I don't think they are necessary. I got fairly lean without them. Just well planned diet and exercise like Gregsimo mentioned.
That said, if you are already lean and need the final boost to burn calories and keep metabolism high, they might be worth it.
James 1:16-17 ESV
Do not be deceived, my beloved brothers. Every good gift and every perfect gift is from above, coming down from the Father of lights
With God's help...Mens sana in corpore sano