Dietary assessment - Weighed food diary

Weighed food diaries (diet diaries or food records) are a detailed prospective dietary assessment method. They have traditionally been used in the United Kingdom for the assessment of dietary intakes in national surveys (Gregory et al, 1990 and Henderson et al, 2002), compared to the United States where the 24-hour recall is the usual method. An individual undertaking a weighed food diary records details of food and drink eaten at the time of consumption. Instructions and record sheets or booklets are provided, together with a set of weighing scales for the food and drink. Portions of food are weighed on to a plate and described in detail in the record booklet. Brand names, a complete description of the method of preparation, cooking and recipes for composite dishes should be recorded. Plate waste is often weighed and recorded separately. A short questionnaire can be included to aid interpretation of the record and provide details of core foods regularly eaten e.g. type of milk and to inquire about non-food items such as dietary supplements.

The food record may also be maintained either by digital audio recording or electronically. In the 1980s PETRA scales were developed in Cambridge, UK; these scales had a cassette recording facility to allow the recording of verbal descriptions of intake (Bingham, 1987). PETRA scales had the advantage that individuals could not see the amount of food being eaten (Stephen, 1990). However, the scales were heavy and difficult to transport and were also very labour intensive as the tapes needed to be transcribed and then coded for data entry. Transcribing 7 days of intake took at least 2 hours. While an innovation to the method at the time, PETRA scales are now rarely used.

Seven days of recording is historically most common, but longer or shorter periods can be used depending on the aims of the dietary assessment. Seven days of recording mean that the assessment is not biased towards certain days of the week and this is important for infrequently eaten foods. Non-food items such as water, dietary supplements and alcohol are also recorded. The 7-day weighed food diary was assumed to be the ‘gold standard’ method of dietary assessment until studies validating weighed food diaries with doubly labelled water, in the early 1990s, showed that underreporting is common and is not confined to obese indivduals (Livingstone et al, 1990). Weighed dietary records completed by parents of young children aged 7-9 years, showed good agreement with estimates of energy expenditure made by doubly labelled water, whilst under-reporting was apparent in adolescents (Livingstone et al, 1992) and adults (REF). The method is used as a standard in dietary calibration studies and is still believed to be the most precise method of dietary assessment but a conscious or subconscious change to eating habits may be made (Livingstone et al, 1990) making it potentially less accurate and reliable.