Lentils may have been one of the first agricultural crops grown, with cultivation beginning more than 8,500 years ago, researchers from the Universities of Wisconsin and Minnesota report. Both lentils and rice have been food staples for many centuries. Eaten together, they offer a low-fat, nutritious source of complete protein, fiber, iron and other essential minerals.

Rice is the seed of a plant called Oryza sativa. Available in different grain sizes, the Food website explains that long-grain rice is light and separates easily when cooked, while short-grain rice has a higher starch content that makes it stickier. The lentil, a plant called Lens culinaris, contains seeds that can be eaten whole or sprouted.

Nutritional information in this article comes from the U.S. Department of Agriculture, as relayed by the eLook website. The serving size is 1 cup cooked, for both lentils and unenriched white rice.

One cup of white rice contains 199 calories; the same serving of lentils has 229. Rice is fat free; lentils have only 1 gram of total fat. They both contain 4 to 5 milligrams of sodium, an amount so low it does not register as a percentage of the daily value for a 2,000 calorie-a-day diet. White rice is low in fiber, at 1 g per serving; lentils, however, contain 16 g of dietary fiber.

Eaten together, rice and lentils provide thiamin (0.38 mg), riboflavin (0.18 mg), niacin (4.55 mg), vitamin B6 (0.38 mg) and pantothenic acid (1.87 mcg). Lentils alone are a rich source of folate, packing 358.38 micrograms per serving, or 89 percent of the recommended daily value, eLook reports. Lentils also provide vitamin C (2.97 mg) and vitamin K (3.37 micrograms).

Rice and lentils contain all the essential dietary minerals, but in significantly different amounts. Lentils are a great source of iron (6.59 mg; 36 percent of daily value), magnesium (71.28 mg; 17 percent), phosphorus (356.4 mg; 17 percent), potassium (730.62 mg; 30 percent), zinc (2.5 mg; 16 percent), manganese (0.9 mg; 48 percent) and copper (0.49 mg; 24 percent). Rice offers more selenium (14.35 micrograms; 20 percent), and both provide about the same amount of calcium (33-37 mg; 3 percent each).

A complete protein is one that provides all the essential amino acids in the proper amounts. When eaten together, rice and lentils create a complete protein. Though lentils are slightly low in two amino acids, they contain one of the highest amounts of protein found in plants, according to the University of Maryland Medical Center. One cup of lentils has 18 g, or 35 percent of the daily value of protein.

The glycemic index indicates the impact of carbohydrates on blood sugar levels; the higher a food's number, the more it increases blood sugar. On a scale where 100 is pure sugar (glucose), white rice has a glycemic index of 64, while lentils are low on the index with a rating of 29, Oregon State University's Linus Pauling Institute reports.

Some nutritional values increase if you use brown rice. Compared to unenriched white rice, brown rice has four times the fiber, five times more thiamin, eight times more vitamin B6 and three to four times more iron, magnesium, phosphorus and manganese.