One in two is/are

This is a tricky one. Ordinarily I would agree that you need subject/verb agreement and, therefore, it should be 'is', not 'are.' The subject, after all, is 'one.' But, on the other hand, you are in fact speaking of a number of people here, despite the singularity of the subject. I think the sentence can stand the way it is.

Not sure what your question is regarding 'be' and 'am.'

They mean British English ('BE'). 'AM' refers to American English. I notice the other short forms are BrE and AmE.

If you consider the "One in two" to be the subject, then use "is"

If you consider " Malaysians" to be the subject (and "One in two" an adjective describing Malaysians) then use the "are".

I can't say to a certainty, but there might be a difference between the American language and British usage here. For example, Americans tend to say "the staff is," seeing staff as a singular, unitary concept. The English would say "the staff are," recognizing that staff is a singular noun which represents a plurality of people. I am not exactly certain, because i am uncertain how an Englishman would approach this specific sentence.

By the way, many Americans would say "their" (as in " . . . with their weight . . .") even if they treated the subject-verb agreement as requiring the singular--preferring that to saying "his or her weight." Many Americans use the third person plural prounouns in order to avoid the clumsy repetition of "his or her" or "him or her." So, for example, in speaking of a mixed group of men and women, an American might say: "Choose just one, and give them a good chance to make their case" in preference to "Choose just one, and give him or her a good chance to make his or her case." How this arose in American usage, and whether or not the English do this, too, is a matter about which i know nothing.

Also, if one thinks subject-verb agreement requires a third person singular verb form, it seems to me that the second occasion of the use of the verb "to be" (which you have highlighted, and which is the form "are") ought to have the same agreement, and be the singular "is."

I would write the sentence:
One in two Malaysians is not satisfied with their respective weight and is attempting to shed some, according to market research company, A.C. Nielsen.

Also, in lieu of "their respective weight" I might just put "his/her weight."

I think somewhere I read/heard that the use of "their" instead of "his/her" is allowed in speech, but in writing it might seem less than correct?

Foofie, better tell that to "Jane Austen: Geoffrey Chaucer, Edmund Spenser, William Shakespeare, the King James Bible, The Spectator, Jonathan Swift, Daniel Defoe, Frances Sheridan, Oliver Goldsmith, Henry Fielding, Maria Edgeworth, Percy Shelley, Lord Byron, William Makepeace Thackeray, Sir Walter Scott, George Eliot [Mary Anne Evans], Charles Dickens, Mrs. Gaskell, Anthony Trollope, John Ruskin, Robert Louis Stevenson, Walt Whitman, George Bernard Shaw, Lewis Carroll, Oscar Wilde, Rudyard Kipling, H. G. Wells, F. Scott Fitzgerald, Edith Wharton, W. H. Auden, Lord Dunsany, George Orwell, and C. S. Lewis."

http://www.crossmyt.com/hc/linghebr/austheir.html#authlist

Oh, what these silly prescriptions have wrought.