On his website Kevin Klatt, Ph.D., R.D., notes that a double-blind, randomized, placebo-controlled trial is the gold standard for research, but when it comes to food it's not so simple. There is no true placebo when a sugar pill has calories.

"This lack of placebo makes designing calorie-containing nutrient studies pretty tough. But it makes food studies even more challenging. Most randomized controlled trials in nutrition only have funding for 2 arms, so what is the appropriate comparator arm for our commodity group (Dairy/Meat/Egg/Cereal) funded trials? The lack of a placebo has made food focused studies so easy to slant in the direction of whatever results you care about (compare anything to a high sugar/high refined carb/high satfat diet and you can make most foods look "good")." 

Want To Buy: A Placebo by Kevin Klatt, PhD, RD: Nutritional Sciences: Basic Science and Clinical Perspectives