Change it to a positive degree: Milk is the most nutritious food.

• Superlative degree কে positive degree তে এ রূপান্তর করার নিয়ম: 
- No other + Superlative degree এর পরের অংশ + Verb + So + Superlative degree এর Positive form +as + প্রদত্ত Sentence এর Subject.  
Example:
Superlative degree: He is the wisest man in the village. 
Positive degree: No other man in the village is so wise as he. 

Superlative degree: She is the best housewife. 
Positive degree: No other housewife is so good as she.

Superlative degree: Milk is the most nutritious food. 
Positive degree: No other food is as nutritious as milk.

Source: Advanced Learner's English Grammar & Composition by Chowdhury & Hossain.