Commercial and home mead makers use staggered nutrient additions to support yeast health and make mead that is drinkable in a shorter amount of time. If you do not want to add nutrients to your mead, it may take longer to age.
For some sources, it may not be easy to determine the nitrogen content of the additive substance. In such cases, you can send a sample solution to a lab for analysis or - much more easily - rely upon trial and error by adding more of your chosen nitrogen source if fermentation becomes too sluggish or threatens to stop early.
Dead yeast cells can themselves be "cannibalized" by a growing yeast colony. There are several additives available in stores. Yeast Hulls or Ghosts contain only the hulls of dead yeast. Yeast Extract contains whole and partial dead yeast cells, and is the richer nutrient. You can also make your own yeast extract slurry by boiling cheap bread yeast. See the section on boiled bread yeast as Fermaid O substitute under nutrition for details and how to make it.
Fruit added to primary fermentation will provide yeast with nutrients that they need. The amount of yeast assimilable nitrogen varies with the fruit (from field to field or even tree to tree!) and there is not any convenient way for home mead makers to measure this. Adding a smaller dose of nutrients than you would use in a traditional mead is recommended, although it is possible to ferment a mead with just fruit (a lot of it), honey, water, and yeast. Many ancient mead recipes included fruit, possibly because honey and water did not ferment well by itself.
Some companies produce yeast nutrient additives that are specifically designed and marketed as organic and do not contain synthetic chemicals. Look for a trusted organic certification. The use of most of these products in mead is not well documented, with the exception of Fermaid O.
These were a common ingredient for older mead recipes (which also advocated for months-long fermentation and years-long aging), but are rarely advocated today. The amount is usually given as "a handful" or "about 25" for 1 gallon of mead. What follows is some math exploring just how much nitrogen content they add.
Raisins are approximately 1% nitrogen.^†
Dried California raisins are approximately .5 grams each. It varies a lot, obviously, but assuming 25 raisins weighs roughly 12 grams:
12 g x .01 [1%] = .12 g Nitrogen
.12 g N = 120 mg N
1 gal (us) = 3.8 liters
120 mg N / 3.8 l = 31.6 mg/l N
31.6 mg/l N == 31.6 PPM N
Raisins don't add much nitrogen, but "a handful" does contain some slight nutrition; 31 PPM N is enough to double the amount of nitrogen in a honey must with no adjuncts assuming that all of the nitrogen from fruit is consumed by the yeast. ^† This is not necessarily true - not all nitrogen content is yeast assimilable.
To reach the necessary YAN for a healthy fermentation, you would need to add a lot of raisins, which can impart an unwanted vinous quality to mead. A traditional normal strength mead requires 180-300 PPM YAN for a healthy fermentation, depending on the yeast strain and alcohol content.
Raisins or other small amounts of fruit (dried or fresh) can act as a stealthy YAN boost if you don't have other options.