No matter how many different dietary theories there are out there, pretty much everyone agrees that vegetables are "good for you". But how good they truly are has been debated - there are plenty of observational studies linking vegetable consumption to favorable health outcomes, but other studies have made headlines by casting doubt on how powerful plant foods are for preventing disease. The data from these observational studies is often flawed simply because the majority of people in the Western world don't eat enough vegetables to have a measurable impact on their risk of chronic disease - only about 25% of Americans eat the recommended three one-cup servings of vegetables each day. Also, total vegetable consumption isn't necessarily an accurate indicator of the healthfulness of one's diet, since some vegetables are far more nutrient-dense than others. Of course, long-term controlled trials of consumption of a high-nutrient vegetable-based (nutritarian) diet have not yet been published (with the Nutritional Research Project, I aim to fill this gap in the medical literature). Some long-term observational studies, however, do provide clear, high-quality data demonstrating that vegetable consumption is an important factor in chronic disease prevention - a recent study on serum α-carotene levels and risk of death provides such data.
Alpha-carotene is one of over six-hundred different carotenoids, a family of antioxidants that also includes β-carotene, lycopene, lutein, zeaxanthin, and astaxanthin. Carotenoids help to defend the body's tissues against oxidative damage, which is a natural byproduct of our oxygen-dependent metabolism.  Oxidative damage to DNA, proteins, and lipids is a known contributor to chronic disease and an accepted mechanism of aging. The body's defenses against oxidative damage consist of naturally produced as well as diet-derived antioxidant molecules.
Many prospective studies in the past few years have supported the epidemiologic association between plasma carotenoids and reduced risk of disease and/or death. [3-6] However, these studies didn't differentiate between carotenoids from food and those from supplements. Carotenoid supplements have failed to duplicate this effect in clinical trials. In fact, supplemental carotenoids are likely to be harmful. A recent meta-analysis of several trials found a 7% increase in mortality risk in subjects taking β-carotene supplements. [7, 8] Also, high serum β-carotene has been associated with decreased lung cancer risk, but β-carotene supplements may increase the risk of lung cancer, especially in smokers. Attempting to duplicate the beneficial effects of carotenoid-rich foods with isolated nutrients is foolish - it completely neglects the contribution of additional and/or synergistic effects of other nutrients contained in those foods.
Beta-carotene is the most widely studied carotenoid, but α-carotene more accurately reflects vegetable intake because α-carotene is not present in most multivitamins and supplements. It is also an excellent marker of high-nutrient vegetable intake, since dark green and orange colored vegetables are the richest sources of alpha carotene. Green vegetables are the highest in overall nutrient density, according to my ANDI scoring system and of course they are the foods richest in alpha carotene.
This study measured baseline serum α-carotene and tracked deaths in the 15,318 participants over a fourteen-year follow-up period. After controlling for potential confounding factors, the researchers found a significant trend - increasing serum α-carotene associated with decreased risk of death from all causes. Those with the highest serum α-carotene had a 39% decrease in risk of death compared to those with the lowest serum α-carotene. Similar relationships were found between serum α-carotene and risk of death from cardiovascular disease, all causes other than CVD, and cancer.