Low carbohydrate (carb), high fat diets continue to be popular and women of reproductive age are following them. They will often change their diet to a more ‘conventional’ one during pregnancy, but are often keen to return to a weight-loss diet, that they have had success with, after the pregnancy. So, what do we know about the effect of low carb, high fat diets on breastmilk composition and on the metabolism of the breastfeeding woman?
The lack of research on this type of diet, and the possible effect on breastmilk composition, breastfeeding women and their infants, was highlighted by both breastfeeding experts and dietitians in 2004 (Heinig & Doberne, 2004; Wood & Hilldebrandt, 2004) at the height of the popularity of the low carb, weight loss diet advocated by Dr RC Atkins — the Atkins diet.
Research on low carb, high fat diets has been undertaken, but surprisingly, only one study has been done. Mohammad, Sunehag, & Haymond (2009) looked at maternal breastmilk production, infant intake and maternal glucose metabolism and broadly looked at the macronutrients in breastmilk.
Two moderate hypocaloric diets (1785 ± 22 kcal/day) were assessed in a cross-over design: seven, healthy lactating mothers and their infants were studied on 2 occasions in random order for 8 days separated by 1 to 2 weeks. On one occasion, the subjects received the low carb, high fat (LCHF, 30% of energy as carbohydrate and 55% as fat) and on the other occasion received the high carb, low fat diet (HCLF, 60% of energy as carbohydrate and 25% as fat). It is important to note that the amount of carbohydrates consumed during the LCHF was around 150 g per day (Table 1). This is not considered a true low carb diet — during a strict low carb diet around 20 g of carbohydrates are eaten, increasing to 40 g to maintain health (Dashti et al., 2006).
Table 1. Daily intake of macronutrients during the HCLF and LCHF diets
Energy intake (EI, kcal/d)
1782 ± 16
1787 ± 31
51 ± 1
107 ± 2*
(% of EI)
26 ± 0
54 ± 0*
265 ± 4
137 ± 2*
(% of EI)
59 ± 1
31 ± 0*
69 ± 2
69 ± 1
(% of EI)
16 ± 0
15 ± 0
* Significantly different from HCLF, p < 0.01 (paired t test).
The researchers found that:
- daily breastmilk production was the same regardless of diet: 829 ± 41 mL/d (HCLF) and 821 ± 33 mL/d (LCHF)
- daily infant breastmilk intake was the same regardless of diet: 611 ± 41 mL/d (HCLF) and 612 ± 41 mL/d (LCHF)
- there was no effect of diet on either breastmilk lactose or protein
- there was no difference in infants' intakes of carbohydrate (lactose) or protein between the two diets.
- the milk fat concentration (4.8 ± 0.3 and 4.3 ± 0.3 g/dL, p = 0.01) and the corresponding daily milk fat content (39 ± 2 and 34 ± 2 g/d, p = 0.02) were higher during the LCHF diet
- the energy content of milk was higher (p = 0.02) during the LCHF diet (654 ± 24 kcal/d) than during the HCLF diet (619 ± 23 kcal/d)
- infants' energy intakes expressed as kcal/d (486 ± 24 compared with 452 ± 23) or as kcal/kg/d (87 ± 4 compared with 81 ± 3) was higher (p < 0.05) during the LCHF diet
- the estimated average maternal energy expenditure was higher (P < 0.03) during the LCHF diet (1368 ± 15 kcal/d) than during the HCLF diet (1283 ± 24 kcal/d)
- the sum of maternal energy expenditure plus milk energy content was higher (P < 0.01) during the LCHF diet (2022 ± 35 kcal/d) than during the HCLF diet (1910 ± 40 kcal/d)
- the deficit in energy balance [intake — (energy expenditure + milk output)] was greater (P < 0.01) during the LCHF diet (−235 ± 39 kcal/d) than during the HCLF diet (−119 ± 48 kcal/d)
- lactating mothers who consumed the LCHF diet decreased carbohydrate oxidation and increased fat oxidation to liberate glucose.
This study, the only good-quality study of a LCHF diet in breastfeeding women, has shown that maternal diet had no short-term effect on milk volume or its water-soluble components, such as protein and lactose. However, it did show that the LCHF diet increased the milk fat concentration and content by about 13% and 15%, respectively, when compared with the HCLF diet. This resulted in an increase in the calorie content of the milk by an average of 35 kcal, leading to increased fat and calorie intakes by infants. The increase in calories was statistically significant, but it is not known whether it is biologically significant.
The estimated average daily energy expenditure was higher during the LCHF diet than during the HCLF diet. When increased milk energy output is added to the maternal energy expenditure, the deficit in energy balance was twice that during consumption of the LCHF diet when compared with the HCLF diet. The researcher suggests that breastfeeding women may lose more weight while consuming an LCHF diet than a HCLF diet.
Weight loss and/or diet and/or unwell and not eating
31-year-old woman breastfeeding a 10-month-old-infant
nausea fatigue vertigo malaise and had vomited once
Serum anion gap 32.3 mmol/L
Blood glucose 3.8 mmol/L
Serum ketones ++
Urinary ketones +++
No specific illness or event described, other than lactation.
2L iv sodium chloride
Next day feeling better, tolerating normal diet and pH, anion gap and blood ketones had returned to normal.
Advised to ensure sufficient energy intake and to avoid prolonged fasts while breastfeeding.
Stopped breastfeeding for 12 hours.
Gleeson, Mulroy and Clarke (2016)
32-year-old woman breastfeeding a 3-week-old infant
Serum anion gap 28 mmol/L
Blood glucose 3.8 mmol/L
Urinary acetone +++
Had not eaten solid foods for days because of nausea and vomiting.
Infusion of sodium bicarbonate (8.4%), potassium phosphate (50mmol/50ml), glucose 20% and saline and refeeding.
Discharged from hospital after 10 days.
Hudak et al., (2015)
32-year-old woman breastfeeding a 10-month-old infant
pCO2 2.8 kPa
Blood glucose 3.8 mmol/L
Blood ketones 7.1mmol/L
Followed a low carb, high fat diet for 10 days, lost 4kg.
10% glucose infusion (3L in 48 hours)
Insulin (4 units in 24 hours).
Discharged after 3 days.
No mention of advice around breastfeeding.
Von Geijer & Ekelund (2015)
36-year-old woman breastfeeding a 5-week-old infant
vomiting for 48 hours
Serum anion gap 35 mmol/L
Blood glucose 133 mg/dL
Urinary ketones <80 mg/dL
Limited her diet to several small high-protein, carbohydrate-free meals in an attempt to return to her pre-partum weight.
Had intentionally lost 30 lbs (13.6 kg) since birth.
Fluid resuscitation with 3 ampoules of sodium bicarbonate per litre of D5W was initiated for 24 hours
Improvement in laboratory results occurred within 12 hours.
Started 1800 kcal diet during hospital stay, supplemented with food from home.
Breastfeeding ceased on advice from lactation consultant.
Sandhu, Michelis & DeVita, 2009
35-year-old woman breastfeeding and formula supplementation of 12-week-old twins
Two day history of progressive fatigue and shortness of breath followed by palpitations and light-headedness with light exertion
Blood glucose 68 mg/dL
Serum ketones moderate
Urinary ketones >80mg/dL
Reported attention to diet selection but denied fasting, carbohydrate avoidance, ketogenic diet and attempted weight loss or meal intolerance.
Initial intervention included iv isotonic saline, thiamine 100mg, magnesium sulphate 2 g and 5% dextrose in normal saline infusion, followed by a meal.
Symptoms improved within 6 hours and patient tolerated a normal diet within 24 hours
Nutritional consultation calculated caloric requirements of lactation and discovered mild carbohydrate and protein dietary deficiency. The patient was discharged home on a normal diet and multi-vitamin supplements.
Limited lactation was continued via breast pump on day 1.
Heffner & Johnson 2008