There are a number of things that can cause a person to DNF, DID NOT FINISH, in an ultra; inadequate training, an injury, not ready for the mental toughness that is essential and poor nutrition. Even though several of those reasons 
can happen all at once, the number one thing that causes people to DNF is poor intake of nutrients. When our bodies are fueled correctly, we are capable of amazing things; the trick however is knowing what to eat and when to eat it.
There are a number of things that can cause a person to DNF, DID NOT FINISH, in an ultra; inadequate training, an injury, not ready for the mental toughness that is essential and poor nutrition. Even though several of those reasons can happen all at once, the number one thing that causes people to DNF is poor intake of nutrients. When our bodies are fueled correctly, we are capable of amazing things; the trick however is knowing what to eat and when to eat it.
Nutrition is comprised of macronutrients and micronutrients. Macronutrients are the ones that contain calories, such as carbohydrates, proteins and fats. Micronutrients are vitamins, minerals and trace elements these are needed in much smaller amounts than macronutrients and are found with the macronutrients. For example eating a spoonful of peanut butter gives you both macronutrients and micronutrients....make sense.
Let us look at the macronutrients. Protein is used for the building of pretty much everything in our body, it is essential for our survival. Carbohydrates are broken down and used for energy. Fats are involved in an array of body functions one of which involves storage of energy. Your body will go to carbohydrates first for energy and in the absence of these, go to fat. Now with that said your body does and can burn both at the same time. With low intensity exercise, like an ultra, you burn quite a bit of fat along with carbohydrates, but your focus needs to be on carbohydrates.
It is important that you not only train for the race with running but you also train with nutrition; on race day you need to have a plan going in and one you have practiced. One week before the event, you need to really focus on consuming enough water and carbohydrates. The goal here is to top off your glycogen stores, AKA carbohydrate stores, so that on race day you are fully charged. Meals should include breads, pastas, fruits, vegetables especially starchy ones and rice. Make sure and include healthy fats such as avocados, nuts and olive oil. Protein sources are any type of meat products and dairy. Think of it this way if it walks on the ground it has protein; if it grows from the ground it contains carbohydrates.
In regards to meal timing, I like to recommend your largest meal not be consumed the night before. Food can actually set in our system for up to 72 hours, for example a steak. A large lunch is fine the day before but your biggest meal needs to be two days before the race. This ensures that you are topping off your energy supply but also that you do not have a gut full before the race causing you to be sluggish.
Most ultras start early in the morning and honestly it’s very hard to have a meal, before the event. Consuming .5-1 gram of carbohydrates per pound of body weight 1-2 hours before the race is recommend; however if you can get in a bagel with cream cheese or a granola bar you will be good to go. Remember most of your energy will most likely come from what you have done the week prior to the race. Think about what you did before your long runs....did you eat a big meal or did you throw something in the backpack for later?
The best piece of advice for eating during the race is to start early, do not wait until you are already depleted..........not good, that increases your chances of hitting the wall; which is literally running out of stored carbohydrates and completely utilizing fats for energy. This is not a bad thing if you have trained your body to do this beforehand but not on race day. My recommendation is to consume around 60 grams of carbohydrate per hour. During an ultra you most likely lose your appetite but you need to consume calories be it via a drink or food. In these situations, I think chips, pretzels; M&M’s and even sodas are OK. Not the best sources but they will deliver the calories needed to keep going. One way to ensure you will consume enough calories is to pack your own. Granola bars, cliff bars, sport beans, bonk bars and gels are easy to carry and tell you how many carbohydrates you are getting, so it is really easy to get the 60 grams or so recommended. Hydration is also essential, you need to be drinking enough fluids right from the beginning and continue all the way through the race. I recommend drinking to your thirst. If you follow a strict plan on hydration, you may feel water logged and sluggish; this is also something you need to practice before the race.
Post-race meals are very important for recovery. Eating a meal rich in protein and carbohydrates is recommended post-race. Many races have a huge smorgasbord ready for you when you finish offering bars, bananas and sometimes-even pasta, perfect for post-race foods. Even though you might want to celebrate, alcohol should not be at the top of your list. The sugar in alcohol is not storable and therefor does not assist with recovery.
Proper nutrition is absolutely essential to finish an event like an ultra. Consuming enough calories and hydrating appropriately are just as important as your training. Be thinking about what you will consume the week before the race and what you will consume, both fluids and food, during the race and your chances of success will increase dramatically.