When the water is poured down the sink, those nutrients go with it.
The longer the vegetable remains in the water, the greater the opportunity for certain compounds to be lost.
Overcooking also changes the physical qualities of broccoli.
The bright green color can become duller. The florets may become very soft, and the characteristic fresh flavor can disappear.
This doesn't make the vegetable dangerous.
It simply means that the cooking process may not be the most efficient way to preserve everything broccoli has to offer.
If boiling is your preferred method, using less water and cooking for a shorter period can help reduce unnecessary nutrient loss.
Another option is to use the cooking liquid in soups or other dishes so that some of the nutrients that moved into the water aren't simply discarded.
Does Broccoli Actually “Cause” Health Problems?
This is where sensational headlines can become misleading.
For most people, broccoli is considered a nutritious food rather than something that should be avoided.
However, individual responses to foods can vary.
Broccoli contains fiber and certain carbohydrates that may cause digestive discomfort in some people, particularly when eaten in large quantities.
People who are not accustomed to eating many high-fiber vegetables may experience gas, bloating, or abdominal discomfort after suddenly increasing their intake.
That doesn't necessarily mean broccoli is harmful.