A couple of points here.......

Weakness, or the perceived weakness during a fast, is most commonly caused by the release of toxins into the bloodstream for elimination via the kidneys and processing via the Liver.
There are also psychological causes as to the "feeling of weakness".
It is a general rule of thumb for many that strength will increase up until about the 3rd to 4th week and thereafter strength will diminish.
A case in point is that of Succi on his own extended fast.

On my recent 4 day Water Fast I had no experience of weakness at all, or any other symptoms of detox either, as I do not suffer from toxemia.

We may also ask ourselves just how low BP can be before it becomes dangerous or potentially dangerous on a water fast?

Only orthodoxy "measures" the BP and how this affects health whether in a case of Hypertension or Hypotension.

We should bear in mind that the body is a self-regulating organism, and where in the absence of food, High BP will be lowered and even where Low BP will rise. This was true in my own case on my first fast of 25 days where previously I had suffered from Hypotension for most of my 24 years.

The load on the Heart is reduced as evidenced by its reduction to about 60 beats per minute and even lower on some days throughout its length.

The body will not allow BP to fall below a certain level on a Water Fast because of its innate power of self-preservation: unless due to congenital defect of the Heart, and where Saras experience with Low BP (in my opinion) was a case of "scaremongering" by ignorant attendants who knew no better.
The "mind" then plays havoc....."you are going to die" via the suggestion from others.

The greatest fear of fasting is the actual fear of fasting itself.

Regards

Chrisb1.