Before you slap chop your low-calorie leafy green on the cutting board or hit the salad bar, you may want to know which has the highest nutritional value of lettuce - iceberg or romaine?

According to the United States Department of Agriculture (USDA)'s Economic Research Service (ERS), lettuce is the leading vegetable crop in the U.S. in terms of its production value and the second most consumed vegetable — with potatoes ranked first.

The main types of lettuce include romaine and iceberg, also known as "head lettuce," which is the most widely used given its low cost and extended shelf life. The U.S. consumption of head lettuce is estimated to be 23 lbs. per person every year, says the University of Arizona College of Agriculture and Life Science. However, the consumption of romaine lettuce and loose leaf lettuces has been increasing throughout the years due to the increased popularity of caesar salad and salad bars, says ERS. The efficiency of packaged salads sold at markets also accounts for this dramatic increase since the mid-1980s.

Lettuce, whether consumed as iceberg or romaine, is rich and abundant with nutritional value. In the battle of iceberg versus romaine, however, who will claim the title of the best and healthiest lettuce?

Battle of the Nutrients

Vitamin A

Romaine lettuce is super rich in beta-carotene, with approximately 4,094 international unit (IU) of vitamin A in one cup, says the USDA. One cup of iceberg lettuce, however, only contains 361 IU of vitamin A. Vitamin A, a fat-soluble vitamin, is important for your vision, immune system, and reproductive system. It has been linked to aiding the heart, lungs, kidneys, and other organs in proper functioning, says the National Institutes of Health (NIH) Office of Dietary Supplements (ODS).

Winner: Romaine

Vitamin K

Romaine lettuce provides 48 micrograms of vitamin K in a one-cup serving while iceberg lettuce only provides 17 mcg, says ODS. Vitamin K helps your blood to clot appropriately and regulate insulin. The American Diabetes Association (ADA) reported on a study published in Diabetes Care, where the effect of vitamin K supplementation was examined in 350 nondiabetic men and women between the ages of 40 and 60 for 36 months. The participants of this study took vitamin K supplements that were five times more than the recommended amount to measure their level of insulin resistance. The results of the study showed that taking vitamin K supplements for 36 months reduced the progression of insulin resistance in older men but not women.

Winner: Romaine

Lutein and Zeaxanthin

Romaine lettuce contains 1,087 mgs of lutein and zeaxanthin — carotenoids that filter harmful high-energy blue wavelengths of light on the retina, says the American Optometric Association (AOA). Iceberg lettuce only contains 199 mgs of these two nutrients. The recommended amount of lutein is six to 10 mg per day, whereas the dose for zeaxanthin is 2 mg per day, to maximize the health benefits for your eyes. Lutein and zeaxanthin can reduce the risk of age-related macular degeneration (AMD), a condition that occurs when the fragile center of the retina, the macula, deteriorates over the years. In a National Eye Institute (NEI) press release, the Age-Related Eye Disease Study (AREDS) conducted in 2001 and then in 2006, known as AREDS2, proved that the use of the supplements vitamins C and E, beta-carotene, and the minerals zinc and copper, with the addition of lutein and zeaxanthin, are safe and protective against the advancement of AMD.

Winner: Romaine