Time restricted eating is limiting the time that you eat within every 24 hours. For example, on a 16:8 time restricted eating regime, you would fast for 16 hours and eat all of your meals within 8 hours. There are many variations of this such as 12:12, 14:10, 20:4 etc. All have been shown to have health benefits. For example, in one study, participants that stuck to a maximum of a 10-12 hour feeding window of their own choice lost 7lb in weight over 16 weeks, had more energy, better sleep and 18 months later had maintained these improvements. Other reported benefits from time restricted eating trials include lower blood pressure, decreased inflammation, improved blood sugar control and lower heart disease risk.
As the body’s ability to handle glucose deteriorates in the evening, a meal eaten at 8 pm can cause twice the blood sugar response as a meal eaten at 8am, so it may be best to choose a feeding window earlier in the day.
Fasting is a period of not eating, and prolonged fasting is when you have periods of 24 hours or more without eating, drinking only water, or unadulterated tea and coffee. We usually run out of glycogen (sugar stores) within 12-36 hours of our last meal and then shift to rely more on our fat stores for energy. Water fasts can be prolonged, for 24 hours up to many weeks, if medically supervised. Or they can be intermittent, such as alternate day fasting, or the 5:2 fasting method, where you eat on 5 days of the week and fast on 2 days, or the fasting mimicking diet, where you fast for 5 days out of the month. Again, studies of fasting have shown reductions in blood pressure, weight and body fat, decreased inflammation and insulin growth factor 1 (a cancer promoting molecule).
If trying prolonged water fasting at home, it is not recommended to do more than 72 hours without medical supervision. If you have diabetes, no period of fasting should be undertaken without medical supervision as your medication may need to be reduced or stopped on the days of the water fast and your blood sugars will need to be closely monitored.
Women should be aware of their menstrual cycle and only undertake a prolonged fast during days 1-10 of their cycle (day 1 being the first day of your period); prolonged fasting later in the cycle can reduce the production of progesterone and disrupt menstruation.
So what is happening during these fasting periods?
8-12 hours - blood glucose begins to dip and your body dips in to glycogen stores, blood pressure lowers and insulin sensitivity increases
12-18 hours – fat stores are used for energy, increase in brain derived neurotrophic factor (BDNF), which protects brain cells, encourages the growth of new brain pathways, enhances learning and boosts mood.
24 hours – autophagy occurs where your cells replace any old, damaged or poorly functioning parts, which reduces inflammation, clears out proteins linked to Alzheimer’s disease and may help kill cancer cells.
36-48 hours – human growth hormone rises and increases muscle mass and stimulates faster muscle repair. It may speed up the healing process for wounds and more serious injuries.
72+ hours – An overhaul of the immune system as stem cell production stimulated by this length of fasting creates new immune cells to replace old ones. Interestingly, in a study where patients with cancer were fasted during their chemotherapy for over 72 hours, their immune system, normally disseminated by this process, remained strong throughout.