ANIMAL PRODUCTS AND THE ENVIRONMENT
-Deforestation: Animal products are the main cause of deforestation, as they require a lot of land to raise the animals and also to grow food to feed them. Specifically it takes 18 times more land to produce meat than vegan foods. 
Resource Overconsumption: Animal agriculture currently occupies 45% of all habitable land on earth, uses over 30% of all freshwater, and over 30% of all food produced. If we eliminated this industry, we would only need to use 25% of all this land and still feed everyone on earth, giving us a lot more land (the 75% left) to use either for building cities, solving the overpopulation crisis, or to preserve wildlife. To learn what would happen in a vegan world, go to the Vegan World section. 
Species Extinction & Others: Because of all the deforestation caused by this industry, as well as global warming, it is the leading cause of species extinction. Fishing for example, murders over 600,000 dolphins, whales, sharks, turtles, and other marine animals as byproducts from their overfishing, which also murders over a trillion fish every year.
Animal agriculture is also the leading cause of: land desertification, ocean pollution - due to old fishing nets, which make up 45% of all pollution in the ocean - as well as ocean dead zones. 
Global Warming: According to a peer-reviewed study done by Oxford University and the magazine Science, known as the Poore and Nemecek Study, animal agriculture is one of the leading causes of global warming. This is because the burps, gases, and manure of these farm animals, especially cows, are by far the main producers of two gases: Nitrous oxide, a gas 297 times more destructive than carbon, and methane, a gas 86 times more destructive than carbon. These gases stay in the atmosphere for over 100 years, propelling global warming forward at an unfathomable rate. Producing over 65% of all nitrous oxide emissions, animal agriculture produces 150 billion gallons of this gas each day. 
VEGANISM AND THE ENVIRONMENT
Animal products are the main cause of basically every current environmental issue. So, veganism can help immensely with every single one:
A vegan saves each day:
1100 gallons of water
45 pounds of grain (food)
30 sq. ft. of forest
20 pounds of CO2
1 animal's life
***According to the Poore and Nemecek study by Oxford Univeristy and the magazine Science, we would only need to use 25% of all land used for animal agriculture in the moment to feed everyone on earth, ending global hunger, and leaving lots of land left to create cities, ending the overpopulation issue. 
This also helps with:
Deforestation: It takes 18 times less land to produce vegan food than animal products. Since eliminating animal agriculture solves the food and overpopulation issues, there is no more reason to need more land, ending any need for deforestation to take place. 
Water shortages would end, as we would get 30% more water for us, which is currently used for the farm animals. 
Species extinction, ocean dead zones, and ocean pollution would greatly if not completely stop, as animal agriculture is the leading cause of every one of those issues. That would save the lives of trillions of different species all over the world. 
Global warming would greatly decrease its effect on the planet, preventing many catastrophic events from occurring because of it. That is because a lot less carbon, methane, and nitrous oxide is emitted through vegan food production in comparison to animal-based food production. 
ANIMAL PRODUCTS AND HUMANITARIAN ISSUES
Due to the overconsumption of resources to produce animal products, lots of humans are left lacking those resources, which are provided to the animals raised for animal products instead.
One case is food:
Over 30% of all food produced is fed to the animals in animal agriculture instead of being fed directly to humans, causing global hunger. We produce enough plants to feed 70 billion land animals, all of which eat a lot more than we do. That means we have way more than enough food to feed the human population, if we eliminate animal agriculture and redirect that food to humans instead. 
Another case is water:
Again, over 30% of all freshwater is fed to the animals in animal agriculture, instead of the humans who need it. This causes tons of water shortages and deaths and suffering from dehydration, which would all be nonexistent if animal agriculture was eliminated and the water was redirected to humans instead. 
Another case is the overpopulation issue:
Currently 45% of all land on earth is used by animal agriculture, and that percentage will only increase as our population increases. There's not enough room on this planet for a growing human population and an unsustainable food industry that barely feeds 1/4 of the human race. If animal agriculture was eliminated, an Oxford University and Science Magazine study known as the Poore and Nemecek study found that we would only need to use 25% of all land used today in animal agriculture, and still feed everyone on earth, ending global hunger, and leaving a huge piece of land left for us to inhabit, solving the overpopulation issue. 
Another case is our future:
Animal agriculture is by far one of the main contributors to global warming, as explained in the previous section (environmental issues). By eliminating this industry our society could reduce global warming's temperature increase to the level it needs to be for us to be safe, preventing the sixth mass extinction and the end of the human race. 
*********All of these problems seem to be only solvable by fully eliminating this industry, but going vegan yourself contributes immensely to that. Last year, the two main dairy producers of the United States went bankrupt because of individuals such as you and I going vegan. You can easily make a difference by changing whats on your plate each meal.
ANIMAL PRODUCTS AND DISEASE CAUSATION
Animal products are the main cause of heart disease, and increase the risk of one getting various cancers, such as bladder, prostate, breast and other cancers, as well as type 2 diabetes and heart disease, by a significant amount even when consuming relatively small amounts of these products.
Even moderate intake of red meat increases your risk of cancers such as bowel cancer: The risk increases by 20% with every extra slice of ham or rasher of bacon - roughly 25 grams - that one eats, and increases by 19% for each thick slice of roast beef or the edible part of a lamb cutlet - about 50g. 
This is because of certain compounds in these foods:
Toxins, saturated fats, and cholesterol, are directly linked to a significant increase in the risk of getting heart disease, strokes, certain cancers, diabetes, and even Alzheimer's. 
Cholesterol is a compound found only in animal products, directly linked to major increases in risk of atherosclerosis and heart disease. [2.5]
When one ingests proteins that have a higher proportion of the amino acids, located solely in animal products, it results in our bodies producing higher levels of the hormone insulin-like growth factor-1 (IGF-1). This hormone stimulates cell division and growth in both healthy and cancerous cells. Thus, high levels of this hormone, caused predominantly by high intake of animal products, is associated with increased cancer risk, proliferation, and malignancy. 
Animal protein intake also results in the body having higher circulating levels of trimethylamine N-oxide (TMAO). This substance injures the lining of our vessels, creates inflammation, and facilitates the formation of cholesterol plaques in our blood vessels. This is all very problematic and dangerous for cardiovascular health. 
Animal protein contains high levels of phosphorous. This high level, when consumed, is normalized in our bodies by a hormone called fibroblast growth factor 23 (FGF23). This hormone consequently becomes more abundant in the body, and has been directly linked to abnormal enlargement of our cardiac muscle, as well as heart attacks, sudden death, and heart failure. FGF23 has been found to be harmful to our blood vessels too. 
Animal products have abundant amounts of heme iron, which convert less reactive oxidants into highly free radicals, which can damage different cell structures such as proteins, membranes, and DNA. Heme iron can also catalyze the formation of N-nitroso compounds in our bodies, which are potent carcinogens. So, unsurprisingly, high intakes of animal products and heme iron are directly linked to increased risks of many kinds of gastrointestinal cancers as well as other diseases. 
Animal proteins finally also have, in general, higher concentrations of sulfur-containing amino acids, which can cause a state of acidosis when metabolized. One mechanism in our body used to treat this acidosis is leaching calcium from our bones to help neutralize the increased acidity. This can have a very detrimental effect on bone health. 
**********The only relatively healthy aspect of animal products are the nutrients that they contain, yet all of these nutrients can be found in plants, but without the disease causation part, as plants do quite the opposite - they contain many compounds that help prevent the very diseases caused by animal products.
: A five year study with half a million men and women: https://academic.oup.com/ije/article/49/1/246/5470096
: Xiao Y, Peng C, Huang W, et al. Circulating fibroblast growth factor 23 is associated with angiographic severity and extent of coronary artery disease. PLoS One. 2013;8(8):e72545.
VEGANISM AND DISEASE PREVENTION
A well-planned vegan diet has no increased risks of diseases, unlike an omnivorous diet.
In fact, when planned properly, a vegan diet has many health benefits:
1. Phytochemicals, found only in plants, are provided in a vegan diet and some are known to reduce the risk of heart disease, cancer, and other diseases. 
2. Plant foods contain fiber, which is linked to a lower risk for cancer, heart disease, and obesity. Animal foods contain no fiber at all. 
3. Plant foods contain no cholesterol, a compound found only in animal products and abundantly so. This compound increases the risk for heart disease, cancer, diabetes, stroke, and other diseases. 
4. Plant foods have compounds found only in plants such as Lycopene, that reduce the risk of heart disease and other diseases such as prostate cancer. This compound is predominantly found in tomatoes. 
5. Plant foods are low in saturated fat, whereas animal products are high. Plant foods are excellent sources of unsaturated fat, and replacing saturated fats for unsaturated ones helps reduce blood-cholesterol levels. 
6. Plant foods are the best and predominant sources of copper, which helps significantly prevent respiratory diseases. 
7. Plant foods are the best sources of antioxidant vitamins, such as vitamins C and E, and beta-carotene, which protect your cells against free radicals - commonly caused by animal products and refined carbohydrates - which cause cancers and heart disease, thus greatly reducing the risk of catching those diseases. 
8. A vegan diet can reverse coronary heart disease 
9. A vegan diet can greatly lower your blood pressure, reducing your risk for hypertension. 
10. A vegan diet greatly reduces your risk of getting type 2 diabetes. 
**** A vegan diet can provide every nutrient except for one: vitamin b12. So, vegans must take a supplement of 1000 grams twice a week, which is a simple either liquid version or under-the-tongue drop that one can buy for a cheap price at any and every pharmacy. It is essential to take this supplement as not doing so could lead to strokes in the long run (taking the supplement eliminates that risk). A well-planned vegan diet that takes these factors into consideration will be very beneficial to everyone who undertakes it, sometimes even reversing chronic illness such as heart disease, and greatly reducing risks of getting cancers such as prostate cancer. 
VEGANISM FOR ATHLETES
There are many benefits in a vegan diet for athletes. First of all, vegan foods unclog your arteries, which were clogged due to the animal products previously consumed. This unclogging allows blood to flow more smoothly and easily throughout the body, directly leading to improved physical performance, as well as reduced risk of heart disease. 
For a lot more information on atheltics and veganism, check out the wonderful documentary: Game Changers on Netflix!