This can be very confusing, particularly if your client is overweight, but it is not at all unusual (1) and there are a number of simple reasons why this can happen.
Under reporting of total intake on food diaries is incredibly common. Studies have found that under-reporting of up to 50% of total energy is not at all unusual and the most common groups to misreport are people who are over-weight, adolescents and female athletes (2).
The first thing to say is that although this statistic is pretty shocking, it may be a simple feature of ‘day selection’. The average variation in energy intake between days is known to be between 20% and 30% (3) and that variation is at its greatest in obese or young people. A self-selected three-day food diary, particularly one that is being used to monitor weight loss is quite likely to be done on ‘good’ days, lowering the recorded calories and prevalence of high energy foods. In other words you are unlikely to choose Christmas Day or your birthday as one of your monitored days. In my own research I have found people rarely ‘choose’ Fridays as monitored days. I always ask for at least one day to be a weekend and this is usually Sunday.
Another common cause of low reporting on a food diary, is sloppy reporting, the non-deliberate missing off or misreporting of some items you have eaten or drunk (4). These are often snack food or things that people had not planned to eat, like a biscuit offered to them at work, a mid morning coffee they don't normally have or the left-overs on their children's plates. Searching through household rubbish and comparing it with food diaries, Rathje (1984) found that most misreporting is totally accidental, due to confusion with package sizes, misdescription of an item or wrong assessment of portions size (5).
Portion sizes tend to be underestimated when large and overestimated when small (6), which might explain increased under reporting in overweight individuals, especially as portion under-reporting has been found to increase in line with degree of obesity (7,8). When people doing food diaries are secretly monitored, there is an uneven distribution in the under-reporting with higher calorie foods being more commonly missed. In a large survey of the UK adult population, the consumption of biscuits, cakes and pastries were the most likely items to be overlooked (8) and drinks are also easy to forget.
Another reason for a lower than expected calorie intake on a food diary is nothing to do with mis-reporting. Quite often the very act of writing down what you eat can change the way you eat. This is very well documented in scientific research and it is well accepted that as soon as a person knows that someone is monitoring what they eat, they start to think about what they eat (9). This is the reason that food diaries are such an effective tool in weight loss – they increase accountability (10). Quite often people don't realise how much they eat before doing a food diary or how much little things like a teaspoon of sugar, a slosh of ketchup or a biscuit with your tea can add up.
There are a number of ways to improve the accuracy of reporting on food diaries. The first method is to give very detailed instructions when you give out the diary. This could include a sample sheet showing exactly how you would like the diary to be done and an idea of portion size measurements. Weighted food diaries, where the client is asked to weight everything, are more accurate and have also been associated with greater weight loss which may be an incentive (11).
Another way of improving accuracy is to double survey. In research studies this is done by asking the participant to do a daily food diary and then to do 24hr recalls diaries on some of the days. This would not be possible to do in private practice as it would take too long but it is possible to get your client to take pictures of some meals. This would have a similar effect, giving you far greater insight into the detail of the diary. Seeing your client’s meals allows much more accurate identification of portion size and prevents confusion. Another way to double survey is by going over your client’s diary when you get it back and ask about the entries. Was there spread on the toast or in the baked potato? Was it 100g dry spaghetti or 100g cooked? Did they really eat nothing between 8am and 3.30pm or drink nothing all day?
It is completely accepted that food diaries are not particularly accurate, but this does not mean that they are not important. As I mentioned above, the act of filling in a food diary has been reliably shown to impact the quality of what we eat and to improve weight loss outcomes in a wide variety of different population groups (10,11). Knowing what your client is eating is vital when giving advice and will give structure to any programme, whether it is for weight loss or to improve sport performance. Trying to improve accuracy is also important. Studies looking at weight loss over time have found that dieters who fail to lose weight are generally reporting only 50% of their measured energy expenditure (12), that successful dieters report 60-80% (13,14) and that individuals who maintain weight lost report 80-100% total energy intake (9).
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