If fat and sugar was reduced in an average persons diet would it prevent the risk of later having the risk of suffering from heart disease?

It would be impossible to say that it would PREVENT a heart attack, but it would be accurate to say IT REDUCES THE RISK, of heart attack.

The risk factors include:
Family history of premature heart disease - obviously you cannot modify this.

High blood pressure
Smoking
Diabetes
High cholesterol

These an individual can modify. Like you said, a low fat, lower refined sugar diet is a great way to help, in conjunction with a lifestyle that incorporates aerobic activity.


Risk Factors and Coronary Heart Disease (http://www.americanheart.org/presenter.jhtml?identifier=235)