Yeast cannot live on sugar alone. Yeast also need nitrogen, and amino and fatty acids to enable them to live and grow. The primary source for these building blocks is the free amino nitrogen (FAN) and lipids from the malted barley. Refined sugars like table sugar, corn sugar or candy sugar do not contain any of these nutrients. And, it is common for extracts (especially kit extracts targeted toward a particular style) to be thinned with refined sugars to lighten the color or reduce the cost of production. An all-malt beer has all the nutrition that the yeast will need for a good fermentation, but all-extract beers may not have sufficient FAN to promote adequate growth. Since malt extract is commonly used for yeast starters, it is always a good idea to add some yeast nutrients to ensure good yeast growth.
If you use ion-exchanged softened water for brewing, the water may not have adequate calcium, magnesium, and zinc for some of the yeasts metabolic paths. Magnesium plays a vital role in cellular metabolism and its function can be inhibited by a preponderance of calcium in the wort. Brewers adding calcium salts for water chemistry adjustment may want to include magnesium salts as part of the addition if they experience fermentation problems. Usually the wort supplies all the necessary mineral requirements of the yeast, except for zinc which is often deficient or in a non-assimilable form. Additions of zinc can greatly improve the cell count and vigor of the starter, but adding too much will cause the yeast to produce excessive by-products and cause off-flavors. Zinc acts as a catalyst and tends to carry over into the succeeding generationtherefore it is probably better to add it to either the starter or the main wort but not both. The nutrient pouches in the Wyeast smack-packs already contain zinc in addition to other nutrients. For best performance, zinc levels should be between 0.1-0.3 mg/l, with 0.5 mg/l being maximum. If you experience stuck fermentations or low attenuation, and you have eliminated other variables such as: temperature, low pitching rate, poor aeration, poor FAN, age, etc., then lack of necessary minerals may be a significant factor.
You will see three types of yeast nutrients on the market that can supplement a wort that is high in refined sugars or adjuncts.
Di-ammonium Phosphate - This is strictly a nitrogen supplement that can take the place of a lack of FAN.
Yeast Hulls - This is essentially dead yeast, the carcasses of which act as agglomeration sites and contain some useful residual lipids.
Yeast Nutrient or Energizer - The name can vary, but the intent is a mixture of di-ammonium phosphate, yeast hulls, biotin and vitamins. These mixtures are a more complete dietary supplement for the yeast and what I recommend.
Servomyces (tm) - This product from Lallemand is similar to yeast hulls but differs by having a useful amount of rapidly assimilable zinc, which is an essential enzyme co-factor for yeast health. This product falls within the provisions of the Rheinheitsgebot.