"Some people choose to eat no meat or fish. They believe that this is not only better for their own health, but also for the world as a whole"

Historically, humans from all cultures have thrived on a meat-based diet. Some cultures have developed many other food sources (bread, rice, ...) but overall meat was always the primary nutrient source. However, with the first quarter of the 21st century behind us, we are seeing an increase in the number of people who decide to give up meat and fish. There are multiple reasons for their choices, ranging from claimed health benefits to animal rights.

First, there is the health argument. Many vegetarians/vegans claim that their diet makes them healthier than "meat eaters", by reducing cancer risks. However, those claims have not been proven yet. On the contrary, we know from a scientific and biological standpoint that humans are omnivorous, and they need nutrients from both plant-based and animal sources in order to operate at peak performance. Iron for instance is primarily found in red meat and is a necessary part of our diet. 

Then, vegetarians/vegans claim that their lifestyles have a smaller impact on the environment. While that is true, there are some caveats. We know that fishing has a negative impact on aquatic biodiversity, and that meat is one of the key green house effect gas producers out there. However, while the vegetarian lifestyle's impact is smaller, it is still significant. Crops take a lot of space, and cultivating some types of crops can hinder local biodiversity.

Finally, there is the animal rights argument. Most vegetarians stopped eating meat and fish because they feel empathy towards the animals we kill. They believe that because the animal is conscious and has feelings, it deserves to live. We won't discuss the precondition (consciousness of animals) here, because consciousness cannot be proven as of right now. However, we can discuss the conclusion. Why would having a consciousness necessarily equate to a right to live ? After all, wolves eat rabbits every day all around the globe, and it's not for a lack of carrots. For all we know, broccolis might be conscious and might have feelings too. After all, plants have a nervous system, defense mechanisms and as far as I'm aware, they haven't communicated with us about liking being eaten yet. How dare you kill an innocent plant to consume it ? 

Have a heart. Eat rocks.