COSA INCREÍBLE

5.4.2.1 Methods available. Three main methods for estimating group and indi­ vidual consumption of food have been used in epidemiological studies (although they do not constitute the only valid methods). Firstly, a dietary history is directed to­ wards obtaining a qualitative and quantitative assessment of a subject’s usual eating pattern. Documentation of a dietary record is undertaken by the subject with the aim of measuring current dietary intake and usually spans 7 days or 20 consecutive meals.665 An alternative method is the diet-recall interview which is directed toward detailing food consumption for a specific period, usually 24 hours. Longer periods are subject to defects in memory.665 A dietary recall method has been combined with a history of the frequency of the consumption of various foods in order to obtain a more accurate measure of the subject’s usual diet.666,667 These methods have been evaluated and have been found to have a high degree of correlation between esti­ mates of food consumption and all are suitable for case control studies.665 The accurate retrospective measurement of the long-term diet is very difficult and may not reflect the current diet.472,668 However, the recall method is the best predictor for estimation of past food and nutrient consumption.669 The limitations of the utility of these methods must be acknowledged because of the important influence that a dis­ ease itself, and knowledge of the disease, may exert on current food intake.665

Ideally the intake of energy and nutrient content of dietary food should be analysed chemically since the energy values and nutrient content of food documented in food tables are only estimates from averages. In group survey work, the cost and time required for chemical analysis makes the method impractical for most clinical and epidem iological research. Assessment of nutrient intake using food tables has become an established method for dietary surveys.670

The 24 hr dietary recall method provides reliable estimates of the average intake for groups, and the data has been comparable to that obtained by more time consuming m ethods.668 Nevertheless, the reliability and validity of the 24 hr dietary recall method is reduced when used to characterise individual patterns of food consump­ tion, or identify individuals having some form of nutritional risk.671 Variation in food intake over seasons of the year, days of the week and work periods are major sources for intra-individual variance for the short recall methods. The recall method is prone to over-reporting low in-takes and under-reporting high intakes with the attendant bias of the "flat-slope syndrome".672

A dietary record covering 7 consecutive days, or 20 consecutive meals, has been recommended as an adequate time to obtain a degree of validity.665

5.4.2.2 Methodological limitations. The accuracy of self-report methods of as­ sessing food intake had been questioned and the validity of methods for measuring normal dietary intake have been notoriously difficult.668 However, there appears to be an acceptable correlation between measured food intake and a dietary report, and self-reported food intake may be more accurate than previously thought.667

Reliability problems arise when an individual’s "usual" intake has sources of vari­ ability that cannot be attributed to true differences between individuals.673 Such error

can arise for many reasons,673 as discussed above. The basic assumption that an

habitual diet exists may be incorrect. There may be inherent errors within food composition tables. Recall accuracy of food consumed may be inaccurate. Interview variability is a well documented area of bias. A training effect may occur with re­ peated interviews or diary entries. There may be differences in absolute nutrient content for non-generically named foods. The day of the week effect due to day to day variation can be a substantial confounding factor. Work related differences and data handling errors add to these more commonly encountered sources for introduc­ tion of bias described above.666,671,673 Food composition tables are useful and have a

well recognised utility, although their limitations are conspicuous.666,668

5.4.2.3 Food measurements and estimations. Food diaries can accurately in­ dicate the food eaten by an individual at a given time, if the subject is properly moti­ vated and is able to record carefully the intake with appropriate instruction before­ hand and checking of the record by interview afterwards.674 However, comparison

of estimated food portions with actual measurements have revealed potentially large errors in portion measurements,668 although others have found an acceptably reliable

agreement. 667 Conducting a dietary survey requiring the subject to measure the

quantities of foods means creating new practices in the kitchen and at the table which may introduce bias in the form of modification of the "normal" diet. An individual’s food intake can be affected by the record-keeping procedure itself.472

An estimate of nutrients of special interest can also be difficult. For example, the fat content may vary between meat cuts and fat may be added or lost during cooking. However, when fat intake is assessed by a dietary history, the correlation between repeated measures is high for the same observer, but low for different observers.472

The contribution of systematic errors, such as coding, to the total error of calculated intakes for a seven-day period is likely to be small for persons with variable eating patterns.675 However, it still will be necessary to standardise the procedures and the

coding, preferably by using the same interviewer, to reduce variance associated with subjective interpretation, thereby increasing the reproducibility of the calculated in­ takes.675

5.4.2.4 Methods of statistical inference. Correlation between dietary informa­ tion and measured parameters will be made with the knowledge that intra-individual variation and random errors of measurement may lead to large confidence intervals, misclassification and reduced accuracy of correlation coefficients. These effects can reduce the power of statistical inference and produce false negative correlations.671

In such circumstances, the interpretation of any observation, in particular negative results, must be considered with caution in the absence of a highly reliable and validated estimate. However, if intra-individual variation and measurement error are random, there will be no bias in the mean value. 668 Comparison of the results of

dietary survey measures of samples from specific populations can be performed, assuming the variations are random about the mean, by comparing the means in the upper and lower quintiles of the characteristic being considered.668

Dietary survey methods may show differences in the averages among samples of people when there are large differences in the diet at the time of the survey. 668

classification of an individual with respect to their diet, or specific nutrient, may give rise to major methodological limitations. For the dietary survey methods, the more important areas of bias include the misclassification resulting form wide intra­ individual variation, instability or inconstancy of the diet over time, such as in seasonal variation within the distribution of the population over longer periods of time.668 As a result of these and other causes of error described above, the intra­ individual variation may be as large as the inter-individual variation.668 If the nutri­ ent content is reported in proportion to total energy consumption, total variance tends to fall, inter-individual variance falls, and intra-individual variance also im­ proves, but to a lesser degree.673

5.4.3 Methods

5.4.3.1 Rational for the methodology employed. Since, within a homogeneous population, the variation for each individual for any particular dietary factor may be as large or larger than inter-individual variations,671 a seven day dietary diary was obtained. This approach was undertaken to improve the estimate of the true mean intake for any dietary factor for the individual, thereby increasing the ability to dis­ tinguish group differences for homogeneous study populations.671,668 Indeed, the methods described above are all applicable to case-control studies. The diet history was selected since the current food intake may be influenced by the disease and/or knowledge of the diagnosis.665 The main purpose of the dietary assessment was to determine if any major differences existed between the case and control group since dietary differences have the potential to influence the measurement of a number of CHD risk fa c to rs .150'469,636,651 as well as some of the p latele t function measures.663,676 Assessing specific areas of possible differences between the two groups is therefore necessary to determine if areas of bias have arisen from differ­ ences in specific dietary factors, particularly if affecting platelet function parameters. The main value of the method used will be to determine differences in grouped sample averages. The observations will need to be viewed with caution, particularly because of the potential for false negative results.

5.4.3.2 Methodology. The nutritional information obtained was the content of the subject’s diet for the 7 days before the blood sampling. The diet was assessed as a food group analysis. The subject estimated the amount of food by describing the number of portions of specific foods and food groups. This was estimated by the subject with the assistance of photographs of food models and descriptive measures from food tables. All data on nutrient composition of foods for approximated por­ tions was obtained from published sources although the comparison was made only with the groups of food types and the number of portions in that group.

The subjects were carefully instructed about the methods of documentation, impor­ tance of continuing their normal diet, on the use of the food model descriptions and photographs, and of what a portion of any specific and commonly used food consist­ ed. The initial instructions on how to keep an accurate diary of all food and beverage consumed at each meal and between meals were given to the subjects by the same individual (the author). The food diary was documented on a single subdivided sheet and on a daily sequential basis. The week consisted of meals which were routine and

Standard for the subject and his family. A follow-up review of the recorded intake with the participants was undertaken in order to check for accuracy and complete­ ness. The follow-up assessment was also carried out by the same interviewer.

The dietary diaries were then coded by another person to blind the author to the identity of the subject for all the diaries. These were randomly coded and mixed so that it was unknown to the author whether the subject was from the case or control group. The number of portions of specific foods and groups were documented and are outlined in Tables 5.7, 5.8 and 5.9.

5.4.4 Results

The number of portions of meat taken by the case group was lower than the control group with a borderline level of significance (Tables 5.7 and 5.9). There was no dif­ ference in the number of portions for individual types of meat, that is, beef, chicken, pork and lamb. Nor were there any significant difference in the way the meals were prepared, whether fried, grilled, baked or in casserole form. In particular, no dif­ ference was observed for fried foods (Table 5.8). The case group tended to have more fish in a baked, grilled or casseroled form, although this was not statistically significant because of the high inter-individual differences leading to the high var­ iances for the groups (Table 5.7). The case group consumed significantly less full cream milk, substituting low fat milk (Table 5.9). In addition, the case group consumed significantly less sweet foods high in fat, such as pastries, cakes, sweet pies and so on, than the controls (Table 5.9). Therefore, the total fatty food intake in the control group was greater than in the case group.

Of similar importance, the case group consummed significantly more fruit than the control group (Table 5.9).

Because the relevance of alcohol to platelet function, this is examined in more detail in the next section.

5.4.5 Discussion

The cholesterol level for those with a previous measurement was significantly great­ er than the level measured at the time of the study in the case group (Table 5.1). It would be reasonable to assume that the diet for the case group had been modified since the diagnosis. To confirm such an assumption it would be necessary to extend the dietary assessment by obtaining a previous and current food group recall estima­ tion. This was not performed as the main purpose of the food intake evaluation was to determine if there were, in the current diet, any differences which might have influenced platelet function, given the potential differences in diet after a diagnosis and the possible influence of that change on platelet function. The potential for such an effect is highlighted in the introduction of this section which illustrates some clear examples were this could occur, For instance, an altered saturated fat, or other fatty acid intake, has the potential to measurably influence platelet function.

The presence of a lower intake of food with greater saturated fat and cholesterol content in the case group could potentially result in reduced platelet reactivity and

aggregation. Such an effect could lead to a false negative result when comparing the case and control group, or reduce any differences found. T herefore, any m easures indicating increased platelet reactivity or aggregability in the case group may be less than at the tim e of, or before the diagnosis o f CHD was established. This dilution bias is unavoidable in the current study because o f the case-control design, and the lim itatio n has to be fully recognised and taken into account when interpreting the primary study results.

A nother im portant bias that has not been controlled for in this evaluation is response bias. The presence o f acquiescence, or socially desirable responses, by individuals undertaking self-response questionnaires is well docum ented.471,677,678 Ideally, some form o f assessm ent o f this potential bias should be adm inistered sim ultaneously as any other self-response inventories.677,678 The lack o f such an assessment does pose a p ro b lem in in te rp re ta tio n o f the resu lts. H o w ev er, the in te rn a l co n sisten cy w ith regard to the reduction of the total blood cholesterol levels since the time o f diagno­ sis in the case group does support a real change in eating behaviour as documented. Sim ilarly, the "halo-effect" would also influence the control group, particularly in regards to alcohol intake.

5.4.6 Summary

1. The subjects in the group with CHD, on the average, appear to eat less fatty foods, less dairy products and m ore fru it than the control group. There was also a reduction in the contemporary cholesterol level compared to previous levels in the sam e group, suggesting that a change in habitual diet has o ccu rred since the diagnosis.

2. T he in flu en ce o f the d ietary d ifferen c es c o u ld have lead to red u ced p latelet reactivity and m easures of aggregation in the case group com pared to the control group.

5.5 ALCOHOL, PLATELETS AND CORONARY HEART DISEASE