Blood contains about 700 kcal/litre, so an average adult would need to drink around three litres to meet their energy requirements each day. Would this much blood also contain enough of the essential micronutrients though? Let’s consider vitamin C, as a worst-case example, because humans can’t manufacture it or store it. Average blood serum levels of vitamin C are around 5mg/litre and the World Health Organisation recommends at least 45mg per day. So you’d need to drink nine litres of blood to avoid getting scurvy, which would give you so many calories that you’d rapidly become overweight. If you dosed your victims with lemons before biting them however, you could raise their serum levels to 15mg/litre and 3 litres of blood would just be enough.
It’s been suggested that the high iron content of blood would lead to iron overload in vampires. But healthy humans can tolerate up to 45mg of iron a day and you’d need to drink at least 26 litres of blood to absorb that much. The real danger then? Salt. Human blood contains about 9g of salt per litre. Those 3 litres you’re drinking each day would give you around four and a half times your RDA, which would lead to dehydration and kidney failure if you didn’t also drink lots of water, and hypertension if you did.