Is There Any Harm In Eating Lots of Vegetables?

I recently made a statement to a colleague that eating an excessive number of vegetables could be harmful. He asked for supporting documentation to back up my comment.  As my statement was based on previous readings, I sought out the clinical literature that associated too many vegetables consumed with what I believed to be the following seven potential problems.  

1.    Certain vegetables, such as cassava, are goitrogenic, which means they cause goiter, a thyroid condition that results in an enlarged thyroid that may or may not impair function. 

2.    Certain vegetables, such as kale, are rich in purine, which can possibly lead to the build up of uric acid in joints, a painful condition known as gout. 

3.     Certain vegetables, such as carrots, are rich in beta-carotene, an anti-oxidant, which can lead to carotenosis, a  benign condition in which the skin turns orange. 

4.     The fiber, particularly insoluble fiber, in vegetables can cause intestinal blockage, gas, and    bloating if over consumed. 

5.      Minerals in vegetables, such as potassium and phosphorus, can be difficult for the kidney to  process in high amounts if the kidney is not working properly such as in certain people with high blood pressure. 

6.      Excessive vegetables can lead to hypervitaminosis, which means too much vitamin, such as excessive Vitamin A, which can be harmful to the body. 

7.      Certain vegetables, such as potatoes and corn, contain high calories from starch  that can contribute to weight gain if eaten excessively.

8.    Certain vegetables, such as spinach, can interfere with calcium absorption.

9.    Certain vegetables, such as tomatoes, contain oxalates which can bind to calcium to form calculi of calcium oxalate. Calcium oxalate build-up can lead to kidney stones.

So are the statements above true? Even though in making my statement I relied on my previous recollections, the question asked deserved specific citations of valid clinical studies. However, after spending three hours searching the literature for supporting documentation, I was disappointed to discover that the literature I could find showed limited support.  I went as far back as the 1950s and still could find almost nothing on the topic. Based on this search, It appears that there do not exist any reliable studies demonstrating harms associated with excessive vegetable consumption, other than for reported individual case reports of reactions to specific high intakes.  The goiter and gout arguments had the weakest supporting evidence. The oxalate, a little stronger. 

Therefore, after reviewing the literature, I have to modify my statement about vegetables to say it is inconclusive if eating too many assorted vegetables for people without underlying medical conditions, such as kidney or liver disease, Chrohn’s disease, irritable bowel syndrome, etc. is necessarily harmful, particularly as it appears that most people would have difficulty eating too many vegetables due to the high fiber content. 

That said, I still think that even vegetables should only be eaten in moderation because there are no good studies to show what happens when they are eaten in excess. The absence of evidence is not always evidence of absence.  Sometimes it is, but I prefer not to speculate in this case having already made one misstatement.  

The lesson here for me is what I always say: Caveat lector--let the reader beware. Don't trust what you read unless there is good science to support it.

I'm glad I was asked to support with proof my inaccurate statement.