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Pocket Guide to Clinical Nutrition Q&A

To help answer any queries you may have we have started to collect and share some questions (and associated answers) in relation to the PENG Pocket Guide to Clinical Nutrition. This is a ‘live’ document so we will continue to add to as appropriate.

Question: Fluid provision - Should I use the total volume of artificial feed or the water content?

Answer We noted that some artificial feed compendia state the water content of feed (e.g.1000ml of feed = 850g of water); and some dietitians have asked if they should use total fluid volume (e.g.1000ml) or water content of feed (e.g.850ml) when matching feed with estimated fluid requirements. PENG committee recommend the use of total fluid volume (e.g. 1000ml) because:

  1. Compared with normal physiology of the body’s fluid content, the difference between total fluid versus water content of feed (e.g.150ml) is negligible:
    • The stomach produces up to 2 litres a day of gastric juices
    • In the small intestine adds a further 7-8 litres a day, most of which is reabsorbed in the small bowel leaving 1-2l which is almost completely absorbed in the large bowel.
  2. 30-35ml/kg is ONLY an estimate for fluid requirements and is quite generous (other guidelines use 25-30ml/kg). As fluid requirements are a general estimate it is disproportionate to apply a such a restrictive approach to intake.
  3. Rather than micro-managing fluid intake, dietitians should focus on monitoring the individual patients’ signs and symptoms for hydration.
  4. Using the water content of feed is inconsistent as we don’t use the same approach for standard oral nutritional supplements or food (e.g. milk, Horlicks and Hot chocolate)
  5. Even in people where it is necessary to severely fluid restrict (e.g. renal, cardiac or liver) we would focus more on salt to manage thirst.
  6. If fluid requirements are being micro managed to this extent it is important to also remember:
    • body composition has a big influence on actual fluid requirements. In adults fat free mass is approximately 70 – 75% water whereas adipose tissue is only 10 – 40% water so a lean individual will have higher fluid requirements than an obese individual of the same weight1. Monitoring and adjustment of the initial estimate are therefore of paramount importance.
    • Consideration only of the water content of feed does not take into account metabolic water as metabolism of nutrients produces significant amounts of water. Oxidation of carbohydrate, protein, and fat produces metabolic water of approximately 15, 10.5, and 11.1 g/100 kcal of metabolisable energy, respectively so the average adult on a mixed diet produces around 350mls of water per day 1

Reference.

1 Medicine, Institute of; Board, Food Nutrition; Intakes, Standing Committee on the Scientific Evaluation of Dietary Reference; Water, Panel on Dietary Reference Intakes for Electrolytes and (2005). 4 Water | Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate | The National Academies Pres. P74 – 165. doi:10.17226/10925. ISBN 978-0-309-09169-5.

Question: Protein requirements - are PENG requirements guideline group recommending the figures given in the protein table, particularly the very high figures for the elderly?

Answer The figures in the table are a summary of published guidelines that have made recommendations on protein intake in clinical conditions.In this edition of the requirements chapter of Pocket Guide to Clinical Nutrition the approach taken has been to provide evidence to allow dietitians to make decision about applicability to their patient. In general, most of the guidelines recommend between 1 to 1.5g protein/kg/day with higher intakes recommended in burns, older adults and enterocutaneous fistulae. The PENG requirements guideline group also recommend 1.0 to 1.5g/kg for conditions where there is no published guideline. So, there is general consensus that most patients protein requirements fall between 1.0 to 1.5g/kg, probably the controversy lies around the evidence supporting the higher intakes (up to 2g/kg) in clinical practice. There are a couple of points that should be taken into consideration when considering protein requirements:

  • Whereas with energy requirements we have indirect calorimetry which can give a reasonably accurate estimate of energy requirements, there is no equivalent for protein. Studies referenced by the various guidelines use a variety of methods including comparison of actual intake with estimated intake and weight change. The most commonly used are nitrogen balance or labelled amino acid studies. Both have inaccuracies, nitrogen balance can be difficult to measure accurately, does not reflect the changes in protein redistribution (e.g. shifts between muscle and splanchnic tissues in the elderly). Labelled amino acid studies measure uptake by muscles but the impact on clinical outcomes is unclear.
  • Protein balance is affected by energy intake but many studies to not report or control for energy intake.

When dietitians are considering use of the higher intakes they should taking into consideration the supporting evidence. Was the guideline a systematic review and is there a clear link between the evidence and the recommendation? Were the studies cited of good methodology and in an appropriate patient group? For example in the elderly, the higher protein recommendations appear to have come from one study and guidelines in the critically ill adults, so these may not be applicable to patients who are elderly and not critically ill.

The tables in the Pocket Guide to Clinical Nutrition give a brief summary of the numbers and types of studies to inform dietitians, and additional more in depth information will be/is provided on the PENG website, but ultimately dietitians are encouraged to read the original guidelines and papers.