As a functional Dietitian, I am not necessarily for or against the Keto diet. I recognize it is a highly effective treatment for certain conditions, while others should stay away from this diet as it poses significant risk to them. How effective (and safe!) it is really depends on your body's physiology and current metabolic state. Metabolic stress, for example, makes the keto diet a dangerous diet for some, while healthy adults may be able to live 'keto-adapted' for months or even years without problems. It may even help to prevent neurodegenerative disease.
However, as I've done some research in order to write my blog post "Is Keto Safe While Breastfeeding?", I came across the pro-keto argument often that claims "babies are in ketosis" because "breast milk is keto", and therefore, "keto must be the way we were all meant to eat". Naturally, I was intrigued by this and had to dig deeper: Are babies really in ketosis? Is breast milk really 'keto'? Are we all meant to be in ketosis, as the 'optimal' metabolic state?
Super interesting concept, so let's dig deeper!
Breast Milk is High Fat - but not Keto
If you've followed me for a while, you probably already know that breast milk is dynamic in its composition. Things like your diet, age, your weight, the stage of lactation you're in and even the time of day change your breast milk's exact composition. Heck, breast milk even changes during your baby's feeding as milk in your 'hind milk' is higher in fat and fat soluble vitamins compared to 'foremilk', at the beginning of a feeding.
However, there is a 'range' for the macronutrients in your breast milk. Your body will attempt to keep your milk's macronutrient content (the fats, protein and carbohydrates) within a certain range to make sure your baby is growing optimally and not missing out on any of those important nutrients.
Mature breast milk (the breast milk that is produced after the second week postpartum) contains roughly 650 to 700 calories per liter. It contains about 60 to 70 grams of carbs per liter, 35 to 40 grams of fat per liter and only about 8 to 10 grams of protein per liter. Converted into calories, the macro distribution of breast milk is roughly 50 to 53% fat, 38 to 40% carbs and 6 to 8% protein. Colostrum is a bit higher in carbs, higher in protein and lower in fat compared to mature breast milk (2).
As you can see, breast milk is hardly keto. Yes, it's high in fat, but it's not really low in carbs. It's also quite low in protein, which is something that goes against today's keto diet trends (although the traditional keto diet as done in medical nutrition therapy for the treatment of epilepsy is both low in protein and carbs!). Per definition, breast milk is not 'keto' and eating this way would likely not put any adult into ketosis.
Your breast milk also won't be keto if you eat a keto diet. Your diet has an effect on your breast milk, but the effect is more on certain vitamins and minerals, antioxidants and the types of fat and amino acids in your milk, rather than the its fat, protein and carb content. The total amount of these macronutrients (fats, protein, carbs) changes over the course of your lactation and throughout a feeding, but it's not variable to the point that your diet can significantly change it's macronutrient composition (fats, carbs and protein).
Nonetheless, Newborns Develop Mild Ketosis
Even though neither colostrum nor transitional milk nor mature breast milk are ' very low carb', newborns start to produce ketone bodies about 2 to 3 days after birth (3). In fact, breast fed babies produce more ketone bodies than to formula fed babies (5)!
Ketone bodies help with baby's brain development and provides baby with a 'backup' fuel, should glucose levels get too low, so this is a desirable effect.
Ketones play an important role in a newborn's metabolism. For one, they are important for baby's brain development. Studies have shown that the newborn brain is much better at using ketone bodies as fuel compared to an adult brain (3). Ketones are also a precursor for important important metabolites which help in brain development, including cholesterol and certain amino acids (2).
Another reason newborns produce ketone bodies is because their metabolism is adjusting to 'life outside the womb'. Consider that a fetus is essentially on a 'continuous glucose drip' inside of the womb, and that constant influx of glucose suddenly stops after birth. Naturally, baby's blood glucose will drop right after birth, and then recover. However, since baby now all of a sudden has to go minutes to hours between feeds, ketones and lactate are used as 'backup' fuel, should blood sugars go low (2).
Essentially, newborns develop a mild state of ketosis shortly after birth. This happens, however, not because of a low-carb diet. Glucose (either from baby's own production of glucose or from mom's milk) is still necessary and plays a major role in the metabolism of newborns. Also important to remember: babies are not in ketosis because of a low carb diet, but because their metabolism is adjusting to life outside the womb!
Researchers estimate that as much as 25% of a newborn's energy requirements are met by ketone bodies (3), but glucose still supplies the majority of a baby's energy requirements.
So, what's the optimal diet?
I think one thing is clear: Breast milk is the optimal diet for newborns.
It's high in beneficial fats, contains all of the nutrients babies <6 months need to thrive, contains beneficial hormones and growth factors, seeds baby's gut with good bacteria and prebiotic fibers and it has other beneficial effects researchers can't even explain yet. The macro distribution in breast milk is set around 53% fat, 39% carbs and 8% protein, which seems to be the optimal ratio for baby's growth and development.
But what about adults? Should we all eat a diet made up of 53% fat, 39% carbs and 8% protein, because it seems to have worked for us as babies?
I think that's a stretch.
Our metabolism is different as adults. We are not growing anymore, and our brains are already developed. Our needs are different as adults compared to when we were babies. We need energy to get us through the day. We need to prevent weight gain (rather than promote it). We need to eat to prevent chronic disease.
Our hormonal household and metabolism is different in the adult stage, and our body reacts different to the food we eat. A diet made up of 53% fat, 39% carbs and 8% protein would hardly put us into ketosis, but it would be lacking in enough protein to prevent muscle loss.
I don't doubt that a lot of us feel better on a lower carb diet, but to claim that we were all meant to be in ketosis because we were in mild ketosis as newborns is a stretch. Consider that a newborn's metabolism is so different that they produce ketones despite a diet moderately high in carbohydrates.