1/27/19
Previously, we went over some of the health benefits that come with implementing an intermittent fasting regimen into your diet. So we know how it can improve our health, but what happens to our bodies when we fast? When are these responses triggered? The literature on fasting is not completely fleshed out yet, and everybody will respond differently to a fast. With that said, there is a general sequence of metabolic responses that ensue after finishing a meal. Some begin immediately, and some don't start for days, but I'll try to cover the relevant events that take place over extended time without eating.
Immediately after eating, the macronutrients (fat, carbs, and protein) from your meal are broken down into their simplest forms (fatty acids, glucose, and amino acids) in order to be used for energy or stored. If you ate a lot of carbs, the spike in your blood glucose will tell your liver to secrete insulin, allowing glucose to enter cells. If it's not being used, it'll be stored as glycogen or adipose tissue. Meanwhile, grhelin will fall and leptin will rise. These are hormones that regulate your appetite. Since you just ate, they will send signals to your brain, which convinces you that you're full. About 3 hours after eating, your blood glucose has returned to normal a normal level.
Around 5 hours after eating, your blood sugar is still dropping, and to keep in within a homeostatic range (70-120 milligrams per deciliter), your body starts to recruit those stores of glycogen that it set aside before. Since blood sugar is low, so too is insulin release from the pancreas. Instead, it starts to release glucagon at greater rates to break down that glycogen into glucose. As you near a full day without food, you'll be running out of glycogen stores, and the main source of energy pivots to fat stores and ketones. The timing of all of this is of course subject to change depending on how quickly your body is using energy. This is why exercising can speed up entrance into ketosis.
After a day of fasting, your body is in full fat-burning mode. It converts the fat stores around your body into fatty acids through lipolysis, and these fatty acids are then made into ketone bodies via beta-oxidation in the liver. The ketones are the fuel version of fats. There are a few different ketone types, but the one that's measured in the blood is BHB, and at this time it'll be well within ketosis range (1-2 millimole). At this point, glucose cannot be found in the body, but it's still needed in your brain, so the body creates its own out of fat and protein through a process called gluconeogenesis (hint: if it has genesis, it means making... see ketogenesis etc. All these terms are understood really easily with a little bit of latin skill, and your doctor won't sound so smart anymore). Revisiting the hunger hormone, you'll find that it's actually dropping now. That's why you get less hungry so far into a fast.
OK, this is insanity, it's been 3 days without food. Here, a lot remains the same. Blood glucose, insulin, and hunger are at baseline lows. Steady ketosis is indicated by BHB levels of more than 2 millimole. Some real deal phenomena start to kick in around now. For one, a growth hormone called insulin-like growth factor (IGF-1) is largely depressed. So what? Well, this seems important because it has been shown to decrease oxidative stress. That's a fancy way of saying that toxins are causing much less inflammation and damage in your body. This part is linked to a slow in aging and lower incidence of cancer. This stage is even linked with a higher production of neurons. It's important to note that these effects have not been seen in fasts shorter than 3 days.
Now you're saying, "This is getting ridiculous, I never would have thought I could go 5 days without food." Now, your blood sugar and insulin are down more than 30% below normal levels. Contrary to what you may have thought, that's good for your metabolism. IGF-1 is down 60% from normal levels now. After a week of fasting, you could have IGF-1 levels that would put you in a group with people that have a growth hormone deficiency. Interesting to note that this group also dies less... in general and from diseases like cancer and diabetes.
Competitive athletes need glucose to be at their best and see peek performance. However, without a need for athletic performance, it may behoove you to cut back on carbs where possible. It makes sense that low-carb and ketogenic diets have gotten plenty of praise in the dieting world, as lower insulin largely means better metabolic health. But it doesn't help if you're eating frequently in large volumes. Partner these diets with fasting, and you'll be a health phenom. Of course, do it to an extent you're happy with. Food is meant to be enjoyed, after all.
For me, I'm not so good at restricting calories. Once I get eating, I eat a ton of food. I'm better at just turning my appetite off completely, so I restrict my time eating more than volume. I've also done my best to keep an eye on carb intake and cut out sweets and sugared drinks that I don't absolutely love.
I thought it was really interesting and helpful to see when each of these metabolic and hormonal effects start to take hold. This knowledge can help you better plan the timing of your fasts and maybe even believe in the benefits a little more. The proof lies in the concentration measurements of these substances. The numbers are all there. So go out there and try it for yourself.
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