Sunday, 21 April 2013

Biologically speaking, when should we stop breastfeeding?

Biologically, we are designed to breastfeed our young. As are, and do, other species with mammary tissue (an organ in female mammals that produces milk to feed young offspring).

This blog isn't about whether we 'should' be breastfeeding our babies - there is no question that we should be, it's why women have mammary glands (breasts) and it provides the nutrition babies needs. Instead what I have done is take the novel approach of comparing between a range of species, the fraction of their lifespan spent feeding their young with milk to determine when, based on other similar species, humans are 'biologically' designed to wean (stop breastfeeding their babies).


Representative data from a number of species showing weaning age, lifespan and the % of lifespan spent drinking mothers milk.


Species
Weaning age (years)
Lifespan
Estimated % of lifespan spent drinking milk from mother
Human
X
70-80
X
Ape
3-6
35-40
12%
Chimpanzee
4-5
35-40
12%
Sheep
0.5
10-12
4.5%
Horse
1
25-30
3.6%
Cow
0.7-0.9
20-25
3.5%
Rat
0.05
2-4
1.7%
Dog
0.15
10-16
1.2%
Rabbit
0.07
9-12
0.7%


Where a range is given, the average was used to calculate the fraction of life-spent suckling.
Human values for weaning age and estimated fraction of life-spent suckling are given as X as this data varies widely across and within populations.
Weaning age and lifespan values are approximate only and some strain/breed variation exists within species, data obtained from Wikipedia.

The data in the table shows that for the most part, the higher order animals, that is, those species with larger brains and long-life spans, like the human, breastfeed their young for a significantly larger portion of their lifespan than do the lower order animals, that is, those with smaller brains and shorter-life spans. An oddity is the sheep, who spends a large portion of lifespan 4.5% drinking milk in comparison to the dog who has a similar lifespan, who only spends 1.2%.

What is most surprising however is how short, relative to lifespan the majority of Australian women are breastfeeding their babies. The World Health Organisation recommend exclusive breastfeeding to 6 months of age and continued breastfeeding with appropriate complimentary food up to 2 years or beyond (2 years = ~2.6% of lifespan). The Australian National Health and Medical Research Council reported in 2005 that 22% of infants less than 3 months of age were receiving formula and that no (0%!!!) of infants at 6 months of age were exclusively breastfed. Additionally, they reported that only 23% of babies are being breastfed at 12 months of age.

If we take the data in the table above and use it to extrapolate to the human, if we invested 12% of our lifespan (~75 years) feeding our young, as do apes and chimpanzees (our closest genetic relatives), this is approximately 9 years of feeding our young milk and if we invested the same amount of time as cows and horses (~3.5%) this would equate to 2.6 years. 
What it seems we are doing however (based on the NHMRC study), is investing a similar amount of time (relative to lifespan) as rats, dogs and rabbits (~0.7-1.5% of lifespan = 0.6-1.1 years). Species that are strikingly different to the human in terms of growth and development after birth and brain function throughout life.

It is time for more targeted research on the appropriate weaning age for humans - what are the consequences/advantages of short duration breast feeding vs long duration breast feeding? Most importantly it is time that we, as a society, change our attitudes so that breastfeeding, for the biologically appropriate timeframe is accepted, encouraged and supported. 


For further reading see this blog by Kathryn Dettwyler which provides a good summary of many of the studies done by anthropologists to try and determine the appropriate weaning age for humans.

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