Doctors are being advised to prescribe oily fish or omega-3 fatty acid supplements to heart attack patients.
The National Institute for Health and Clinical Excellence (NICE) believes this is an effective way to cut the risk of further heart attacks.
It is the first time NICE has recommended lifestyle change - alongside drugs - in guidance on preventing repeat attacks.
Patient representatives said the measures could have a big impact.
The guidelines update the previous set produced six years ago, and the new lifestyle section says patients should be advised to give up smoking, be physically active for 20-30 minutes a day, and eat a Mediterranean-style diet.
Professor Gene Feder, Chair of the Guideline Development Group, said: "The growing evidence that cardiac rehabilitation and specific lifestyle changes reduces the risk of second heart attacks, as well as improving quality of life, is not widely recognised."
He said that while drugs could make an enormous difference to patients who have had a heart attack, there is now also compelling evidence that lifestyle changes can play a role too.
Evidence from trials suggests that eating more oily fish can improve survival rates in heart attack patients.
The new guidelines recommend that patients who have a had a heart attack in the last three months eat more oily fish or are prescribed certain preparations of omega 3 fatty acids.
Research has found that one specific omega-3 supplement - Omacor - cut the risk of a patient dying suddenly by up to 45%.
Omacor is currently the only omega-3 supplement with a licence for post-heart attack treatment.
However, the costs of this to the NHS are likely to be quite high, as around 260,000 people have a first heart attack every year, and of those around a fifth may need supplements as they are intolerant to oily fish.
NICE estimates the cost could be around £7million in the first year.
John Walsh, a patient representative on the Guideline Development Group, said: "There is really a lot of good advice in [the report] that we must ensure gets to patients.
He said there was good evidence that secondary heart attacks could be avoided if patients changed their lifestyles.
He said: "The changes that the average person needs to make are really quite small.
"In my case for example, I've decided that I'm not going to choose to live without cheese, but what I do now is eat very much less of it."
"I'm quite sure that from the patients' point of view, if we can get this information into their hands, and their minds and into their hearts, there will be a big health gain in this area."
Professor Mayur Lakhani, chairman of the Royal College of General Practitioners, said: "Having a heart attack used to be a life sentence for patients; now it is possible for most patients to lead healthy and fulfilling lives.
"This requires a high standard of modern medical care and for patients to follow lifestyle advice."