Doubt about pre‐operative carbohydrate supplementation

We commend Fawcett and Thomas for their review of pre-operative fasting recommendations 1 and applaud their candid acknowledgement of the mounting evidence of lack of clinical benefit for oral pre-operative carbohydrate loading (preCHO), considered an essential element of the enhanced recovery after surgery (ERAS) programme 2. We wish to mount further direct challenges to the concept of preCHO, on several grounds. The subjective benefits of preCHO, namely the reduction in anxiety, distress, thirst and hunger 1-3, are relative to the dietary habit of the subject during the preceding weeks, as much as they are to the immediate duration of restriction of food and water. A predominantly carbohydrate-based ‘standard’ diet (such as has been advised by national advisory bodies for several decades) accentuates these symptoms, whereas the widespread adoption of reduced fasting times for both food and water (6 h and 2 h, respectively), reduces the impact of this acute deprivation. Indeed, preCHO has been shown to be of benefit only when compared with fasting without water, but negligible when compared with water 1. The analogy drawn between surgical stress and exercise, with respect to lactate production and carbohydrate loading, is both false and out-dated. In the context of exercise, lactic acid is produced when glycolysis outstrips mitochondrial capacity for aerobic metabolism, whereas intra-operative lactic acidosis is likely to represent hypoperfusion. This does not imply a primary substrate lack and therefore is not improved by preCHO. The objective metabolic benefit of preCHO is purported to be a reduction in peri-operative insulin resistance, as evidenced by an improvement of insulin sensitivity assessed by the hyperinsulinaemic euglycaemic clamp test (HIEG) 3. We contend that this appears to be based on a divergence from the metabolic standards of that test. When the HIEG was first described in 1979 4, the pre-conditions required that all subjects not exercise for 48 h and all consume ‘at least 200 g of carbohydrate per day for 3 days before study’, implying that some standardisation of carbohydrate consumption is necessary. However, in all of the preCHO-studies’ application of the HIEG 3, precisely the opposite occurs: the preCHO group is administered 150 g maltodextrin within 12 h of the pre-operative test, whereas the fasted control group is not. It is our contention that this preCHO augments the disposal of glucose during subsequent HIEG, through mechanisms that are not yet fully understood. By way of analogy, this is akin to comparing sprint time-trial performance between those that are sprinting at the start, vs. a stationary-start control. Since preCHO has no significant demonstrable clinical benefit compared with placebo, we propose that it be relegated to be an optional, rather than obligatory, component of ERAS. Furthermore, in light of current understanding of the incidence and consequences of peri-operative hyperglycaemia 5 and the growing recognition of the incidence and implications of covert primary hyperinsulinaemia 6 in the population, we would caution against preCHO in the majority of surgical patients.

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