Results

The average energy:BMR ratio of the survey participants was 1.13 (SD=0.3) for women and 1.25 (0.3) for men. Two hundred and sixty women (48%) and 163 men (29%) fell below the cut off level of 1.1 and were classified as LERs. The difference in proportions of LERs between the sexes was highly significant (p<0.001), therefore all subsequent analyses were performed separately by sex.

SOCIOECONOMIC, DEMOGRAPHIC AND BEHAVIOURAL CHARACTERISTICS

There were a number of significant differences between the LERs and the non-LERs in both sexes (table 1). For both men and women, the LERs were more likely to come from the manual social classes (64%v 49% among men, 55%v 42% among women). They were also more likely to be in receipt of benefits and more likely to report physical mobility problems, defined as needing aids to walk. The LERs of either sex were less likely to report being in good health and less likely to have received higher education. Body mass index was also significantly higher among the LERs of both sexes (27.5 v25.7 among men, 27.9 v 25.4 among women). No significant differences were observed in age, smoking habits, numbers living alone, home ownership, receipt of private pensions, problems in chewing food or geographical area of residence.

Multiple logistic regression models showed that among men obesity (body mass index⩾30), social class and home ownership were all independently associated with LER status at the 95% significance level, while among women only obesity remained significant (table 2). There was also some evidence of education among men and health among women being associated with LER status, the probability decreasing with education and increasing with poor health.

REPORTED FOOD INTAKE

A comparison of the reported food intake of LERs and non-LERs is provided in table 3. A similar comparison with food intake expressed as a percentage of total energy intake is presented in table 4.

Male LERs reported lower consumption than non-LERs of 25 food/drink groups, the largest proportional differences arising with alcohol, butter, non-low fat spreads, fruit juice, wholemeal bread, biscuits, cakes and pastries (table 3). The differences reached statistical significance (p<0.05) for 17 groups. Women generally reported lower intakes than men, and LERs reported less than non-LERs in all but five food/drink groups. The differences reached statistical significance in 13 groups with the largest proportionate differences arising in consumption of alcohol, butter, sugar and biscuits, cakes and pastries.

Adjusting for energy intake, male LERs reported higher intakes in 16 food/drink groups, the largest differences occurring with poultry, soup and fried fish (table 4). The 13 food/drink groups for which male LERs reported lower intakes included full fat dairy products, biscuits, cakes and pastries, fruit and nuts, alcohol and fruit juice. Female LERs reported higher intakes of 17 food/drink groups and lower intakes of 12 groups. The food/drink groups reported higher by male LERs were exactly the same as those reported higher by female LERs with the exception of sauces and pickles.

NUTRIENT INTAKE

Over the four day period the average daily intakes of energy, macronutrients, eight minerals and 12 vitamins were significantly lower (p<0.01) for LERs compared with non-LERs of either sex (results not shown). For macronutrient densities (intakes as % total energy or per 1000 kcal), significantly higher intakes of protein and starch were reported by LERs of both sexes, as were significantly lower intakes of alcohol, total fat, saturated fat and transaturated fat (table 5). In addition, sugar intake was lower and fibre intake higher among both sexes, although differences were not statistically significant. The largest observed differences were the increased protein intakes of both sexes. For the micronutrients examined, energy adjusted intakes of all minerals and vitamins were higher among female LERs than non-LERs, significantly so for seven of eight minerals and 9 of 12 vitamins. Among men, intakes of all minerals and 9 of 12 vitamins were higher among LERs, significantly so for five minerals and five vitamins. The largest differences in micronutrient intake were observed in vitamins D and C among women, and vitamin D and niacin in men. These results refer only to micronutrient intake from food and drink as supplements were not included in the analysis.

Discussion

The UK National Diet and Nutrition Survey of people aged over 65 years was affected to a large extent by low energy reporting. In the free living sample 48% of women and 29% of men were classified as LERs. Using a comparable cut off (1.1 × BMR) LERs comprised 21% of the sample studied by Price.5 In the NDNS of the younger 16–64 year old adults, 46% of women and 29% of men were classified as LERs despite using a cut off value of 1.2 × BMR,3 a similar cut off with the elderly would increase the LERs to 61% of women and 41% of men. The levels observed among the free living subjects were considerably higher than those observed among those living in institutions (19% men, 19% women).

There are several possible explanations for the reporting of low energy intakes by participants in dietary surveys. The survey period may coincide by chance with a period of low energy intake, however the probability of this effect causing the number of LERs observed here is extremely small when using a stringent cut off value. Alternatively, participants may have changed their diet during the period of the survey, but again the cut off value used here makes this an unlikely explanation for the large numbers of LERs observed. The primary reason for low energy reporting in this survey is most likely to have been incomplete recording of food intake.

Weighed food diaries are potentially the most accurate way to measure food intake,9 however their completion requires greater effort than other dietary survey methods and this has previously been found to present a barrier. In assessing the accuracy of weighed diaries Livingstone10 reported that snack foods were most likely to be missed. While the weighed diary methodology may be a factor here it would not explain why this survey was so much more affected than the previous NDNS of younger adults, unless snack consumption is a larger feature of the elderly diet. Poorer memory should not have been an issue as all subjects completed memory tests before being given food diaries, those failing being given assistance. Increased errors in food weighing may be a factor, although such errors would be expected to be unbiased with over-reading as probable as under-reading.

The NDNS of younger adults was carried out eight years before this survey. Hirvonen6 compared low energy reporting in two similar studies carried out 10 years apart. The prevalence of low energy reporting rose from 27% to 42% among men and from 33% to 46% among women, increases of a similar magnitude to those seen when comparing the two NDNS surveys.3 The rise was attributed to increased health consciousness and a reluctance to report consumption of foods known to be unhealthy. This effect may have been a further contributory factor to the low energy reporting observed here. A further possibility is that the observed increases in low energy reporting result from decreasing energy requirements, however evidence from elderly UK women suggests that this is not the case.11

The suitability of equations used to define a low energy reporter must also be questioned. Calculation of the BMR used the Schofield equations,⁷ the derivation of which included only small numbers of elderly subjects. The equations also assume a linear relation between body weight and BMR, as fat tissue is less metabolically active this seems likely to be an oversimplification. While verification of the equations is required urgently, they continue to be the most suitable for an elderly European population and inaccuracies are unlikely to be the sole cause of the large numbers of LERs observed here.

As with other studies of low energy reporting, women were more likely than men to be classified as LERs.3 5 6 This may partly be attributable to women obtaining higher percentages of energy from fat than men, while fat is most affected by low energy reporting, however the difference between men and women was small (36.2% women, 35.3% men; p=0.02), and would not explain the large difference in LER prevalence. Health consciousness, which is thought to be higher among women, may also be a factor.

Obesity was most strongly associated with LER status in both men and women, in agreement with other studies.3 5 6 This could be the result of the linear relation assumed by the Schofield equations between BMR and body weight, which as already mentioned seems likely to be an oversimplification. Significant univariate associations were observed between LER status and both education and social class, however results from a multivariate analysis were less clear and differed from those of other studies. Hirvonen6 observed that LERs were more likely to be highly educated in a group of 16–64 year olds, while Price5 found no effect of current social class in a cohort of people born in 1946. The only significant association we found in a multivariate model was with social class among men. This suggests social class is more important than education among older populations while the reverse may be true in younger groups.

Food groups in which LERs reported lower intakes than non-LERs included full fat dairy products, sugar and sweet foods, and alcoholic drinks. Interestingly, they also reported lower intakes of fruit and nuts, wholemeal bread and wholemeal cereals, the differences in alcohol consumption were also greater for wine and spirits than for beer. This could indicate that two effects are observed when comparing the food intakes of LERs and non-LERs, one being a tendency to under-report mostly those foods that are known to be unhealthy, the other being true dietary differences associated with the socioeconomic characteristics of the two groups. Few studies have compared food intakes of LERs and non-LERs but a similar pattern was reported from the NDNS of British adults.3

Absolute intakes of all macronutrients were significantly lower among LERs, as seen elsewhere.3 5 12 Comparison of nutrient densities gave results broadly consistent with those of other studies: protein was higher among LERs3 5 12; total fat was lower5 12; total carbohydrate did not differ significantly5 but sugar was lower3 and starch higher3; alcohol was lower.3 A further study by Hirvonen6 also found higher densities of protein and carbohydrate and lower densities of fat among LERs, but results of significance tests directly comparing the two groups were not reported.

Low energy reporting causes major problems in the interpretation of dietary data. The probability of a single factor causing the level of low energy reporting observed in this survey is extremely small. Rather it is probable that several factors coalesced, these being primarily a tendency to omit certain items from weighed diaries, a reluctance to report foods known to be unhealthy, and possibly a weakness in the definition of elderly LERs. By adjusting for energy intake the effect of low energy reporting is reduced in analyses of both food and nutrient intakes, although as food groups are not uniformly affected this is not a complete solution. Validatory biomarkers such as doubly labelled water provide a more complete answer to the problem and would have been particularly useful in this survey given the scale of low energy reporting that took place.