Sunday, January 19, 2014

The Minnesota Coronary Survey

Studies Associated with the Trial

The Minnesota Coronary Survey: composition of their diets, adherence, and serum lipid response (1975)
The Minnesota Coronary Survey: methodology and characteristics of the population (1975)
The Minnesota Coronary Survey: effect of diet on cardiovascular events and deaths (1975)
Test of effect of lipid lowering by diet on cardiovascular risk. The Minnesota Coronary Survey (1989) [1]

Participants and Diets

A total of 4393 male and 4664 female patients of several mental hospitals were put in two groups an experimental group or a control group.

“The original population was initially stratified into 512 cells on the basis of eight variables. These were: age, sex, length of stay in the hospital, weight, blood pressure, diabetes, cigarette smoking, and evidence by electrocardiogram of a previous myocardial infarction. When new subjects were admitted later, they were divided among four cells, based on only age and sex.”

The experimental diet was “a compromise between the B and C diets of the National Diet-Heart Study with target values of 45% of calories from fat, a polyunsaturated/ saturated fat (P/S) ratio of 2.5, and less than 150 mg of cholesterol daily”.  The control diet was what was normally served at the institutions.  Unfortunately I don’t have access to what the diets in the NDHS were or further dietary information most likely provided in the first paper

The replacing on SFA with PUFA in the experimental diet was achieved by “filled milk and ice cream, a whole egg substitute, soft margarine, whipped topping, filled cheese, low fat ground beef with added vegetable oil, and filled sausage products”

The only other dietary information they give is:

Experimental
Control
Fat (%)
SFA/PUFA (%)
P/S
37.8
9.2/14.7
1.6
39.1
18.3/5.2
0.3
Cholesterol
166
446

Results

Cholesterol levels decreased (by 14.5%), and were lower in the experimental group.  Adherence to the experimental diet resulted in greater reductions of cholesterol (fairly proportionally), but not much (decrease of 15.4 if < 5% meals were missed).  Triglycerides were lower in the experimental group, but increased after 3.5 years on the diet and were higher than the control group after 4.0 years on the diet (may not be significant)


Primary end-points for CHD were slightly lower in men consuming the experimental diet (10.5%), but higher in women consuming the experimental diet (31.7%).  The difference in men is probably not significant, whereas the difference in women may be, but this wasn't tested


Deaths from all-causes were similar for men, but slightly higher in women consuming the experimental diet (16.4%).


The causes of death are presented in table 7.  There doesn't seem to be a consistent pattern with large enough numbers to draw any conclusions, but we can use this to calculate CHD mortality ('atherosclerotic heart disease' + 'cardiac arrest, heart block').  We can also estimate the number of person years based on table 4 and table 5, then report CHD mortality in person years.  The results are in the table below


Men
Women
Exp
Con
Exp
Con
Atheriosclerotic heart disease
18
13
12
11
Cardiac arrest, heart block
21
21
10
9
CHD mortality
39
34
22
20
Estimated person years
2454
2357
2367
2360
CHD mortality per 1000 person years
15.89
14.42
9.30
8.48

The increase in primary end-points and deaths in women consuming the experimental diet led to a very modest (probably non-significant) total increase in primary end-points and deaths in both sexes, which you can see in the tables and these life tables.


An advantage of the trial was the large sample size, but a disadvantage was that “the mean duration of time on the diets was 384 days, with 1568 subjects consuming the diet for over 2 years”, which is only 17.3%.  The trial was not randomised, but the initial stratification and the large group size would have controlled for individual variation.  This leaves bias, particularly in the later stratification, as a potential factor.  However, potential bias would likely be in favour of the experimental group.

No comments:

Post a Comment