This study found that alternate day fasting (ADF) damages the heart muscle – decreases its size, decreases pump function, makes it “stiff”:
Chronic Alternate Day Fasting Results in Reduced Diastolic Compliance and Diminished Systolic Reserve in Rats, Journal of Cardiac Failure, October 2010
The six-month long alternate day fasting (ADF) diet resulted in a 9% reduction (p<0.01) of cardiomyocyte diameter and 3 fold increase in interstitial myocardial fibrosis. … Left atrial diameter was increased 16%, and the E/A in Doppler-measured mitral flow was reduced (p<0.01). Pressure-volume loop analyses revealed a “stiff” heart during diastole in ADF rats, while combined dobutamine and volume loading showed a significant reduction in LV diastolic compliance and a lack of increase in systolic pump function, indicating a diminished cardiac reserve.
Conclusion: Chronic ADF in rats results in development of diastolic dysfunction with diminished cardiac reserve. ADF is a novel and unique experimental model of diet-induced diastolic dysfunction. The deleterious effect of ADF in rats suggests that additional studies of ADF effects on cardiovascular functions in humans are warranted.
There are a few other things that go on which make this particular method of weight loss challenging for an older person or for someone who has diabetes, heart disease, dementia, or other chronic condition.
Raises Blood Glucose
When you eat, the body secretes insulin, an anabolic or building/storage hormone which enhances uptake of glucose into cells. Insulin lowers blood glucose. When you don’t eat, the body secretes insulin’s complement, glucagon, a catabolic or break-down hormone which releases glucose from cells. Glucagon increases blood glucose. For people with diabetes or insulin resistance, that increase in blood glucose while fasting is a problem, because levels can stay elevated. High blood glucose damages tissue, especially the small blood vessels in the eye and kidney. (If you’re testing: over 100 mg/dl designates prediabetes, over 126 mg/dl designates diabetes.)
When you fast, your body depletes its stores of carbohydrate for energy and burns more fat. But it also burns more protein, leading to muscle loss over time. I don’t know if this contributed to the heart damage in the study above, but you would expect heart muscle to be sacrificed during a fast.
When your body starts burning protein for energy, it enters a conservation mode which lowers metabolism. A lower metabolism can thwart weight loss in the long term. It can also make exercising difficult.
Strain on Kidneys
As your body breaks down protein for energy, it releases protein’s nitrogen which is removed from the body as urea via the kidney. In a healthy person, the kidney can keep up. In an older person, or one with diabetes or kidney disease, it is more challenging.
Many people lose their appetite as they age, whether from chronic disease, medication, depression, dementia or other mental health disorders. It becomes difficult to provide enough food and nutrients when meal times are restricted. Older people also experience problems with dentition, digestion, absorption, metabolism, and dehydration, any of which can compromise nutrition. For example, even people in their 50s can have decreased vitamin B12 status because of decreased absorption.