Fruits And Vegetables: Eating Colors For Optimal Health
Peter Palmieri, MD, MBA, dives into what the colors of fruits and vegetables tell us about their nutritional value.
Have you ever wondered what makes blueberries blue, raspberries red, and oranges orange? More importantly, can the color of fruits and vegetables tell us something about their nutritional value? Mounting evidence suggests that the natural colors of plant-based food can provide a helpful guide for selecting the "five-a-day" fruits and vegetables recommended as part of the Dietary Guidelines for Americans<sup>1</sup>. Combining foods of different colors at every meal is a straightforward approach to a diet rich in various nutrients.
Why are Fruits & Vegetables So Many Colors?
Plants, including fruits and vegetables, get their color from a large class of natural chemical compounds known as phytochemicals (phyto is Greek for plant). In human nutrition, a phytochemical is any biologically active molecule of plant origin that provides health benefits<sup>2</sup> beyond basic nutrition, such as lowering the risk of a particular illness.
Generally speaking, phytochemicals produce their disease-prevention effects by acting as antioxidants. Which raises the question: What exactly are antioxidants, and why are they essential to our health?
Oxidative Stress: A Quick Overview
You may have heard of harmful chemicals that go by the names of free radicals and reactive oxygen species (ROS). You are probably aware that molecules called antioxidants protect us from the damaging effects of free radicals. Without getting bogged down in biochemistry, let's take a closer look at these substances. This will help us better understand why antioxidants are so important to human health.
Reactive oxygen species are by-products of many chemical reactions essential for normal cell function. These molecules play a crucial role in cell communication and contribute to our immune response to infection. They provide the weaponry that allows our white blood cells to vanquish bacterial invaders. But in excess, they can injure tissue by damaging proteins, fat tissue, and DNA<sup>3</sup>.
The hallmark of too many free radicals is inflammation. Its consequences are protean: cancer, heart disease, respiratory illnesses such as asthma and chronic obstructive pulmonary disease, kidney disease, rheumatoid arthritis, cataracts, neurologic afflictions such as Parkinson's and Alzheimer's, and even aging.
Cellular metabolism is not the only source of harmful oxygen species. Extreme physical activity, exposure to certain chemicals, radiation<sup>4</sup>, and ultraviolet light can all lead to a toxic accumulation of these molecules.
Colors of Fruits and Vegetables
Different Colors, Different Benefits
Nature has color-coded food for us. Fruits and vegetables may not come with a nutrition facts label, but their pigment can tell us a great deal about their nutritional content. Let's take one color at a time.
Red fruits and vegetables tend to be high in Vitamins A and C, and a variety of phytonutrients. Tomatoes are rich in lycopene,<sup>7</sup> which is associated with a lower risk of cardiovascular disease and prostate cancer. Ellagic acid,<sup>8</sup> found in grapes and pomegranates, is a potent anti-inflammatory that has beneficial effects on lipid and glucose metabolism. It has also been shown to have anti-cancer effects through various mechanisms.
Blue and Purple
When you eat eggplant, leave the peel on. You don't want to miss out on an important class of phytonutrients known as anthocyanins<sup>9</sup>. Also found in blueberries, black currants, and red grapes, anthocyanins have been shown to improve insulin sensitivity, inhibit platelet aggregation, and suppress the growth of many tumors.
Sometimes I feel that chemists who name molecules are trying their hardest to come up with tongue twisters. That's certainly the case with cryptoxanthin, the pigment that gives oranges, tangerines, peaches, papaya, and persimmons their distinctive hue.
Cryptoxanthin is a carotenoid. It’s related to beta-carotene, which is found in carrots, sweet potatoes, cantaloupe, mangoes, and pumpkins. Beta-carotene is considered a provitamin because it is a biological precursor to Vitamin A. Whereas taking excessive Vitamin A in the form of a nutritional supplement can result in toxicity with potentially serious side effects, that is never the case with foods containing beta-carotene. The body can put the brakes on the conversion of beta-carotene to Vitamin A when it senses that levels are sufficient.
Carotenoids<sup>10</sup> are essential for eye health and cardiovascular health. They have been linked to improved cognition<sup>11</sup> and may play a role in preventing certain cancers<sup>12</sup>.
Be aware that prolonged eating of foods rich in beta-carotene can result in the skin turning an orange hue. This is known as carotenemia and is frequently seen in infants and toddlers. Though it can be alarming to parents, it is harmless and certainly not permanent. My pediatric mentor was the legendary Dr. James Raettig. He would frequently remind pediatricians-in-training that the best advice to give parents of children with carotenemia is, "Keep up the good work!" The color change can be seen as a litmus test for a diet rich in vegetables.
In adults, high serum beta-carotene levels can interfere with certain medications, so this should be discussed with a physician. One must not confuse carotenemia with less benign mimics that bronze the skin. These conditions include jaundice, Addison's disease, and hemochromatosis.
Folic acid (also known as Vitamin B9) gets its name from the word foliage. Spinach, kale, and arugula are all bursting with this nutrient essential for cell growth and development. Green leafy vegetables are also a source of quercetin, as are asparagus, green peppers, and green tea. Quercetin and kaempferol (also found in green leafy vegetables) are part of a family of plant-derived compounds known as flavonoids<sup>13</sup>. If consumed regularly, these may reduce the risk of several chronic illnesses, including cardiovascular disease, certain forms of cancer, and neurodegenerative diseases.
White and Brown
Even plants that seem to be devoid of color contain beneficial phytonutrients. Among these is allicin<sup>14</sup>, the substance in garlic that is a potent killer of fungi and bacteria, including certain antibiotic-resistant strains such as methicillin-resistant Staphylococcus aureus (MRSA).
White onions contain quercetin (which we already encountered in the green category) and Vitamin C, Vitamin B6, and folate. Cauliflower and bok choy are rich in glucosinolates<sup>15</sup>—yet another class of anti-inflammatories that may help reduce certain tumors.
Tips for a Colorful Diet
I don't know about you, but my head is spinning with the names of all of these phytonutrients. The good news is you don't have to learn these chemical names by heart. Remember, plant-based food is color-coded for your convenience.
The next time you're at a farmer's market or in the produce department of your grocery store, look at the rainbow of colors surrounding you. As you fill your basket, be sure to include fruits and vegetables of different colors. The broader the palette of colors available to you at home, the more balanced your intake of antioxidants will be. And if you can't get fresh vegetables, rest assured that the frozen form is an excellent alternative.
When you get home, have fun creating colorful combinations. Mix different hues in your vegetable medley. Shred carrots, red cabbage, white cabbage, and add chopped cilantro and green onions for a healthy take on coleslaw. Add fresh fruit, seeds, red onion, and nuts to your salad. Be adventurous! It's hard to foul up fruit and vegetable combinations.
What About Dietary Supplements?
More than half of the U.S. population uses dietary supplements<sup>16</sup>. The two most common reasons cited for their use are to fill nutritional gaps and to support overall health<sup>17</sup>. Frequently, this is not done in isolation. In self-reports, users of supplements are more likely than non-users to say they engage in healthy lifestyles such as eating a balanced diet, exercising regularly, and getting a good night's sleep.
As a source of phytonutrients, supplements may not be as effective a choice as eating whole foods that contain macronutrients and micronutrients. Observational studies show that regular dietary intake of plant-based micronutrients leads to a lower incidence of heart disease, cancer, and other illnesses. But these benefits are hard to reproduce in individuals who get these nutrients in the form of nutritional supplements<sup>18</sup>.
There could be several explanations for this. It's possible that what is most important to good health is a general balance of antioxidants that can be achieved through a nutrient-dense diet. Phytonutrients seem to act synergistically to produce their antioxidant effect<sup>19</sup>. A combination of these nutrients, as can be obtained through a diet rich in fruits and vegetables, produces a more potent antioxidant effect and better health outcomes than individual phytonutrients taken alone as supplements.
Scientists are busy studying the synergistic and potentially antagonistic effects of micronutrients. For example, they have found that a biologically active form of Vitamin E called alpha-tocopherol produces additive antioxidant effects when taken in conjunction with beta-carotene<sup>20</sup>. Findings such as these may lead to the development of improved dietary supplements in the future. In the meantime, your best bet is to fill up your plate with lots of colorful veggies.