This section is from the book "The Hygienic System: Fasting And Sun Bathing", by Herbert M. Shelton. Also available from Amazon: The Hygienic System Vol III Fasting and Sun Bathing.
The pulse varies greatly during a fast. It may run up to 120 or even higher, or it may drop as low as 40, per minute. Indeed, Mr. Macfadden records a case in his practice in which the pulse went down as low as 20 and was so feeble it could scarcely be felt. It is the usual thing to have the pulse rate increase at the beginning of the fast and then, after a day or two, to drop. In chronic cases that are confined to bed during the fast, the pulse usually, after its temporary rise, drops to 48, or 40, where it may remain for a day or two days and then mounts up again to 60. After a few days it will settle at 60 and remain there until eating and activity are resumed. It is, of course, understood that the pulse is subject to all the variations, while fasting, as at other times of life, and that where there is "disease" of the heart, or nervous troubles, it will often vary greatly from the above standard. Where stimulants are employed during a fast, these occasion more heart activity than if taken when one is eating.
Discussing what, to the uninitiated, are alarming heart symptoms which may arise during a fast, Mr. Carrington says: "I may here remark, however, that such extreme variations invariably denote some profound physiological change taking place at the time--a crisis, in fact. The fact that hitherto weak hearts are actually strengthened and cured by fasting proves conclusively that any such unusual symptoms, observed during this period, denote a beneficial reparative process, and not any harmful or dangerous decrease or acceleration, due to lack of perfect control by the cardiac nerve."--Vitality, Fasting and Nutrition, p. 464.
Mr. Macfadden records the case of a man whose heart beat fell to twenty and was so faint it could scarcely be felt, after three weeks of fasting. It quickly rose after the patient took some nourishment. (Natural Cure for Rupture--p.p. 36-37). Abnormally slow pulse is seen in rare cases of extreme debility, especially in those who have for weeks, months or years prior to the fast, been in the habit of taking stimulants. A complete withdrawal of the stimulants results in a great slowing up of the habitually excited activities of the body. Mr. Carrington says that some of these cases die, although I have not seen it. He says: "The long-deferred crisis is at hand. Either the patient will recover the expended powers, and live; or, if wasted to such an extent that recovery is impossible, will die, during the fast. This is the most frequent, if not the only cause of death that occurs during cases of protracted fasting, when death occurs before the return of natural hunger. Such cases never die from starvation; it is a physiological impossibility for them to die during a fast before the return of natural hunger--unless the vital powers have previously been so wasted as to render their recuperation impossible--death being due to this failure; and it will thus be seen that the real cause of death is, again, previous mal-treatment--death occurring in spite of, and not on account of, the fast. Had the fast been instigated, in such cases, at an earlier period, the vital powers might have been sufficiently strong to have withstood the shock--recuperation instead of failure would have resulted; i.e., instead of death."--Vitality, Fasting and Nutrition.
A very rapid pulse is seen in exercise, excitement, nervous shock, and gas pressure, etc. Exertion may increase the pulse rate more in the faster than in the regular eater. As there is nothing essentially abnormal about this and the pulse rate soon settles back to its regular fasting level, there need be no concern over it.
Let us not forget, in considering the unusual cases, that the vast majority of patients will not experience any of these inconveniences during a fast. Abnormally high or low pulse rates during a fast are exceptions and not the rule and do not denote any danger from the fast in itself. They should cause no alarm, so far as fasting, per se, is concerned. As a rule, the heart beat is steady, forceful and in keeping with the activities of the body.
While in acute disease it is usual to have temperature and pulse rate rise and fall together, in fasting and chronic disease this is not always so. For example, in many cases of heart disease or in goitre, the heart may be beating at an enormous rate and temperature remain normal or fall below normal. In fasting, on the other hand, the pulse rate may fall very low and temperature remain normal. Or, if the fasting patient becomes active, his pulse rate may rise to 110 or 120 a minute and his temperature remain normal.