Whew, we’re on a nutrition kick over here at Functional Phyzio! We’ve talked about fats, proteins, supplement support for arthritis, but I want to talk about something most people wouldn’t consider a part of their “diet,” which is the timing of when they eat.
Fasting has popped up in the last few years, and it’s fairly popular, with individuals touting its effects in supporting weight loss, wakefulness, bloating, and even immune system conditions.
There are loads of different types of fasts, so some studies use a full 24 hour fast, only drinking water. And some studies count fasting as consuming light, low calorie meals (under 300) that dont’ spike blood glucose as fasting. There’s also research into dawn-till-dusk fasts, which have their own benefits.
We’re going to focus more specifically on the 16-8 fast, because it’s one of the most common. This is also commonly known as “Time Restricted Feeding.” When we talk about fasting in this blog post, we’re speaking to this specific set type of fasting, but it is by no means the only kind.
Let’s dive into the science behind intermittent fasting, and talk about what it’s useful for, because if you’ve been with me long enough, there isn’t a good or a bad. There are strategies and there are effects. Align your strategies with your desired effects. I might recommend intermittent fasting for someone with an inflammatory disease, like RA, but for my crossfit athlete looking to improve their performance? Probably not, so let’s talk about.
The idea behind intermittent fasting, as it’s touted, is that allowing for extended periods between eating allows for the inflammation that comes with digestion, allowing the body to become less ‘irritated’ or ‘inflamed,’ and also allows improved regulation of the hormone involved with digestion. It’s essentially a rest and reset for your body. This also impacts the immune system, with some studies of fasting finding that inflammatory modulators actually increase; IE, the body clears out any active inflammation faster.
Time Restricted Feeding has been associated as a boon for numerous metabolic disorders, increasing insulin sensitivity (reducing the likelihood of diabetes), reducing inflammation, boosts the immune system, supports brain health– The list actually keeps going. It supports the gut microbiome and its diversity, which supports overall health, and supports the stomach lining, so reducing any stomach disorders or discomfort, and improves digestion. It also supports the circadian rhythm, our biological clock, because of the consistency and timing of the meals supports release of specific hormones, improving sleep quality and long term memory.
Why does this happen?
Great question!
With so many effects, lots of different things are happening. Let’s break it down into a few specific effects, and let’s also talk about why time restricted feeding is hard on athletes.
Increased Insulin Sensitivity
Bear with me, because this is about to get nerdy. Our body has two key hormones that help regulate blood sugar, which is essentially the amount of energy flowing in our blood. Insulin is our “Store it,” hormone, which is why it spikes when you eat a piece of candy. You eat candy, it goes into your bloodstream, and your body needs to bring that number down by stashing it. Glucagon is the hormone that our body releases when our blood sugar is low, and we need to mobilize energy to stay awake and active.
When we’re in a fasted state, the body needs to release more glucagon, and the Pancreas is in charge of this. With an increased fasting state, the hormones that are sensitive to glucagon, and generating glucose are more active.
The body is working on breaking down its current glucose stores (often in the Liver), and releasing them into the blood stream. The exact mechanism of how intermittent fasting promotes increased insulin sensitivity may have to do with shifting the hormonal balance of glucagon/insulin. But if you step back and think about it, fasting decreases how often our blood sugar ‘spikes’ when we eat, and increases the impact of its counterweight. So it makes sense that when there are more spikes, insulin is more active and effective, but of course there isn’t evidence for why.
There is consistent evidence for this effect, using the classic 3-hour Oral Glucose Tolerance Test. Adult men who were pre-diabetic participated in a Restricted Time Feeding Trial, and had their insulin levels change by approximately 26 ± 9 mU/l (p=0.01) and 35 ± 13 mU/l (p=0.01) at 60 and 90 minutes. What do those numbers mean, and why do they matter?
Normal Insulin LevelNormal Insulin Level (SI Units*)Fasting< 25 mIU/L< 174 pmol/L30 minutes after glucose30-230 mIU/L208-1597 pmol/L1 hour after glucose18-276 mIU/L125-1917 pmol/L2 hours after glucose16-166 mIU/L111-1153 pmol/L3 hours or more after glucose< 25 mIU/L< 174 pmol/L*SI Units: Conversional units x 6.945
You can use the chart above to see what normal values are. A 26 point change is actually quite significant, easily approximately a 10% change. That’s a pretty significant shift in metabolism.
TLDR, you limit how often your blood sugar spikes, you decrease the amount of insulin release and how often it’s released. Your body then becomes more sensitive to when insulin is released. This is helpful for two reasons. 1) Increased insulin sensitivity fends of diabetes and the host of health complexities that can cause, and 2) reduced circulating insulin levels mean you’re storing energy (fat) less, and burning through your body’s natural stores more.
Circadian Rhythm
Circadian Rhythm refers to the natural cycle of hormones that determine our wakefulness. Often with TRF, people are much more consistent of when they eat, and their digestive hormones are speaking more regularly, and are limited to a specific time. The hormones of digestion and rest/wakefulness are interlocking, and this makes sense; when we eat (or don’t) is essentially determing energy we take in (or don’t). While our circadian rhythm determines if we spend energy, or don’t. They should talke to each other, because they’re deeply related.
And to put it simply
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If you’re fed at a certain time, your body starts to expect food at that time, and will pre-emptively release digestive hormones. That’s why if you usually eat at 12:00, but you decide to skip lunch until 2:00 or 3:00, your body is cranky and your stomach is rumbling.
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If your body expects to be fed at 12:00, but it doesn’t get fed until 3:00, you will be cranky, and your body will adjust your circadian rhythm and hormones, to expect new food at that time. Which is to say, the next few days, you will get hungrier at 3:00 o’clock.
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Because your body is expecting food and mobilizing digestive hormones, it can most easily integrate food and nutrients when digesting at an expected time, and prevent undue stress on the body.
IE, your body needs to know when food is coming so it can better accommodate the rigga-ma-roll of digestion. It reduces the stress on your system, and improves integration and absorption of nutrients.
Insulin is particularly sensitive to this (IE, the hormone that stores sugars), which is another reason why TRF may be so effective in increasing insulin sensitivity; it helps keep insulin on a regular circadian clock.
Part of the reason you might be so sluggish at 2:00 am is not just because of fatigue, but because your body expects to be in a resting and fasted state, with low energy expenditure.
Weight Loss
This is a spicy topic, because there’s some of what the research says, and there’s also what most people experience in the real world.
One common effect when people perform IF in the real world, is that reducing the window of calorie consumption leads to reduced calorie intake. It prevents the midnight snack, the early breakfast, the 2nd dinner. Over time, this reduces the amount of calories averaged out over a year, and therefore can lead to a decrease in weight. This is what many people experience when they pick up TRF, is that they’re benefitting from having a reduced calorie intake.
Now, many of the studies out there attempt to account for this, and insure that even with TRF, there’s similar caloric intake.
What they find is that, even when consuming the same amount of calories, individuals utilizing TRF experience weight loss, of an average of about 5-10% of the bodyweight, per year. This is comparable to the calorie restriction model.
That is to say, choosing when you eat is just as an effective strategy as how many calories you eat. That’s not to mention that not all calories are the same (but that’s a different discussion entirely).
One thing I want to emphasize is that these people experienced weight loss when their feeding window was earlier in the day. IE, people with a feeding window from 8-4 experienced a more consistent weight loss than those from 10-6. If you’ve been paying attention, this might actually make sense; the timing of our daily rhythm should coincide with the timing of our eating, and it seems it’s more efficient to digest food earlier in the day.
So, intermittent fasting is kind of working on a 3 pronged approach here; it’s restricting the number of calories you eat, and it’s lining up when you eat them for a better match with your circadian rhythm, and it’s doing so more consistently.
That all sounds great!
Unless you’re an athlete.
Can you guess why?
Athletic Performance
I don’t typically recommend TRF for my athletes. Often times, we’re worried about actually getting enough calories and promoting adequate recovery. The same things that make TRF helpful for weight loss are the very things that make it a yellow flag for athletes, by reducing the number of calories consumed. And, that increased insulin sensitivity means we’re also in a fasted state more often, which means it is harder to mobilize the necessary energy for a workout. IE, ever worked out on a super empty stomach when you haven’t eaten for six hours? Sucks, right? Because your body has burned through it’s more accessible stores.
There is a study that found TRF had little impact on performance. However, they only used 18 people. That is tiny, and they only did it for 4 weeks. That’s not a full strengthening cycle, so we can’t actually track muscle hypertrophy or muscle adaptations. It’s actually pretty poor study design.
Current recommendations actually suggest avoiding high intensity workouts during a fasted state, because of the impaired energy metabolism and increased risk for central nervous system fatigue. And as an athlete, the consistent decrease in body weight can have negative effects because you might not just be losing fat, but hard earned muscle. When we have low blood sugar (because we haven’t eaten lately), gluconeogensis is the process that mobilizes energy to the blood stream, and it is the number one cause of lean muscle loss in athletes.
There is clear and consistent evidence that Ramadan fasting (Sunrise to sunset) is hard on the body, decreases lean muscle mass, and decreases the performance. The data on true performance factors is more mixed for TRF.
You can ameliorate many of these effects of TRF if you’re an athlete (and still get the bang for your buck) with two easy strategies (easy).
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Workout within your feeding window. Don’t workout fasted. This way your blood sugar is high enough that you can efficiently mobilize what you need to.
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Make sure that you’re eating enough calories
However, honestly, I find that when most people care about performance, are tracking their macros, work a job, track their training schedule– adding in a time restriction is too difficult, and hard on their bodies.
TLDR: Intermittent fasting is helpful for weight loss because of how it syncs our bodies, promotes a regular eating patterns, align it with our circadian/waking rhythm, and it can promote weight loss. In athletes, especially endurance athletes, it can promote lean muscle mass loss because of the reduce fat and glucose stores.
Time Restricted Feeding clearly illustrates that when you eat matters. Transitioning to eating more calories earlier in the day keeps your digestive hormones aligned with your day/night cycle, and eating consistently helps your body work most efficiently. However, you can still get some of the benefits, even if you don’t have the shortest eating window.
And like with all things, the best diet is the one that makes you feel good, that you can stay consistent with. So, take what works for you, and don’t get too stuck on one idea of what’s right or not.
