NEW YORK (MainStreet) — Experts have long thought that many people with low incomes often eat poor diets – processed foods, refined grain and meat, which are heavy in fat and salt – because they are less expensive than healthier diets – those filled with fresh fruits, vegetables, nuts and fish. Yet, a new study shows the difference in cost between an unhealthy diet and the healthiest diet is just $1.50 a day, on average, for one person.

The research, conducted by the Harvard School of Public Health (HSPH) and published online in the British Medical Journal, is the first to systematically evaluate the scientific evidence for what's become conventional wisdom, according to lead author Mayuree Rao, a junior research fellow in the department of epidemiology at HSPH.

While $1.50 a day doesn't seem like much of a financial burden, the problem arises when that cost per year (about $550) is multiplied out for a family of four (about $2,200). This, the authors note, could burden low-income families. Still, the authors point out, $550 per person per year is a relatively small amount of money when compared with medical costs associated with diet-related chronic diseases that can result from eating an unhealthy diet.

It is a well-known that high-fat diets can lead to heart disease and other chronic medical conditions, but a recent international study, which included researchers from Yale University and the University of Erlangen-Nuremberg, among others, published in the journal Nature, showed that foods high in salt may be a factor in increased numbers of auto-immune diseases. Autoimmune diseases are those in which the immune system, which protects against germs, erroneously attacks healthy tissue. There are more than 100 autoimmune diseases, including diabetes, muscular sclerosis and psoriasis.

A 2011 report by the American Autoimmune Related Diseases Association (AARDA) and the National Coalition of Autoimmune Patient Groups (NCAPG) estimates the cost of just seven of these autoimmune diseases--Crohn's disease, ulcerative colitis, systemic lupus erythematosus (SLE), multiple sclerosis (MS), rheumatoid arthritis (RA), psoriasis and scleroderma, cost an estimated $51.8 to $70.6 billion annually. AARDA estimates that there are 50 million Americans who have an autoimmune disease and say that currently there is not enough data to determine the full cost of autoimmune diseases to the healthcare system.

The Harvard researchers have called for food policies to help offset the differences costs between healthy and unhealthy foods.

--Written for MainStreet by S.Z. Berg, author of College on the Cheap