A remarkable paper, that's how I would describe Caloric cost of normal pregnancy. Obstet Gynecol. 1972 Dec;40(6):786-94, by Emerson K Jr, Saxena BN, Poindexter EL. As you can see, it is not a new one. As a matter on fact, it is hard to find such well done research papers these days.
The study sets out to measure what is the extra metabolic requirement of a normal pregnancy. And they did it not the usual way (fail-prone questionnaires), they did it in a metabolic ward setting. How the authors were able to find 11 pregnant women (before 20 weeks of pregnancy) willing to spend 1 every 4 weeks locked in a hospital, with a constant diet, weight and fluid balance control, nitrogen balance, and (after 3 days of dietary equilibrium) 2 consecutive days of expired air collection for 2 hours, 3 times a day, is simply astonishing. But they did it, and what they found out is really interesting, useful and, in some ways, counterintuitive.
7 patients were in positive caloric balance – they had normal weight before getting pregnant. Four patients, on the other hand, were deemed overweight and were put in a low calorie diet to prevent excess weight gain. As you can see in the table, they had a measured caloric deficit around 200 Kcal/day. However, not only these overweight women gained weight on a calorie-restricted diet, but they gained more weight (10 Kg) than the lean mothers that had a positive calorie balance around 200 Kcal (7.36 Kg). In other words, if you are pregnant, you will gain close to 8 Kg, whether you overeat or undereat by 200 Kcal/day. How is that for calories in, calories out?? Maybe hormones are driving the process? You bet they are. By the way, the 4 heavier women were eating a very restrictive diet in order to produce the aforementioned caloric deficit – only 1440 kCal/day (ranging from 1200-1600). Yet, all these starved mothers managed to gain weight, so did their offspring (not a single baby was born underweight, and these babies were actually heavier). Again, how is that for calories in, calories out?
Now, table 2 is pure gold (see below). Why? One could speculate that the 4 overweight women overate while outside of the hospital, so that they would really be in positive caloric balance. It may be true, but here enters the VO2 measure. It is a measure of the metabolism, by way of measuring how much oxygen is being consumed per Kg. Remember, this was measured after 3 days in the hospital, eating controlled food portions and resting – same (low) level of physical activity for every woman. In table 2 you can see that the calorie restricted women had a mean VO2 of 3.2 ml/Kg/min, versus 4.06 for the positive caloric balance ones. Those of you that specialize in exercise physiology please correct me if I am wrong, but I understand from these numbers that the starved moms' basal metabolism was running 21% slower than that of the well fed ones. If this is correct, this would mean that Nature finds a way. Starve a pregnant woman and the calories will be partitioned towards maintaining a healthy pregnancy, even if it means slowing down metabolism as a whole.
Finally, the authors have calculated the total extra caloric expenditure from conception to delivery: it is 27,120 +/- 2175 Kcal. It seems a lot, but is it?
The mean daily increase for all subjects amounts to 100 Kcal for the whole pregnancy. The median value was 9 Kcal for the first trimester, 84 for the second, and 216 for the 3rd trimester. Interestingly, the authors observed sparing of fat tissue even when the caloric balance was negative. The fact that pregnant women tend to be insulin resistant makes this observation even more interesting.
Now, let's think about this for a moment. 100 Kcal is really nothing. It is one cereal bar. It is a single banana. So, if you really believe the calories in, calories out paradigm, this is what you would get: just eat one extra banana a day, and you will be 10 Kg fatter 9 moths later. But if you undereat by 2 bananas a day, you will still be 10 Kg fatter later. How? May be photosynthesis, who knows?
There is more to the paper (nitrogen balance, and esoteric concepts like “non-protein respiratory quotient”), and I recommend those interested to take a look at the original. But table 1 of this paper is enough to bring down the whole edifice of calories in, calories out. In fact, pregnancy itself already proves that point: one (usually) will be in a positive caloric balance for the pregnancy to advance, but the positive balance is a consequence, not a cause (conception and hormones, of course, are causal). Or else, one could just eat an extra banana a day, and get a baby after nine months!