A sentence from JapanesePod 101:
米は、多くの食事で、重要な糖質源になっている。
Rice is an important source of carbohydrates in many diets.
The になっている seems unnecessary here. Could one simply end the sentence with 糖質源だ without changing the meaning? i.e.
米は、多くの食事で、重要な糖質源だ。
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1だ is a familiar form, it is not generally used in writing but more in informal speech. If you are saying it out then your substitution is correct. But when being written in formal documents, I think even the normal form 「です」 is personal and subjective and hence not generally used in books or academic writing. You can probably write 「米は、多くの食事で、重要な糖質源である。」. I think this is more assertive than 「なっている」.– Mobius PizzaCommented Oct 10, 2013 at 14:41
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1You could end it with である, but it changes the meaning - just as changing 'Rice has become an important source of carbohydrates in many diets' to 'Rice is an important source of carbohydrates in many diets' in English changes the meaning. The translation provided is not wholly literal.– SjiveruCommented Oct 10, 2013 at 23:26
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The literal translation would be something like "Rice, in many diets, has been an important source of carbohydrates". In English, a sentence like that would be said as "Rice is an important source of carbohydrates in many diets", which has a much more diluted meaning. It is much more direct, and in Japanese, not as formal. In cases such as the one you propose, である or です should be used in place of だ. だ has a sense of "It is...", while になっている has a sense of "It is usually the case that...". になっている implies that not all diets receive carbohydrates from rice.– user4060Commented Oct 15, 2013 at 19:09
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1 Answer
You can replace '糖質源だ' with '糖質源になっている' without changing the meaning of the original sentence.
As Mobius Pizza is saying, sometimes '糖質源だ/糖質源である' are used instead of 'になっている'. Meanwhile, '糖質源です' and '糖質源になっている' are also objective and assertive expressions. Examples of subjective expressions are '糖質源だろう/糖質源でしょう'.
The 'has become...' sentence in Sjiveru's comment refers to the role of the rice in the past. But the original sentence only mentions the present state. The sentence in Sjiveru's comment
Rice has become an important source of carbohydrates in many diets
translates to, say,
'米は、多くの食事で、重要な 糖質源となった/糖質源となってきた'
As to PhoenixFox's comment, '糖質源になっている' and '糖質源だ' are both direct expressions. While the original sentence means 'It is 'usually' the case that...', the meaning 'usually' comes from '多くの食事で' (in many diets). If you say, omitting 'in many diets',
米は、重要な糖質源になっている。
this is equal to saying
米は、重要な糖質源だ。
Therefore, you can replace '糖質源になっている' with '糖質源だ'.
However, as mentioned in the comments, 'になっている' seems to be more frequently used than 'だ'. The reason is unclear to me, but there is a similar, widely-used expression '...源になっている/...源になる'. Also, the author might prefer the rhythm (ending always with 'だ' makes sentences too short and monotonic).
