There are studies which show benefits from IF (intermittent fasting) but there are also some studies that raise concerns. The 5:2 book was based on clinical research with breast cancer patients and that initial study seemed to show health benefits but that study was not long-term and did not assess any impact on fertility, or the different impact IF could have on the sexes.
There's a brief summary here of some of the research which probably has enough detail that you could find the original studies if anyone wants to read more.
^Despite the growing enthusiasm for intermittent fasting, researchers have conducted few robust clinical trials, and its long-term effects in people remain uncertain. Still, a 1956 Spanish study sheds some light, says Louisiana-based physician James B. Johnson, who co-authored a 2006 analysis of the study's results. In the Spanish study, 60 elderly men and women fasted and feasted on alternate days for three years. The 60 participants spent 123 days in the infirmary, and six died. Meanwhile 60 nonfasting seniors racked up 219 infirmary days, and 13 died.
In 2007 Johnson, Mattson and their colleagues published a clinical study showing a rapid, significant alleviation of asthma symptoms and various signs of inflammation in nine overweight asthmatics who near-fasted every other day for two months.
Detracting from these promising results, however, the literature on intermittent fasting also includes several red flags. A 2011 Brazilian study in rats suggests that long-term intermittent fasting increases blood glucose and tissue levels of oxidizing compounds that could damage cells. Moreover, in a 2010 study co-authored by Mattson, periodically fasting rats mysteriously developed stiff heart tissue, which in turn impeded the organ's ability to pump blood.^
However, the entire to fast or not to fast argument as regards fundraising is nonsense. If you want to fast to fundraise then do so. If you'd rather eat cakes then dig in. Fasting for one day isn't going to stop us being obese as a nation.