It's really a chemical kinetics problem that I would need to draw pictures for, but I'll try my best do to it with words.

High blood pressure is usually a results of your kidneys bloating and putting pressure on the capillaries and at the same time increasing your blood volume. I.e., there's more blood to push around in a smaller space so the pressure goes up.

If you're a researcher and you look at this, you'll quickly come to the conclusion that your kidneys have a lot of salt in them and this makes sense. The increased level of salt requires more water to dilute the salt so if the kidneys have lots of salt, they'll also have lots of water which will cause your blood pressure to rise. So the simple answer is "salt causes high blood pressure". And this is where everyone stops.

The next step, which we don't talk about, is WHY do your kidneys have lots of salt? Is it because you ate a lot of salt? Or is it something else? If your kidneys are getting the right signals from your other organs (e.g., your liver). They will flush the salt out with your urine. That is, it doesn't matter how much salt you eat (within reason), your kidneys can just get rid of it when you pee.

Conversely, if you're not getting the right signals, then your kidneys will hoard salt (and thus cause high blood pressure). They will hoard all of the salt they see and never (again within reason) dump it in the urine.

So to summarize:

Proper signalling: salt leaves the kidneys faster than it comes in, so excess salt is always purged.

Improper signalling: salt is hoarded in the kidneys, so even minimal salt builds up

So the question is, what's causing the screwed up signalling? Your liver being overburdened with metabolizing fructose. So fundamentally it's the fructose that's driving the high blood pressure, not the salt.

Think of it this way:

If you're getting the signal to purge the salt, then as soon as you see it in the blood, you pull it out of the blood and drop it in the urine. And as long as you can get it out of the blood faster than you can eat it, it never builds up.

If you're not getting the signal to purge (or are getting a signal to hoard), then every molecule of salt you see, you keep. So even if you eat a crazy low sodium diet, there's always a little salt around and when you see it, you keep it.

Insulin causes the kidneys to hang onto salt, so eating less carbs can be very effective at lowering blood pressure. I can tell how much insulin is in my system from the oedema/edema in my fingers - too much carb consumption and my fingers swell. They go down again when I cut down on the carbs. Salt consumption makes no difference to the edema.

Sodium is an essential nutrient. We absolutely need to eat a certain amount (I've seen estimates ranging from 200-1500mg - actual needs are going to vary depending on how your body handles sodium, weather, physical activity level, and so on).

For a person not eating processed foods, even the more stringent limits of the DASH diet (1500mg/day, or a bit over half a teaspoon) are going to give quite a bit of leeway (the question of the validity of the DASH is a totally different question).

1 teaspoon of salt is about how much I'd put in a dish intended for at least 6 servings. I recently added 1/2 teaspoon salt to a cookie recipe that made 24 cookies (non-gluten, of course!), and they were too salty. I sprinkled a few good shakes of salt from the saltshaker into a bowl to simulate salting food, just because I was curious how much that actually adds, and it came out to about 1/16th teaspoon.

Some sodium is present even in non-processed foods, but, for example, a pound of steak is going to be around 300mg. Fruits and vegetables have a negligible amount. Dairy products are a bit more.

So, someone who is eating all unprocessed foods might have a hard time meeting the minimum their body needs, especially during hot weather or when physically active, if they don't salt their food to taste or add the occasional bacon, lacto-fermented veggies, or other processed but reasonably acceptable source of sodium.

To add to the already-great answers above: You may be onto something with the stress, and you may be able to tackle both the salt and the carb issues at once.

Stress: This 2009 review of the literature shows links between stress, sodium retention, and hypertension in some individuals. (Not sure if there's evidence that a broader range of people exhibit this, but I wouldn't be surprised.) Does your dad have relatives or other people who can come in and give him a break once in a while?

Carbs vs. salt: Miked can correct me if I'm wrong here, but my understanding is that part of the reason salt is implicated in hypertension is because of the water we retain to hold onto the salt--the increased fluid causes an increase in blood pressure. My further understanding is that carbs also cause us to retain water, which would also increase blood pressure. I've seen some claims that the carbs cause us to retain more than salt does, but I don't know if any research has verified that yet. (And I don't have time to "connect the dots" between various studies to figure it out for myself!)

Carbs and hypertension: Here's a study that implicates sugar with high blood pressure (but doesn't implicate starches, interestingly enough).

I hope this helps; good luck!

I have a question to add on to this. My mom has high blood pressure and is on medication for it. She avoids salt. I'm not sure if it's because that is recommended when you have high blood pressure, or if it is recommended because of the medication. Either way, if she reduced her sugar intake, and that caused her kidneys to start working properly, would that medication become dangerous to take? Would it make her blood pressure too low? How would she know it was safe to go off of?

I have a question to add on to this. My mom has high blood pressure and is on medication for it. She avoids salt. I'm not sure if it's because that is recommended when you have high blood pressure, or if it is recommended because of the medication. Either way, if she reduced her sugar intake, and that caused her kidneys to start working properly, would that medication become dangerous to take? Would it make her blood pressure too low? How would she know it was safe to go off of?