Because they contain a lot of vitamins and nutrients, vegetables are essential for the proper functioning of our bodies.

Michael Pollan, author of "The Omnivore's Dilemma" and "In Defense of Food", says vegetables can act as preventive medicine - and they offer more health benefits than taking expensive supplements.

"It is much better to get your vitamins from food. Our bodies have evolved to obtain nutrients from food, not pills. So it's no wonder that supplements generally only work if you have a specific deficiency", Pollan told Business Insider.

Many doctors, nutritionists, independent supplement researchers and leading longevity experts agree with him.

Want to add a new supplement to your diet? Try broccoli. Cruciferous vegetables, such as broccoli and cauliflower, are called "cole crops" - term referring to plants in the Brassicaceae family.

Cole is a more general term used to describe various plants in this family, including cabbage, Brussels sprouts, broccoli and cauliflower.

A family of cruciferous vegetables called Brassica, also known as cole crops, offers many health benefits, says Pollan.

Brassicas are some of the crunchiest plants you can find in the grocery store. They are excellent for improving circulation and blood flow and can help prevent a wide range of diseases.

Scientists believe that broccoli and other vegetables in the same family contain sulfur-based chemicals called glucosinolates, which give these vegetables their bitter flavor. When we cut, chew and digest cruciferous vegetables, these powerful chemicals break down into bioactive compounds that can help prevent certain forms of cancer. But squeezing these compounds into a pill is not the same as consuming them in whole foods.

"When they identified this compound and then put it into a supplement, it didn't have the same effect", Pollan said.

Studies have consistently shown that the bioavailability of nutritional supplements - the amount of nutrients our bodies absorb - can vary greatly.

Because supplements are not well regulated, it is often difficult to know exactly what they contain and it can be almost impossible to tell if the supplements you are taking are helping you or doing you more harm than good.

People who eat lots of vegetables tend to have fewer strokes, healthier hearts, less visceral fat, a lower chance of developing type 2 diabetes and a reduced cognitive decline, which scientists don't think it's a coincidence.

"There are things we don't understand about the structure of foods and the relationship between chemicals in foods that are very important", Pollan said.

Scientists agree that no supplement can equal a good diet and exercise routine in helping to prevent many chronic diseases.