Coulston has recently reviewed the clinical experience of modified enteral formulas for managing diabetic patients. Most of the evidence used to support the use of specialised enteral feeds in diabetic management has been extrapolated from the general diabetic literature and is aimed at avoiding hyperglycaemia (11). Promotional literature from the different nutritional companies is primarily based on dietary guidelines and not based on clinical studies. Although some studies have looked at the glycaemic effect of different liquid formulas given as oral test meals (high-fibre, low-carbohydrate, standard formula) (13), long-term studies are lacking. Only short-term studies have been carried out using specialised oral diets in which carbohydrate content is reduced by increasing MUFA content. These short-term studies have been undertaken either as single test meals or over short periods of time involving relatively few subjects.
One of the longer studies examining the metabolic effects and clinical outcomes of a modified versus a standard formula for enteral tube feeding in diabetic patients was by Craig et al. (14). This pilot study was a prospective randomised double group parallel trial in which 34 patients with diabetes were randomised to receive either modified (55% fat, 33% CHO) or standard enteral tube feeds (35% fat, 53% CHO) for up to three months. Glycaemic control was judged to be significantly better following the modified feed during weeks 1, 5 and 7. However this occurred despite any significant differences being found in the HbA1c level, fasting glucose and lipid profile!
Devising a tube feed for people with diabetes based on dietary nutrients known to improve glycaemic control is not a precise science. Many of the nutrients included in tube feeds need to be chemically modified to enable delivery from a tube. As the glycaemic response of a food is dependent on its physical properties, changing nutrients from their solid phase to a liquid phase can radically change the glycaemic properties. With respect to glycaemic control, while there is good evidence of the beneficial effect of fibre in the solid diet, the addition of fibre in the liquid diet has not been shown to be of benefit (12). In addition, fibre supplementation to tube feeding can be problematic as optimal fibre blends increase feed viscosity, making formula flow through fine-bore feeding tubes extremely difficult. The lack of improvement in glycaemic control with tube feeds containing fibre is probably related to the biophysical properties of fibre in a liquid. For tube feeds the post-prandial insulin and glucose responses are related to the carbohydrate load and not to its fibre content.
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