Why Not Just Take Phytonutrient Supplements?

If phytonutrients can be so healthful, why not just take plant extract supplements rather than going through all the trouble of eating the plants themselves? Besides the misidentification, contamination, and adulteration issues rife within the poorly regulated supplement market, there’s the question of dose. Taking polyphenol supplements can result in blood levels up to nearly an order of magnitude higher than that of a polyphenol-rich diet. When it comes to hormesis, less may be more. 
 
If you give cancer-susceptible mice a high fat diet to accelerate tumor development, but feed them a dietary-relevant dose of resveratrol, the tumor burden is cut in half. But at the higher dose of resveratrol you might find in a supplement, 200 times more resveratrol led to only half the benefit in terms of preventing cancer formation. Similar results were found when colorectal tumors from human patients were implanted in resveratrol-fed mice. The anti-cancer effect was lessened or absent entirely. In people, colorectal biopsies after consuming the daily resveratrol dose one might find in 2 cups of wine or grape juice showed greater activation of a protective detox enzyme than biopsies taken from those taking a 200 times greater one gram supplement dose. 
 
More is not better, when it comes to hormesis, where more can most certainly be worse. Many flavonoids function as “nature’s pesticides,” protecting plants from predators like us. We co-evolved to counter these defenses, and thanks to hormesis a smidgeon of toxin can actually be beneficial, but a profusion of toxin can be toxic. 
 
Isolated phytochemicals and plant extracts that are life-extending in model organisms at one dose, can be life-shortening at higher doses. For example, an extract of Siberian ginseng extends the average lifespan of C. elegans by 5 percent at 100 mg/ml, 16% at 250, but at ten times the dose, 2,500, it reduces average lifespan by 23%. Similar hormetic dose-response reversals of fortune have been documented in a variety of other plant extracts and compounds.  
 
Green tea is a good example. Regular consumption of green tea is considered not only safe but salutary, but the consumption of green tea extract supplements was found to cause liver inflammation in about 1 in 15 women, based on the thousand-person Minnesota Green Tea Trial. In contrast, out of more than 50 interventional studies of regular brewed tea or green tea beverages, not a single case of liver toxicity was reported. To paraphrase a review on the anti-aging effects of polyphenols, it’s easier to overdose on supplements than salad.