Maybe not everywhere, so many of us gives our ferrets cat food (dry food with high protein content). But as that's adjusted for cats, ferret food should be better.
This is what a ferret is considered to need during different seasons:
winter spring summer autumn
Protein >=35% >=40% >=30% >=30%
Fat 20-50% 40-50% 35-55% 30-55%
Carbohydrates <=25% <=20% <=30% <=30%
Which can be expressed like this:
Energy requirement:
Protein 40% or more, and most in the spring
Fat 40-50%
Carbohydrates 20% or less, and least in the spring
As a gram of the different components contains different amounts of energy (fat about twice as much as protein and carbohydrates), using average values for energy content and digestibility, this tells us that: Ferret food should contain about twice as much protein as fat, and less carbohydrates than fat. This is where cat foods usually don't measure up, as they often contain too little protein compared to fat.
The absolute values in percent isn't the all important thing, as they vary depending on how much "other" they contain. Food in tins usually contains more water than dry food, so the percentages cannot be as high. And since you don't know exactly which types of proteins and fats they contain, this can only be considered as general guidelines, and you'll have to see which food works best for you.
You cannot live of only protein, fat and carbohydrates. For example a small amount of fibres is good for digestion. The fibre content should never be more than 4%, and should preferably be around 2%. Minerals are necessary too, an ash content of 7-12% is considered to be optimum.
Vitamins are a difficult subject, as the sources differ very much in their recommendations. What can be said generally is that the fat soluble vitamins (for example A and D) can be over dosed, as they are stored in the body, and also that you can live on what is stored for some time. You cannot live on stored water soluble vitamins, for example B, as they disappear from the body every day, so you don't have to worry as much about over dosing them.
The conclusion I draw from all this, is that cat food is OK for the most part, but I don't consider it sufficient for at ferret, at least not year round, as it often contains too little protein compared to fat.