COMMENTARY

Nutrition Support for the Obese Surgical Patient

Jay M. Mirtallo, MS, RPh

Disclosures

January 14, 2008

In This Article

Special Considerations for the Obese Patient

Because obese patients already have an adequate (or excess) intake of calories, it is easy to overlook the need for nutrition support. This can lead to prolonged periods of inadequate nutritional intake and significant protein depletion, especially because the response to injury for an obese patient is different from that for the nonobese patient. Usually, the injury response is manifested by a rapid adaptation to fat as an energy source. In the obese patient, less energy is derived from fat, with most energy being derived from carbohydrates and protein.[6]

Additionally, energy expenditure is increased in obese patients,[7] which is partially attributed to an increase in lean body mass, otherwise known as metabolically active tissue. Therefore, it is important to recognize the need for nutrition support.

Nutrition support can be administered either enterally (through the digestive tract) or parenterally (outside the digestive tract, such as intravenously); both routes require specific access devices. Complications may occur that are inherent to placement of the access device as well as to the delivery of nutrients via this route. As such, the indication for nutrition support is based on a careful evaluation of the risks and benefits.

Generally, a postoperative patient with adequate protein stores may support the metabolic processes required to recover from surgical injury for a period of several days. (The exact length has been debated to be around 3-5 days for the critically ill and 7-10 days for routine surgical patients.) If nutrition is not provided after this time, it is likely that nutrient deficits will begin to negatively affect the patient's outcome.

Indications for nutrition support are similar for the obese patient to those for other patient groups, including an abnormality of the gastrointestinal tract that is likely to persist beyond 7-10 days. Such an abnormality may result in the need for parenteral nutrition if there is a contraindication to enteral nutrition, such as ileus, bowel obstruction, mesenteric ischemia, or fistula. The incidence of patients requiring parenteral nutrition who are obese (weight > 130% IBW) is reported to be about 30%.[4]

The gastrointestinal tract may also be diminished in function to the extent that enteral nutrition must be modified in order to be successful. This may require proper enteral access (postpyloric) and/or the use of advanced enteral formulas designed for patients with digestive or malabsorptive problems.

Obese patients are at an increased risk of complications from nutrition support, especially the detrimental effects of overfeeding.[3] Aggressively high caloric doses have been associated with increased carbon dioxide production, which increases the work of breathing and may prolong the need for mechanical ventilation.[8]

Hepatic complications such as fatty infiltration of the liver and hepatic steatosis are reported when excess calories are administered, especially as carbohydrates.[9] Cautious administration of carbohydrate (dextrose), fat, and fluid is suggested for obese patients with type 2 diabetes, congestive heart failure, and metabolic syndrome.[3]

Therefore, it is important to accurately determine energy requirements in this patient population. However, this goal can be difficult to accomplish. Several predictive equations have been developed to estimate energy requirements, but adapting these formulas for obese patients is problematic.[7] Almost all formulas are based on body weight; so which weight should be used – the ideal, actual, or adjusted body weight?[10,11] And, if the body weight is adjusted, which adjustment factor should be used?

The problem with using the IBW is that lean body mass (metabolically active tissue that closely correlates to energy expenditure) increases in obesity, but there is no linear relationship to the increase. This means that as the severity of obesity increases, the percent increase in lean body mass does not increase in proportion to the increase of fat mass.

Also, energy expenditure is increased in obesity. Therefore, use of the IBW underestimates energy expenditure. On the other hand, because fat is metabolically inert and does not contribute extensively to energy expenditure, use of the actual body weight tends to overestimate calories.

An adjusted body weight using 25% as a factor may be useful at lower classes of obesity (Class 1 or 2), but is likely to be inaccurate in Class 3 obesity. An adjustment factor of 50% has recently been reported to provide a more accurate estimation of energy expenditure for patients in whom the goal is to provide full calories.[11] As there are problems in estimating energy requirements of obese patients, measurement of energy expenditure using indirect calorimetry is recommended.[2,5]

Only a few studies have examined nutrition support in obesity, and most involved parenteral nutrition. Three studies in particular evaluated the use of hypocaloric nutrition support. Dickerson and colleagues[12] provided parenteral nutrition to morbidly obese, critically ill patients who had surgical complications. The caloric dose provided was about 50% of the measured energy expenditure, along with 2.1 g of protein per kg IBW. This group observed a weight loss of about 1.7 kg/week while supporting protein anabolism, as evidenced by positive nitrogen balance and significant increases in serum total iron binding capacity and serum albumin concentrations. Also, all of the mildly to moderately stressed patients exhibited complete tissue healing of wounds, abscess cavities, and closure of fistula.

Choban and colleagues[13,14] designed 2 prospective trials comparing hypocaloric and routine caloric doses of parenteral nutrition in obese (weight >130% IBW) patients. In the first study,[13] energy expenditure was measured by indirect calorimetry, and caloric intake was modified to deliver 100% of that amount in the high caloric group and 50% of that amount in the hypocaloric group. Both groups received protein at a dose of 2 g/kg IBW per day. There were no significant differences between the groups in nitrogen balance.

In the subsequent study by this group,[14] a specific parenteral nutrition formula was used for the hypocaloric group, the dose of which was based on the amount needed to deliver 2 g protein/kg IBW. This approach obviated the need for measured energy expenditure, a technique that is not available to most organizations. This regimen (14 ± 3 calories per kg actual body weight) was compared with a traditional daily dose (22 ± 5 calories per kg actual body weight), with both groups receiving 2 g protein/kg IBW. Again, nitrogen balance was similar and there was a trend toward better glucose control in the hypocaloric group.[14]

Hypocaloric feeding is an intriguing nutrition approach for the obese patient, but it may also have some issues requiring further investigation. One patient in the Dickerson study developed a dry, scaly rash after 20 days of parenteral nutrition and nil per os.[12] The patient responded to supplemental zinc and intravenous fat emulsion, so it is unclear which deficiency was present. Also, reduced efficacy of hypocaloric feeding in older patients has been suggested by the work of Liu and colleagues,[15] but their retrospective study provided insufficient evidence to avoid hypocaloric feedings in these patients. However, the evidence does support the need to increase monitoring of hypocaloric feeding in the obese, older patient.

During hypocaloric feeding, protein doses of 2 g/kg IBW have been found to be effective. This is a high dose of protein that is usually reserved for severely stressed patients or those with large volume protein losses from burns or fistulas. For the less stressed patient with Class 1 or 2 obesity, in whom the goal is to maintain energy reserves, a higher energy intake may be used. Because nitrogen balance is closely related to caloric dose, a higher caloric dose (even if it is considered to be hypocaloric as it relates to the normal nutrition support population) may allow for lower protein doses (1.2-1.5 g/kg body weight) to achieve the established nutritional goals.

The approach to nutrition support is dependent on the goal established for the patient. Because of the detrimental effects of overfeeding, the ultimate goal is to avoid the administration of excess calories. Dickerson and colleagues[12] found that weight loss could be accomplished with hypocaloric feeding, but this approach should be limited to the morbidly obese patient.

Patients with Class 1 or 2 obesity would likely benefit from maintenance of body weight until recovery from the operation is achieved. In this case, hypocaloric feeding as demonstrated by Choban and colleagues[13] is effective, where weight loss is possible but not a primary endpoint; this approach is potentially useful in the obese patient with type 2 diabetes. Other less obese patients may benefit from nutrition support providing 25 kcal/kg actual or adjusted body weight.[7]

All of these approaches are associated with many variables that can influence their success. As such, they provide a reasonable starting point but must be carefully monitored for efficacy or adverse effects. Efficacy is monitored by determining nitrogen balance, response to visceral proteins such as prealbumin, and patient response such as healing of surgical wounds, abscess cavities, and fistula. Complications may be assessed by daily measures of glucose (although more frequent point-of-care glucose determinations may be necessary if the patient is or has become hyperglycemic), as well as weekly measures of triglycerides and liver enzymes. If the patient is receiving mechanical ventilation, measured expiratory volumes and pCO2 should be monitored.

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