapefruits, honeydew melon and kiwi fruit. High fructose fruits include bananas, mangoes, cherries and grapes.