Biologically:
The body keeps the levels of sodium, potassium, calcium and magnesium in pretty tight regulation. It doesn't "like" or "want" to have lower than normal levels and it takes steps to keep that level very regular, either by excreting and/or storing the excess (yes, your body is able to store sodium for future use, just as it can store calcium and magnesium as well) or if you aren't getting enough, it relelases its stores and becomes more efficient at extracting sodium from the foods that you eat and the water you drink. So even though you may eat less sodium, your body simply takes steps to counteract that.
Low sodium levels are unusual and/or temporary (such as severe dehydration) and it's only in people who have serious health issues or are on medication that it is even POSSIBLE. Sodium - blood: MedlinePlus Medical Encyclopedia Back in the early days of the site, we had a few people (including a medical doctor) try to follow the FGD and had their blood levels of sodium, cal, mag, and potassium tested before and after doing a strict FGD for 6 weeks and their levels were virtually identical, all within normal range, before and after doing the FGD for 6 weeks, and the doctor's sodium level actually went UP because her body just got better at extracting sodium from food and water. (BTW she gave up FGD based on that info and did indeed get a girl on LE Diet).
Additionally, low sodium diet cannot be swaying because people of sub-Saharan African descent all carry a gene that causes them to have higher levels of sodium in their blood****** than people of other ethnic heritages (possibly as a defense against dehydration, malaria, or both) and yet they have the highest % of girls conceived of any ethnic group (102 boys for every 100 girls versus 105/106 for every 100 girls for other ethnicities) and this has stayed consistent even as they have moved out of Africa and into other parts of the globe.
Historically, humans and in fact ALL mammals have an insatiable palate for salt. Whenever they find salt, they eat it in huge quantities and virtually all cultures across the globe use tons of the stuff in food preservation and condiments and this has been true for at least dating back thousands of years, yet the gender ratio stays at about 50-50 regardless. Archaologists recently uncovered a salt mine in China dating back to 6000 BC (that's 8000 years ago). There was NEVER a time where humans were having disproportionate numbers of male and female offpring over the long term - not when we were cavemen eating not as much sodium, not in ancient China, and not today. It is VERY likely that the altered gender ratio in China that the FGD book used as "proof" that sodium sways blue, was NOT due to sodium whatsoever but was the result of female infanticide because we now know for a fact that is occuring there today.
Humans are eating more salt than ever before and yet the gender ratio is actually FALLING with more girls conceived than they were even 50 years ago.