Scientists at the London School of Hygiene & Tropical Medicine said eating better would benefit both people's health and the environment.
The diets of the average UK man and woman do not currently conform to World Health Organisation (WHO) recommendations but its study suggested that if they did, greenhouse gas emissions would be reduced by 17%.
Researchers analysed food diaries from more than 1,500 adults in the UK and looked at how diet affected health problems such as coronary heart disease, stroke, type 2 diabetes and a number of cancers.
They said diet-related ill health in the UK is estimated to cost the NHS around £6 billion annually, but calculated that eating more healthily could save almost seven million years of life lost prematurely in the UK over the next 30 years.
They estimated that it would also extend average life expectancy by around eight months (12 months for men and four months for women).
These health gains would come mainly from reductions in coronary heart disease and stroke, they said.
"Encouraging people in the UK to modify their diets to contain fewer animal products and processed foods and more cereals, fruit and vegetables would produce tangible benefits to both health and the environment," researchers said.
But they added that the health benefits and acceptability of such diets is likely to peak at around a 40% reduction in greenhouse gas emissions as greater reductions than this would be likely to result in "unacceptable diets and progressively reduced health gains" - although these would still be an improvement on current diets.
They said "radical" dietary changes such as veganism were not necessary in order for there to be large reductions in emissions and benefits to health.
One of the authors, Dr Alan Dangour, said: "This is the most detailed analysis to date for the UK and our findings show that even making relatively small changes to current diets would have a tremendous impact on both the environment and population health.
"It's clear from our analysis that we do not need to make radical changes to our dietary habits to bring about substantial benefits.
"We hope the detailed information we've compiled about the composition of healthy and low-emission diets will help to prioritise policies and interventions aimed at promoting healthier and more environmentally-sustainable diets."