There is one notable diet intervention trial which succeeded in producing marked improvement in outcome, for both cardiac and cancer mortality. This is the Lyon Diet Heart Study. The final analysis is available in full text here. The sub analysis for cancer protection is available here.
What did the Lyon group do? Well this is a little bit difficult to find out as the early publications are in the Lancet and are not available on line. But, from the subsequent rhetoric, I think we can assume the usual things about increasing fruit and vegetables, skipping fat, especially the dreaded saturated fat, and putting the maximum amount of fiber down the loo were all applied. All well and good, except most of these were done far more effectively by the WHEL study, which failed miserably.
When you look at the macronutrient ratios given in table 3 it's clear that there was a 3% replacement of calories from fat with those from carbohydrate. Not a huge change, but on the basis of the Finland study it was probably significantly deleterious. Vitamin intake? In table 4 of the cancer sub analysis paper you can see that there was minimal difference in daily intake of vitamins E and C between groups.
On the face of it there are remarkably few differences between the two groups, especially when you look at the much larger and completely ineffective changes produced by the WHEL study in nutrient intakes.
So what is so special about the Mediterranean diet in Lyon that is not present in the Mediterranean diet in San Diego?
Surely everyone (in France anyway) knows that there is a centuries long tradition of avoiding all butter and cream in an arc between Perpignan and Nice, and for 50 miles inland. Absolutement mon amis, people there have always, at least since Roman times, eaten an experimental gloop produced by Astra-Calve, a subsidiary of Unilever. Made of partially hydrogenated canola oil. Hopefully that's the low erucic acid version of rapeseed oil. The trans fats listed in table here don't look too traditional but, what the heck, it was free and people ate it, in the study anyway.
As it says in the trial design their Mediterranean/intervention diet included:
"no butter and cream, which were to be replaced with an experimental canola oil–based margarine (Astra-Calve, Paris, France) rich in oleic and alpha-linolenic acids. The oils recommended for salad and food preparation were canola and olive oils exclusively."
The exact composition of this gloop is unclear, except that it had a rather high omega 3 to omega 6 ratio. Skipping back to table 3 from the final analysis paper we can also see that dumping all corn oil and sunflower oil (not allowed for cooking or salad dressing) reduces your omega six intake, while the gloop increases your omega three intake, giving a ratio of 1:4. That's a pretty good ratio. The "prudent diet" diet group trundled along with a ratio, pleasing to any poverty stricken cardiologist who needs more business, of 1:16. Awful.
The message I get from the Lyon study is that an absolute omega three fatty acid deficiency is probably rather bad for you and that correcting the ratio of omega three to omega 6 is probably very good for you.
Want to get hold of Astra-Calve's gloop to correct your fatty acid balance?
Forget it. There are better ways.
PS The Lyon study final report begins with this sentence:
"Recent dietary trials in secondary prevention of coronary heart disease (CHD) reported impressive reduction of the recurrence rate by a range of 30% to 70%."
It cites three references for this statement.
First is the DART study. This found that reducing total fat while increasing PUFA (probably omega 6 back in the 1980s) was useless. No surprise there. Increased cereal fiber was slightly worse, this produced a small but non significant increase in the risk of being dead at the two year mark. But two or three fish meals a week, without all that fruit 'n' fiber rigmarole, was very useful. A drop of 29% in two year total mortality. Good, though hardly world shattering
Second reference is to Singh. Read more about this particular paper from Singh here in the BMJ and you will see why the WHEL trial did so badly!
Third reference is self citation.
I don't see a huge amount of support for that first statement.