>From The Breastfeeding Answer Book, page 296:

"Although ist was once thought that breastfeeding takes more energy than
bottlefeeding, research on premature babies (Meier 1988) and babies with
congential heart disease (Marino 1995) indicates that the oposite is
true: bottle-feeding is more physically stressful than breastfeeding.
Differences between the composition of human mik and formula may also
affect the amount of energy expended.  One study of healthy thow-day-old
babies found that the breastfed newborns had more energy-efficient heart
rhtythms, lower heart reates, and expended less energy thatn the
artificially fed babies, even though the breastfed babies spent less
time sleeping than their bottle-fed counterparts (Zeskind 1992)."

Citations: Marino, B. et al.  Oxygen saturations during bresast and
bottle feedings in infants with congential heart diesease.  J Pediatr
Nurs 1995; 10(6):360-64.

Meier, P. Bottle and breast feeding: effects on transutaneous oxygen
pressure and temperature in small preterm infants.  Nurs Res 1988;
37:36-41.

Zeskind, P. et al. Rhythmic organization of heart rate in breast-fed and
bottle-fed newborn infants. Early Dev Parent 1992; 1(2):79-87.