The two charities have joined forces to explore the impact of diet on mood and behaviour, and launch a campaign called Feeding Minds.
The research, due to be published on 16 January, reveals that food can have an immediate and lasting effect on a child's mental health because of the way it affects the structure and function of the brain.
For example, children with attention deficit hyperactivity disorder are often deficient in iron and the essential fatty acid omega-3. Their behaviour improves when they consume higher levels of these minerals.
However, research has found that people are consuming less nutritious food than in previous generations. People in the UK now eat 59 per cent less fish - an important source of omega-3 - than they did 60 years ago. Over the same period there has been a 34 per cent decline in fruit and vegetable consumption.
The report also raises concerns over changes in food production. New substances such as pesticides, additives and trans-fats, which can prevent the brain from functioning effectively, have been introduced to the diet.
Courtney Van de Weyer, researcher on the Feeding Minds campaign at Sustain, said, 'Unless there is a radical overhaul of food and farming policies, there won't be healthy and nutritious foods available in the future for people to eat.
'The rise of processed food in the west has been staggering. Many people are confused about what constitutes a healthy diet. There is little nutrition education in schools and cooking skills are in decline.
'The Government needs to set more stringent targets for food manufacturers and introduce a ban on the advertising of unhealthy foods to children.'
The Feeding Minds reports and nutritional advice are available at www.mentalhealth. org.uk/feedingminds.
