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Why might formula feeding increase
the risk of being overweight?
Dr Amy Brown
L
evels of childhood and adult overweight and
obesity are growing in the UK. Alongside the
associated impact on the health of the nation, this
is placing a growing pressure on NHS resources.
Obesity is a complicated situation with many
factors influencing why and how people put on
weight. Understanding some of these reasons is
important to enable us to intervene to promote
better long term health. How an infant is fed
during the early weeks and months may play an
important role in this. By understanding the link
between milk feeding and later weight gain we can
work towards promoting healthier early infant
feeding strategies and promote healthy weight gain
from the start of life.
Many studies have shown that formula fed babies
have a greater risk of being overweight as babies,
children and even adults1. Although there has
been lots of research in this area, no one is really
sure as to the exact reason why. As with anything,
it is likely to be a combination of different factors.
Understanding why formula fed babies may gain
more weight is important in both promoting the
importance of breastfeeding but also
understanding how risks can be reduced for those
babies who need to be formula fed.
Exploring the link between breastfeeding and
weight is not straightforward particularly in terms
of separating breastfeeding from wider parenting,
family and health factors. Of course breastfeeding
will not protect a baby from being overweight as
an adult if that child or adult goes on to eat a high
calorie unhealthy diet. However it may be that
breastfeeding makes that less likely to happen in
the first place or means that babies are a healthier
weight to start. Also, some people argue that the
link between breastfeeding and weight is explained
by the mother’s background rather than
breastfeeding itself. They argue that mothers who
themselves are overweight are more likely to
formula feed (and therefore pass on a genetic risk
of overweight rather than it being anything to do
with the milk) or that mothers from more
deprived backgrounds are more likely to formula
feed (deprivation is linked to an increased risk of
being overweight too). However, some important
studies have shown that this does not explain the
relationship – breastfeeding predicts child weight
over and above income, education or weight.
Breastfed infants are therefore more likely to have
healthier current and future weight than formula
fed infants. But why is this? To be truthful, no one
really understands why although there are lots of
suggestions.
1. Growth patterns of breast and formula
fed babies
Firstly, growth patterns differ for breast and
formula fed infants during the first year and into
the second year2. Breastfed babies are significantly
lighter than formula fed babies by one year of age,
but there is no difference in height or head
circumference suggesting a leaner rather than
smaller baby. Babies who are overweight are
more likely to go on to be overweight as children
and as adults and there are also links with later
health problems such as diabetes and heart
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disease. Also, babies who gain weight particularly
rapidly in the first year might be more at risk of
later problems with obesity and diabetes.
2. Differences in energy intake between
breast and formula fed babies
Differences in growth between breast and formula
fed babies can be explained partly by increased
energy intake amongst formula fed babies3.
Formula-fed babies drink greater volumes of milk
by as early as two days old and continue to do so
throughout the first six months. Breastfed babies
also feed more slowly, spending more time resting
in between sucks than formula fed babies which
may mean they are more likely to stop feeding
before they are too full. By eight months of age it
has been estimated that a formula fed baby has
received approximately 30,000 more calories than
a breastfed baby.
3. Differences in milk content
introduced to solid foods later and are less likely
to drink juice and other commercial drinks during
the first year. Children who are formula fed are
also less likely to eat fruit and vegetables as
toddlers and are more likely to be fussy eaters.
One possible explanation for this is that children
who were breastfed have tasted different flavours
through their mums’ milk and are then more likely
to accept them as solid foods. One study showed
that breastfed babies whose mothers were asked
to drink lots of carrot juice were more likely to
accept carrots when being weaned. However,
although breastfed babies are more likely to eat a
healthy diet and be weaned in a healthier way, this
does not completely explain the link between
breastfeeding and weight.
Breast and formula milk also have different
contents and concentrations which may explain
differences in growth patterns4. Formula milk is
based on cows’ milk which is intended for calves
and their pattern of growth not human infants.
Calves double their birth weight by 47 days,
whereas a breastfed infant is expected to double
its birth weight by 180 days postpartum. Also,
whereas early calf growth priorities overall size,
early human growth focuses on brain
development. Cows’ milk therefore has much
greater levels of protein and minerals to support
this rapid growth. At three – six months of age,
formula fed babies consume around 70% more
protein than breastfed infants and, depending on
content of solid foods; intake at 12 months may
be five times as much as is needed. Excess protein
intake has been linked to overweight and how fat
is distributed. Interestingly, a few studies have
now shown that babies who are fed a low –
protein formula are at less risk of overweight
compared to babies fed standard formula. There
are other differences in content too. Levels of
insulin are higher in formula milk increasing risk of
later weight gain. Breast milk of course has active
properties not found in formula milk which might
help balance weight gain and growth.
5. Breastfeeding and ability to control
appetite
Another idea as to why breastfed babies have
healthier weight gain is that breastfed babies get
more opportunity to learn to balance their own
intake of milk according to how hungry they are6.
They learn at an early stage to control their
appetite. One reason for this is that breast milk
changes over the course of the day. Whereas
formula fed babies tend to be given feeds of
similar volumes and milk which will have the same
calorie content, breast milk may be higher or
lower in fat and energy depending on the time of
day. Breastfed babies become used to this change
and adapt to drinking larger or smaller amounts
4. Breastfeeding and future diet
Breastfeeding also appears to encourage a
healthier later diet5. Breastfed babies tend to be
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weight and eating problems9. Mothers who are
very controlling are less likely to have breastfed
suggesting that perhaps learning in the early
months to let the baby control how much they
feed might set up a pattern for life.
dependent on the content of the milk and how
hungry they are. Formula fed babies do not get
this opportunity though and become more used
to drinking set amounts at set times. Studies have
shown that when a mother has a high fat content
in her milk the baby will feed for a shorter time
than those who have lower fat content. Also if a
breastfed baby feeds more frequently they will
feed for shorter periods of time – formula fed
babies are less likely to make the same
adjustment.
In summary there are many different reasons why
formula fed infants may be at more risk of being
overweight than breastfed infants. These are
linked to quantity of milk consumed, the content
of the milk and opportunity to learn to regulate
their own intake of milk. A number of studies
have shown that the longer a baby is breastfed for,
the lower the risk of overweight becomes over
time. Overweight and obesity and the health
issues this brings are continuing to rise, especially
amongst young children and increasing
breastfeeding rates may be one way of reversing
this trend. All the more reason to continue
supporting new mums to not only start but to
continue breastfeeding. 
6. Breastfeeding and maternal feeding style
Another explanation looks at differences in how
mothers who breast or formula feed approach
feeding their baby. Mothers who breastfeed are
more likely to feed their baby on demand whereas
mothers who formula feed are more likely to feed
to a mother led schedule7. One reason for this is
of course that breastfed babies typically need to
be fed on demand to establish milk supply.
However, allowing the baby to feed whenever
they are hungry may encourage them to learn to
regulate their own appetite, whereas feeding at
specific times might reduce this ability. In turn
being less likely to control appetite increases the
risk of becoming overweight.
Key References
Owen, C.G., Martin, R.M., Whincup, P.H., Smith, G.D.,
& Cook, D.G. (2005). Effect of Infant feeding on the risk
of obesity across the life course: a quantitative review of
published evidence. Pediatrics, 115, 1367 – 1377.
2
Dewey, K.G., Heinig, M., Nommsen, L., & Lonnerdal.,
B. (1991). Adequacy of energy intake among breast fed
infants in the DARLING study: relationships to growth
velocity, morbidity and activity level. Research on
Lactation, Infant Nutrition and Growth. Journal of
Paediatrics, 199, 538 – 547.
3
Heing, M.J., Nommsen, L.A., Peerson, J.M., Lonnerdal,
B., & Deery, K.G. (1993) Energy and protein intakes of
breast fed and formula fed infants during the first year of
life and their association with growth velocity: the
DARLING study. American Journal of Clinical Nutrition,
58, 152 – 161.
4
Michels, K.B., Willett, W.C., Graubard, B.I., Vaidya,
R.L., Cantwell, M.M., Sansbury, L.B. & Forman, M.R.
(2007). A longitudinal study of infant feeding and obesity
through life course. International Journal of Obesity, 31,
1078 – 1085.
5
Galloway, A., Lee, Y., & Birch, L. (2003). Predictors
and consequences of food neophobia and pickiness in
young girls. Journal of the American Dietetic Association,
103, 692 – 698.
6
Nommsen, L.A., Lovelady, C.A., Heinig, M.J.,
Lönnerdal, B., & Dewey K. G. (1991). Determinants of
energy, protein, lipid and lactose concentrations in
human milk during the first 12 mo of lactation: the
DARLING study. American Journal of Clinical Nutrition,
53: 457 – 465.
7
Brown, A.E., Raynor, P. & Lee, M.D., The development
of a controlling maternal feeding style: a comparison of
formula feeding and breast feeding mothers: Journal of
Human Nutrition and Dietetics, doi:10.1111/j.1365277X.2010.01145.
8
Brown, G.W., Tuholski, J.M., Sauer, L.W., Minsk, L.D.,
& Rosenstern, I. (1960). Evaluation of prepared milks for
infant nutrition; use of the Latin square technique.
Journal of Pediatrics, 56, 391 – 399.
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On top of this, mothers who formula feed have
more opportunity to try to manipulate how much
milk their baby drinks8. Whereas breastfeeding
mothers usually need to let their baby lead in how
often they feed and how much, if a baby is fed
with a bottle it is easier to track how much they
have drunk and encourage them to finish off a
feed. A few studies have shown that it is fairly easy
to encourage a bottle fed baby to drink more milk
than they would naturally need. Conversely it is
very difficult to persuade a breastfed baby to
overfeed. Other studies have shown that
breastfeeding mums are more likely to recognise
and respond to signs of their baby being full and
not wanting to feed compared. Again this might
reduce the baby’s ability to control their intake of
milk and appetite.
It is also possible that mothers might learn a
certain style of feeding their baby which they then
carry through to when the baby is older and
eating solid foods. Lots of research has shown
that mothers who put lots of pressure on older
children to finish a meal or eat certain foods or
who are controlling about how much food the
child eats are more likely to have children with
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