Someone who has a high-carb diet might never move beyond the catabolic state as they will always have a reserve supply of glycogen. However, someone with a low-carb diet and who regularly exercises might move through it very quickly (the “keto diet”, in which you cut out almost all carbs to maintain low blood glucose levels and glycogen stores, works in the same way). “I would move away from intermittent fasting for fat loss, and if you want to adopt it think about the health benefits,” says Clarkson.
How to fast
“To fast you have to downregulate the feeling of hunger,” says Clarkson. Hunger is felt when ghrelin, a hormone released from our stomach, triggers the production of two other hormones, called NPY and AgRP, in the hypothalamus.
While these three hormones generate feelings of hunger, there are a multitude more that suppress it. Sometimes called the “satiety hormones”, one of the key ones is leptin which is released from fat cells to suppress the production of ghrelin – basically telling the body “there is fat here that you can burn”.
Ghrelin is sometimes called the short-term hunger response because it is released when the stomach is empty and there is less pressure on the stomach wall. It can be overridden to a certain extent by drinking water. Leptin meanwhile works over the long term.
“Our hunger hormones are regulated by many things, genetics being one of them,” says Clarkson. “But thinking about the nerves that are attached to our stomach and digestive tract – if your stomach is not distended your body will think it is hungry.” She adds that staying hydrated can help with the early feelings of hunger until your body has adjusted. “The first couple of weeks will be tough, but you get used to it.”
For most people, ketosis occurs 12-24 hours after eating, so if you have your evening meal between 18:00 and 20:30, the fed state would end between 21:00 and 23:30 and ketosis and autophagy might occur by 06:00 to 08:30 the following morning. “But the majority of people are sitting down and opening a packet of something else after dinner,” says Clarkson. “Snacking or sugary drinks and beer extend the fed state for three hours. If you finish snacking at 21:30-22:00, the fed state is being taken to 01:00-03:00,” she says. This might mean ketosis never occurs before you next have a meal.
“If you can make the informed decision of eating the evening meal an hour earlier and not snacking, you may be getting into that ketosis state by morning, versus someone who is having the high-carb evening meal and snacking, waking up at 06:00 and never getting into that state,” she says. Clarkson suggests starting by eating earlier on a Sunday evening, or having breakfast an hour later and starting from there, building up from one or two days each week.
With a careful approach, intermittent fasting might help your body to perform its own repairs and recoveries. Autophagy appears to decline with age, so giving yourself a boost later in life might be useful. But be aware that it might not be the right strategy for weight loss, and there is no replacement for a balanced diet.
William Park is a senior journalist at BBC Future and is @williamhpark on Twitter.
All content within this article is provided for general information only, and should not be treated as a substitute for the medical advice of healthcare professionals. You should speak to your doctor or healthcare professional if you are pregnant or have a health condition such as diabetes and are considering intermittent fasting, are planning to fast long-term, and you should not avoid fluids while fasting.