Every diet plan that has ever produced results is, at its heart, a strategy for lowering calorie intake. All-fat, no-fat, reduced-carbs, gluten-free, paleo… they may all take different approaches to weight loss, but they all have the same result. By limiting food intake, or even just limiting food options, these diets reduce our calorie consumption. And when calorie intake drops below calorie burn, that’s when we start to drop the pounds.
I am a 65-year-old male who started IF seven weeks ago. I only eat between noon and 8pm. I am obese, but losing about a pound a week so far. Notably, except for time, I have not changed what I eat at all. My diet was never terrible or great, and now it is the same, a mix of raw fruit sometimes and a donut another time. But I only eat it during the appointed hours. Remarkably, I do not feel hungry. I used to eat comfort breakfasts like pancakes or waffles, and I thought I would miss them. But no, I truly am not hungry in the mornings. I often delay lunch, but I still stop eating at 8. That alone probably has cut many calories of desserts. Bottom line: works for me so far.
And while this may sound difficult, it’s really not. After all, the reasons our bodies store fat in the first place is because we, as a species, aren't really designed for constant eating. The three-square-meals (or more) eating plan is a relatively new phenomenon in human history; for much of our past, feast and famine were the order of the day. Our bodies know how to handle long stretches without food, and they know how to make good use of the “feasts” when they come along.
The thing is that if you want to slim down and get rid of excess pounds, then the fasted state is really the best way to do it. If you keep taking food into your body, insulin levels will remain high, and you will keep burning glucose for fuel instead of burning fat. Of course, to remain in the fasted state, you need to not be in the fed state, and that can be a problem. Starving yourself all of the time isn’t enjoyable, and it’s not healthy. To be blunt about it, your body needs nutrients, it just doesn’t need them all day, every day.

What and when you eat during the feeding window also depends on when you work out. On days you exercise, carbs are more important than fat. On rest days, fat intake should be higher. Protein consumption should be fairly high every day, though it will vary based on goals, gender, age, body fat and activity levels. Regardless of your specific program, whole, unprocessed foods should make up the majority of your calorie intake. However, when there isn’t time for a meal, a protein shake or meal replacement bar is acceptable (in moderation).
Conversely, a 2013 study completed by researchers at Washington University School of Medicine, presented the exact opposite, showing increased reactions in receptors on the tongue, when participants were given artificial sweeteners. Their study concluded that if the receptors are capable of detecting sweetness, even though calories aren’t present, a hormonal response (insulin release) is likely.
This is excellent, thank you! One thing I would love to see added to this research is the keto effect of just water over those 120 minutes, and THEN the keto effect of adding a tad bit of protein, such as collagen powder (I use the green tub great lakes powder; it dissolves great in coffee). I feel like that might put your test even more into an annals of science, and for us folks who don’t have the finger prick blood testers, it would be fun to know. Also, sugar free sweeteners like splenda (!!) or stevia–do they have any effect? Anyway–great read, exactly what I was looking for when I googled my question.
Finally, note that fasting to induce ketosis can also trigger a set of symptoms often known as the “keto flu.” Fasting ketosis symptoms may include decreased energy levels, increased cravings, digestive issues, muscle aches and dizziness. These symptoms can last between a few days to a few weeks when starting the ketogenic diet, but they typically subside once your body enters ketosis and begins to adapt.
An intermittent fast is a brief fast where, for 12–16 hours or more, you don’t eat anything except water (a few exceptions apply). And while that may sound incredibly difficult to achieve, you might already be fasting without knowing it if you eat dinner at, say, 7 p.m. and break your fast in the morning between 7—10 a.m. — and if you only have water and black coffee or tea between. 
Expect funny looks if you spend a lot of mornings with breakfast eaters.  A few weeks back I had a number of friends staying with me, and they were all completely dumbfounded when I told them I didn’t eat breakfast anymore. I tried to explain it to them but received a bunch of blank stares. Breakfast has become so enGRAINed (zing!) in our culture that NOT eating it sounds crazy.  You will get weird looks from those around you…embrace it. I still go to brunch or sit with friends, I just drink black coffee and enjoy conversation.
Fasting helps you build a better brain, too. Intermittent fasting increases a protein in your brain called BDNF that researchers have nicknamed “Miracle-Gro for your brain.”[7] BDNF improves learning and memory and can help you forge stronger neural pathways, making your brain run faster and more efficiently, which is especially important as you age.
Because it simplifies your day. Rather than having to prepare, pack, eat, and time your meals every 2-3 hours, you simply skip a meal or two and only worry about eating food in your eating window. It’s one less decision you have to make every day. It could allow you to enjoy bigger portioned meals (thus making your tastebuds and stomach satiated) and STILL eat fewer calories on average.
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