The Price of Milk

I recently saw a post on social media proclaiming that scientists had re-evaluated the metabolic cost of breastfeeding, that original studies had been based on the metabolic processes of men, and that lactating people were being starved based on outdated guidelines. This has all the hallmarks of a good scientific exposé, a misogynistic system, poor methodological practice, and a ‘common sense’ fact that most people feel they know. And who doesn’t love talking about the price of milk? So let’s dive in.

I felt drawn to this topic again while writing my dairy cattle welfare lecture. In 2024 the average UK dairy cow was producing 8,278 litres of milk in a year. It’s fair to say that my sympathy for dairy cows has increased since I myself have started being a lactating animal. I’m coming to the end of my lactating journey with my second child, and to date have spent about 3 years cumulatively nursing babies, for both of my children I was their exclusive source of nutrition for the first 6 months of their lives. I have absolutely no idea as to how many litres I produced in that time, but I can definitely say that, much like the cows I’ve worked with, lactating is hungry and thirsty work.

So what is the actual metabolic cost of breastfeeding?

Firstly, as far as I can see, the NHS doesn’t give a calorie recommendation for breastfeeding parents, instead, guidance often focusses on eating a varied and healthy diet. Off to a Good Start, the Public Health Scotland parent-facing guidance, has all of this to say in its 60 pages

What you eat will not affect your milk supply, but just like any new mum, you need to eat enough and include a variety of foods to meet your needs and those of your baby. It’s also good to get regular exercise

Indeed, in my personal (middle class, highly educated, highly supported) experience, the advice I received was about maximising my ability to eat and nurse at the same time, with my midwife sister in law sending me gigantic boxes of biscuits, and my NCT instructor running an activity for partners where they could think about meals and snacks they would prepare ahead of time if they had to return to work.

600 calories is a pretty common estimation of the caloric cost of breastfeeding. In 1970, an estimate of 600kcal/day was thought to support lactation, and they knocked 100 calories off for the official recommendation ¹. A much more recent trial reported in a conference, looking at breastfeeding times, food intake, and infant weight gain in 3 month postpartum mothers, nursing parents required about 622 +/- 136 kcal/day for milk production alone ². Another study put the estimate at 650 kcal/day ³, noting that approximately 66% of this additional requirement was met via increased intake, but that about 28% was met by reducing energy output, and only about 5% of this requirement was met by tissue mobilisation, meaning that breastfeeding is not likely to help the weight ‘drop off you’ as is sometimes said. Historically, this lack of association between breastfeeding and weight loss has led to some concern over the caloric recommendations in lactation. For example, a 1983 study noted that those who lactated the longest also ate the most and lost the smallest amount of weight, and that perhaps this should lead to lower Recommended Daily Allowances for lactating people ⁴, whereas I think many of us in our current climate of wanting to promote breastfeeding would draw the conclusion that promoting caloric intake might increase breastfeeding length! Physiologically, peoples bodies prefer to use carbohydrates during pregnancy and lactation ⁵, highlighting the energy cost of this process (and potentially flagging why breastfeeding cookies are so commonly shared online). Lactating people are not more energy efficient ⁶ through some metabolic magic, and so the extra cost to grow a baby must be met either through increased intake, reduced output, or eating precious energy reserves in the body.

I will say that in looking up the metabolic costs of breastfeeding, the vast majority of articles were more concerned with weight loss rather than whether lactating people were receiving enough calories.

Breastfeeding rates are commonly disappointing ⁷,⁸ and in my experience as a chronically online mother, I have seen many of my compatriots concerned about ‘under supply’, and there’s some research to back this up, with many people stopping breastfeeding (or expressing and bottle feeding) because of concerns around supply ⁹, ¹⁰.

I admittedly didn’t spend a huge amount of time researching this topic, this is after all, only a blog post, but I was struck by the lack of linkage in the research between the energy cost of lactation and supply concerns as a reason for not maintaining breastfeeding. In the dairy cow world, this is one of the first things we would look at. Not whether or not the cow felt comfortable in the parlour. While the social issues around breastfeeding are undoubtedly important, this does appear to me to be a pretty big hole in the scientific narrative.

The original social media article then started to look at the caloric cost of healing ¹¹ and sleeplessness ¹². These are important considerations, certainly. I would say its generally accepted that healing requires energy, and different labours and the effect of waking up with a baby several times of night undoubtedly ups that energy requirement too. But I think this is starting to chase down a bit of a rabbit hole, of trying to find an objective ‘truth’ to the cost of lactating and breastfeeding. Over and over again when I look into questions around parenting, I’m struck by the huge natural variation in human parenting, across biology, across culture, across experiences.

This should not be a question we approach in search of permission to eat more or less. Instead, this is a minimum requirement for supporting our bodies. How it impacts weight loss should not be considered as important and worthy of research as it how impacts undersupply, and I can’t get over this gulf in the findings.

So, are scientists re-evaluating the price of milk? No, not really, there’s no united front of scientists pushing this. Are lactation consultants, midwives, and post-partum support groups fully understanding the huge biological cost of milk production? Possibly not, and its worth bearing that in mind if you’re breastfeeding or expressing. For me, I was a hungry fiend during breastfeeding, and lost no weight doing so, but I did grow two pretty great little people. Sounds worth it to me.

References

Footnotes

1. Thomson, A.M., Hytten, F.E. and Billewicz, W.Z. (1970) ‘The energy cost of human lactation’, British Journal of Nutrition, 24(2), pp. 565–572. Available at: https://doi.org/10.1079/bjn19700054.

2. Pereira, L.R. et al. (2015) ‘Energy Intake and Energy Cost of Exclusive Breastfeeding: Preliminary Data from the Postpartum Calorimetry Study’, Canadian Journal of Diabetes, 39, pp. S21–S22. Available at: https://doi.org/10.1016/j.jcjd.2015.01.092.

3. van Raaij, J.M. et al. (1991) ‘Energy cost of lactation, and energy balances of well-nourished Dutch lactating women: reappraisal of the extra energy requirements of lactation’, The American Journal of Clinical Nutrition, 53(3), pp. 612–619. Available at: https://doi.org/https://doi.org/10.1093/ajcn/53.3.612.

4. Manning-Dalton, C. and Allen, L.H. (1983) ‘The effects of lactation on energy and protein consumption, postpartum weight change and body composition of well nourished North American women’, Nutrition Research, 3, pp. 293–308.

5. Butte, N.F. et al. (1999) ‘Adjustments in energy expenditure and substrate utilization during late pregnancy and lactation’, American Journal of Clinical Nutrition, 69(2), pp. 299–307. Available at: https://doi.org/10.1093/ajcn/69.2.299.

6. Bender, R.L., Williams, H.S. and Dufour, D.L. (2017) ‘No change in energy efficiency in lactation: Insights from a longitudinal study’, American Journal of Human Biology, 29(6). Available at: https://doi.org/10.1002/ajhb.23051.

7. Diaz, L.E., Yee, L.M. and Feinglass, J. (2023) ‘Rates of breastfeeding initiation and duration in the United States: data insights from the 2016–2019 Pregnancy Risk Assessment Monitoring System’, Frontiers in Public Health, 11(December), pp. 1–9. Available at: https://doi.org/10.3389/fpubh.2023.1256432.

8. North, K. et al. (2022) ‘Breastfeeding in a Global Context: Epidemiology, Impact, and Future Directions’, Clinical Therapeutics, 44(2), pp. 228–244. Available at: https://doi.org/10.1016/j.clinthera.2021.11.017.

9. Heng, W.X. and Azhari, A. (2025) ‘Analysis of Social Media Posts: Breastfeeding Mothers Concerned About Breast Milk Supply, Logistics of Breastfeeding, and Child Development Outcomes’, Women’s Reproductive Health, 12(3), pp. 697–710. Available at: https://doi.org/10.1080/23293691.2025.2489500.

10. Hegney, D., Fallon, T. and O’Brien, M.L. (2008) ‘Against all odds: A retrospective case-controlled study of women who experienced extraordinary breastfeeding problems’, Journal of Clinical Nursing, 17(9), pp. 1182–1192. Available at: https://doi.org/10.1111/j.1365-2702.2008.02300.x.

11. Demling RH. Nutrition, anabolism, and the wound healing process: an overview. Eplasty. 2009;9:e9. Epub 2009 Feb 3. PMID: 19274069; PMCID: PMC2642618.

12. Markwald RR, Melanson EL, Smith MR, Higgins J, Perreault L, Eckel RH, Wright KP Jr. Impact of insufficient sleep on total daily energy expenditure, food intake, and weight gain. Proc Natl Acad Sci U S A. 2013 Apr 2;110(14):5695-700. doi: 10.1073/pnas.1216951110. Epub 2013 Mar 11. PMID: 23479616; PMCID: PMC3619301.