Much of what is to be included in a speech on Western diets will have to revolve around what must be included. I would say that one part of your speech should be on how much Western diets have changed over time. In the last fifty years, what is seen as representative of a "successful" Western diet has undergone significant alteration. One part of a speech outline should include the foods that have been come to be included in the Western Diet. Outlining these will prove to establish the basis for both the Western Diet as well as how it is viewed today.
I think that this becomes the next phase of the speech. The Western Diet is now under scrutiny. What used to be included in a "Western diet" is something that we now understand as not necessarily the embodiment of health:
The way humans have eaten has changed more in 140 years than in the 10,000 years before. And it changes with the processing of grain, the refining of oils from grain, the arrival of free sugar and high fructose corn syrup, lots of processed food and less reliance on whole foods — fruits, vegetables, whole grains. That, in short, is the Western diet.
The idea of looking at a Western Diet should be examined in terms of what it constitutes. The foods and intake that used to constitute a Western Diet now results in unprecedented cases in Type II Diabetes, heart disease, and digestive challenges. This reexamination of the Western Diet would be another part of the speech.
Finally, I think presenting how the Western Diet is viewed today would be an effective conclusion. This can include how the diet is supplemented with a desire for something more healthy and more conscious of the diet's limitations. In being able to present a view of the Western diet in terms of where it was and where it is now with how it is viewed today, a speech on the Western diet can be informative in its resonance with the listener. This might be a good outline to follow in constructing the speech.