Thoroughly read some past reviews , analyse the issue and see the gap , could be one way. Other way could to arrive at some popular strategy of nutrition but still not so well discussed, can also be picked up for review.
First be sure what you want to review, a method, an outcome, an input, or an impact and why you would like to review that specific thing. Based on that, read as many papers you can and make an xl sheet with a jist of the paper and its reference. while reading them, you will know about various research gaps. these will go under the section of future scope in your review. you can also divide your review in chronological sections or sequence of updates.