Health benefits of prolonged “water fasting” (zero calories) or Buchinger fasting (200–300 calories/day) don’t last, according to authors of a review of eight studies.
Five days of fasting lowered weight by about 6%, but this weight was regained after 3 months of regular eating, the investigators found. The article was published in Nutrition Reviews.
“Water fasting led to improvements in blood pressure, cholesterol, and blood sugar levels, but these were short-lived,” senior author Krista A. Varady, PhD, told Medscape.
“Levels returned to baseline…quickly after participants started eating. Most benefits disappeared in 3–4 months,” said Varady, professor of nutrition at the University of Illinois, in Chicago.
“My overall conclusion,” she said, “is that I guess you could try it, but it just seems like a lot of work, and all those metabolic benefits disappear. I would encourage someone hoping to lose weight to try intermittent fasting instead of water fasting, because there’s a lot more data to show it can help with weight management.
“People should consult their doctor if they have diabetes or any other major obesity-related conditions before doing water fasting,” Varady cautioned.
“Healthy people with obesity can probably fast safely for 5 days on their own (if they don’t have any other conditions). However, no one should undertake one of these fasts for more than 5 days without medical supervision,” she stressed.
Eight Studies of Water and Buchinger Fasting
Although several favorable effects of prolonged fasting have been observed, benefits must be weighed against risks, Varady and her co-authors write.
Most medically supervised fasting programs have reported only minor adverse events, which included hunger, headaches, nausea, vomiting, dry mouth, and fatigue. However, more severe events have also been documented, including edema, abnormal results on liver function tests, decreased bone density, and metabolic acidosis.
The researchers aimed to determine the effect of prolonged fasting on weight, blood pressure, lipid levels, and glycemic control, as well as safety and the effects of refeeding.
They examined two types of prolonged fasting: water fasting and Buchinger fasting, which involves consuming 250 mL of fruit or vegetable juice for lunch and 250 mL of soup for dinner every day of the 5- to 20-day fast.
Buchinger fasting is only popular in Central Europe. Water fasting “institutes” exist in the US, such as one in California, Varady noted.
The researchers excluded fasting during Ramadan or fasting practiced by Seventh Day Adventists.
They identified four studies of water fasting and four studies of Buchinger fasting (of which one study of 1422 participants assessed fasting for 5, 10, 15, and 20 days).
The review showed that prolonged fasting for 5 to 20 days produced large increases in circulating ketones, weight loss of 2% to 10%, and decreases in systolic and diastolic blood pressure.
People who fasted 5 days typically lost 4% to 6% of their weight; those who fasted 7 to 10 days lost 2% to 10% of their weight; and those who fasted 15 to 20 days lost 7% to 10% of their weight.
LDL cholesterol and triglyceride levels decreased in some trials.
Fasting glucose levels, fasting insulin levels, insulin resistance, and A1c decreased in adults without diabetes but remained unchanged in patients with type 1 or type 2 diabetes.
Some participants experienced metabolic acidosis, headaches, insomnia, or hunger.
About two thirds of the weight lost was of lean mass, and one third was of fat mass. The loss of lean mass loss suggests that prolonged fasting may increase the breakdown of muscle proteins, which is a concern, the researchers note.
Few of the trials examined the effects of refeeding. In one study, normal-weight adults lost 6% of their weight after 5 days of water-only fasting but then gained it all back after 3 months of eating regularly.
In three trials, participants regained 1% to 2% of their weight 2 to 4 months after fasting; however, those trials instructed participants to follow a calorie-restricted diet during the refeeding period.
Three to 4 months after the fast was completed, none of the metabolic benefits were maintained, even when weight loss was maintained.
The study did not receive external funding. Varady has received author fees from Hachette Book Group for the book The Every Other Day Diet and from Pan Macmillan Press for the book The Fastest Diet. The other authors have disclosed no relevant financial relationships.
Nutr Rev. Published June 27, 2023. Abstract
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