There are several methods for estimating dietary nutrient intake.153,237 The most common methods are food intake records and dietary recalls. The dietary recall (usually obtained for the previous 24 hours) is a simple, rapid method of obtaining a crude assessment of dietary intake. It can be performed in approximately 30 minutes, does not require the patient to keep records, and relies on the patient's ability to remember how much food was eaten during the previous 24 hours. Accurate quantification of the amounts of foods eaten is critical for the 24-hour recall. Various models of foods and measuring devices are used to estimate portion sizes. Advantages to the recall method are that respondents usually will not be able to modify their eating behavior in anticipation of a dietary evaluation and they do not have to be literate. Disadvantages of the 24-hour recall include its reliance on memory (which may be particularly limiting in the elderly), that the responses may be less accurate or unrepresentative of typical intakes, and that it must be obtained by a trained and skilled dietitian.
Dietary diaries are written reports of foods eaten during a specified length of time. A food-intake record, lasting for several days (3 to 7 days), provides a more reliable estimate of an individual's nutrient intake than do single-day records. Records kept for more than 3 days increase the likelihood of inaccurate reporting because an individual's motivation has been shown to decrease with increasing number of days of dietary data collection, especially if the days are consecutive.238 On the other hand, records maintained for shorter times may not provide accurate data on usual food and nutrient intakes. The actual number of days chosen to collect food records should depend on the degree of accuracy needed, the day-to-day variability in the intake of the nutrient being measured, and the cooperation of the patient. When food records are chosen to estimate dietary energy and DPI in MD patients, it is recommended that 3-day food records be obtained for accuracy and to minimize the burden on the patient and/or his family. Records should include at least one weekday and one weekend day, in addition to dialysis and nondialysis days for MHD patients, so that variability in food intake can be estimated more accurately.
The validity and reliability of the dietary interviews and diaries depend on the patient's ability to provide accurate data and the ability of the nutritionist to conduct detailed, probing interviews. The intake of nutrients is generally calculated using computer-based programs. Food records must be maintained meticulously to maximize the accuracy of the diary. Food intake should be recorded at the time the food is eaten to minimize reliance on memory. Special data collection forms and instructions are provided to assist the individual to record adequate detail. Recording error can be minimized if instructions and proper directions on how to approximate portion sizes and servings of fluid are provided.
Food models are also helpful for instruction. The food record should indicate the time of day of any intake (both meals and snacks), the names of foods eaten, the approximate amount ingested, the method of preparation, and special recipes or steps taken in the food preparation. The dietitian should carefully review the food record with the patient for accuracy and completeness shortly after it is completed.
Calculation and Expression of Protein and Energy Intake
DPI can be expressed in absolute units such as grams of protein per day (g/d) or as a function of the patient's actual or adjusted body weight (eg, g/kg/d; Guideline 12). Dietary energy intake (DEI) refers to the energy yielded from ingestion of protein, carbohydrates, fat, and alcohol. DEI can be expressed in absolute units such as kilocalories per day (kcal/d) or as a function of the patient's actual or adjusted body weight per day (kcal/kg/d). Consideration should be given to using the adjusted edema-free body weight (aBWef, Guideline 12) to express DPI or DEI in individuals who are less than 95% or greater than 115% of SBW.
In CPD patients with normal peritoneal transport capacity, approximately 60% of the daily dialysate glucose load is absorbed, resulting in a glucose absorption of about 100 to 200 g of glucose per 24 hours.239,240 Another method of estimating the quantity of glucose absorbed is the following formula240:
where x is the total amount of dialysate glucose instilled each day. Both of the methods described above are based on the observation that (anhydrous) glucose in dialysate is equal to about 90% of the glucose listed. For example, dialysate containing 1.5% glucose actually contains about 1.30 g/dL of glucose and 4.25% glucose in dialysate actually contains 3.76 g/dL of glucose.240 It is probable that the relationship between dialysate glucose concentration and glucose absorbed may be different with automated peritoneal dialysis.
The net glucose absorption from dialysate should be taken into consideration when calculating total energy intake for PD patients.