A year after weight-loss surgery, 73 patients were eating about 1,400 calories a day. Their food gave them less than half the folate they needed

Weight-loss surgery works largely because people can no longer eat much. That is the point, but it raises an obvious question: if the plate shrinks to a fraction of its old size, do the vitamins and minerals on it shrink too? Most studies answer it with blood tests. A study published in Nutrients on 28 October looked instead at the food itself, and followed it through the whole first year after surgery.

The data are older than the paper. Between 2007 and 2009, 85 adults with severe obesity were enrolled at the Military Institute of Medicine in Warsaw, and the 73 who had surgery and completed every follow-up are in the analysis: 47 had a sleeve gastrectomy, which removes most of the stomach, and 26 had a Roux-en-Y gastric bypass, which makes a small pouch and routes food past the first stretch of the small intestine. Seven in ten were women, the average age was 41 and the average BMI 47. Before the operation and at one, six and twelve months, a dietitian took each person through everything they had eaten and drunk the previous day, using a photo album of portions. Researchers at the Medical University of Warsaw have now analysed those records.

Calories dropped hard. One month after surgery, people reported eating around 770 to 860 kcal a day, while they were still moving from liquids and purées to soft food. By twelve months, intake had climbed to about 1,410 kcal after a sleeve and 1,460 after a bypass. The authors are cautious about the pre-surgery figures of roughly 3,750 to 4,250 kcal: one remembered day says little about a usual diet, and self-reported intake is notoriously unreliable.

All 12 vitamins and minerals measured fell after surgery and stayed below their pre-surgery level all year; for most, intake was lowest at one month and recovered only partly by twelve. Against Polish reference intakes, a year's food supplied on average only 41 to 43% of the folate people needed, 48 to 63% of the iron, 52 to 64% of the calcium and 58 to 60% of the magnesium. On the day they described, no more than 15% of people reached their calcium target, 4% their magnesium and 19% their iron, and just one person out of 73 reached it for folate. Vitamin B12, B6, riboflavin and vitamin A, by contrast, were roughly at or above their reference values.

The two operations mostly looked alike. Once the results were corrected for testing 13 nutrients at once, the type of surgery made no overall difference to intake. Only calories and zinc changed differently over time between the two groups, with a slightly steeper early fall after bypass, and the authors warn the bypass group was too small, with only four men, to read much into it.

The limits matter here. A single 24-hour recall at each visit cannot pin down anyone's usual diet. Supplement use was recommended but never recorded, and no blood was tested, so low intake from food is not the same as a deficiency: supplements can make up much of the gap, as other studies have shown. Patients chose their operation with the surgeon rather than being randomised, and bariatric care has changed since 2009. What the study does show is how little food alone delivers in that first year, which is why the authors call for dietary checks alongside the usual supplements and blood tests.