When -ung is a suffix like this it is feminine. (It creates a noun from an equivalent verb, e.g. Ernährung from ernähren.) However, there are a few words where it isn't actually a suffix, such as Sprung and Dung (which are both masculine). There are also a few like this which look like they are suffixed stems but aren't, such as Shantung (which is a Chinese loan word I believe; it's also masculine.)
While a dictionary might be a better place to look, I'll give it a go. As I understand it Ernährung can be translated as nutrition, diet, intake or feeding
Nutrition is the "the science of nutrients" (nutrients being things like carbohydrates, fats, proteins, vitamins, minerals etc). It can also be used to mean the process by which organisms take in nutrients.
As mentioned by chapman-john above, in English you will rarely hear "the nutrition" as abstract nouns tend not to take articles (i.e. it is usually just "Nutrition"). Often you will see it in the form of an adjective or adverb (nutritional). For example, "That food isn't very nutritional" would mean the food doesn't contain many useful nutrients, i.e. it isn't very healthy. Here in the UK we have "Nutritional information" labels on food packaging which list things like how much energy (calories/kilojoules), salt, sugar etc is in a food product; in the US I believe this is labelled "Nutrition Facts".
Diet when used like this is similar to nutrition - it describes what nutrients an organism consumes. Intake is pretty much self explanatory (it is what is "taken in", i.e. consumed).
Feeding isn't really a good word-for-word translation I don't think but is often the best choice in context.
The word alimentation has more uses or various meanings in French than in English. http://dictionnaire.reverso.net/francais-anglais/alimentation
Your dictionary is concise precisely because it does not have all the words. You might want to try the online Oxford: http://www.oxforddictionaries.com/definition/english/alimentation or you might also like to have an American alternative: http://www.merriam-webster.com/dictionary/alimentation
Alimentation does not exactly mean nutrition, but it is "the act or process of affording nutriment or nourishment." http://www.merriam-webster.com/dictionary/nutrition Whereas nutrition is "the process of eating the right kind of food so you can grow properly and be healthy" or "the act or process of nourishing or being nourished; specifically : the sum of the processes by which an animal or plant takes in and utilizes food substances"
Hey Allintolearning - In the end, Duo decides what English words correspond to "die Ernährung". However, my comment that "nutrition" is an everyday English word - while "alimentation" is not - is a practical and useful response to MariaDeLau's question and also supports Mahankr's comment about the term being very uncommon in American English.
I agree that nutrition is the more common word and further I feel that alimentation is used differently than nutrition. So, it is not just a matter of which word is more common, but whether "alimentation" should also be allowed. Only sometimes would "alimentation" be translated as "Ernährung", but it sometimes is. http://www.dict.cc/?s=Alimentation
Yes, we should report "the alimentation" as one possible meaning of "die Ernährung", while "the nutrition" is another different possible meaning for this word. These are not different ways of saying the same thing. It is just that "die Ernährung" has many meanings. I am sorry that you misunderstood my intentions which were not so much unfriendliness as an effort to supply you and inevitably MariaDeLau with more information as she was wondering why it would not be accepted when it possibly could be.
You should use the report feature to explain to Duo that you think "alimentation" should be accepted as a different way of saying "die Ernährung" in English. I don't recommend that you communicate with Duo the way that you communicated with me - your earlier post wasn't particularly friendly/polite.