[{_id:1,nutrients:{vitaminB:5,vitaminC:7}},{_id:2,nutrients:{vitaminC:1,vitaminD:2}}]

db.collection.aggregate([{"$addFields":{"sums":{"$reduce":{"input":{"$objectToArray":"$nutrients"},initialValue:0,"in":{$sum:["$$this.v","$$value"]}}}}},{$group:{_id:null,sums:{$sum:"$sums"},},}])