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It is fairly common for a woman to have one breast larger than the other. I have also heard that, for a breast feeding mother it is common for one of the breasts to yield a creamier milk whilst the other is more watery/less nutritional value.

My question: Is there any correlation between the larger of the two breasts and the one which yields the fattier milk?

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    $\begingroup$ Can you please supply a link to support your claim of breast milk production differentiation? $\endgroup$ – anongoodnurse Oct 1 '14 at 13:42
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    $\begingroup$ The only reason I know of that gives a nursing mother a breast size discrepancy is that the infant favors one breast over the other. Naturally, production will be different between the two, but probably only related to volume, while fat/protein/water proportions would be very close to the same. I've read in a number of places that the milk that comes out first is high calorie, low nutrition, while the milk that follows is low calorie, high nutrition, but that is off the same breast and a different thing. $\endgroup$ – 3970 Oct 1 '14 at 19:44
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Because mammary glands define us (mammals), the genetics of the ability to make milk has been of interest.[1] Because we consume milk, we have a dairy industry. Because the industry places a monetary value to certain milk constituents, the genetics of milk component production has been a viable area of study[2]. Since you are talking of cream production, we'll stick to that.

The effect of nutrition and the environment (e.g. temperature) on milk fat production has long been known, and the ability to selectively breed for higher fat content has been exercised for decades if not longer[3][4], But the molecular genetics of milk fat production is only recently starting to be understood.

Mammals with the highest fat content are those in cold environments where an insulating layer of fat is required, such as gray seal milk (53.2% fat), whale milk (34.8%), and polar bear (31%). Humans and cows have approximately the same milk fat percentage (~4.5% fat).[5] The gene most studied to date is the DTAG1 allele[6] on chromosome 14.[7]

It is very clear that genetically milkfat production is under the governance of part of chromosome 14.[7][8] Since this chromosome is identical in both breasts, genetically one breast cannot make milk with a higher fat content than the other. Environmental and nutritional factors affect both breasts equally, so those can be ruled out as a source of fat percentage differences. The only factors left that can be variables are 1) fat content of one breast over the other, and 2) demand from one breast over the other.

In cows, more frequent milking increased milk production but decreased fat percentage; in the end, the effect on fat in milk was the least influenced by frequency of milking (significance questionable).[9]

Triglycerides in milk are synthesized in the smooth endoplasmic reticulum of the mammary alveolar cell from precursor fatty acids and glycerol (obtained from blood nutrients), and while other cell types are involved in milk let down, etc., there is no known function of interstitial adipocytes (fat cells).[10]

That was the long answer. The short answer is: no[11].

[1] The origin and evolution of lactation
[2] Milk Components: Understanding the Causes and Importance of Milk Fat and Protein Variation in Your Dairy Herd
[3] A review of nutritional and physiological factors affecting goat milk lipid synthesis and lipolysis
[4] Factors affecting milk composition
[5] The Highest Percentage of Fat in the Milk of an Animal
[6] Evidence for multiple alleles at the DGAT1 locus better explains a quantitative trait locus with major effect on milk fat content in cattle.
[7] Identification of novel single nucleotide polymorphisms in the DGAT1 gene of buffaloes by PCR-SSCP
[8] Scientists pinpoint gene linked to fat in cow’s milk
[9] Covariance among milking frequency, milk yield, and milk composition from automatically milked cows
[10] MILK SECRETION: AN OVERVIEW
[11] Does a bigger breast give creamier milk?

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    $\begingroup$ Please add a reference link to support your short answer. :D $\endgroup$ – J. Musser Oct 1 '14 at 17:35
  • $\begingroup$ You should move your tl;dr to the top. $\endgroup$ – David says Reinstate Monica Oct 1 '14 at 18:42
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    $\begingroup$ There's some link recursion here. I better not click source 11 or I'll get stuck in a loop. $\endgroup$ – 3970 Oct 1 '14 at 19:40
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    $\begingroup$ "Since this chromosome is identical in both breasts, genetically one breast cannot make milk with a higher fat content than the other. Environmental and nutritional factors affect both breasts equally, so those can be ruled out as a source of fat percentage differences.".. I don't think so. Environment, which also includes artificial environments such as clothing can affect "symmetric" parts asymmetrically. There is an inherent asymmetry as well. All these can result in different tissue efficiency- just like vision or hearing or muscle strength in limbs etc $\endgroup$ – WYSIWYG Oct 2 '14 at 10:19
  • $\begingroup$ However, I cannot comment on correlation of breast size and milk composition. $\endgroup$ – WYSIWYG Oct 2 '14 at 10:21

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