If you’re looking for ways to lose weight, it’s likely you’ve come across diet plans that recommend fasting. Though fasting diets can vary widely in the amount and length of fasts they recommend, like six- or ten-hour intermittent fasting, or alternate-day fasting, they all have a similar goal: Get your body to start burning stored energy. That stored energy is typically fat, and the more you burn, the more weight you lose.

(Before going further, we acknowledge that mainstream nutrition experts generally disapprove of fasting in any form. But we daresay that’s largely because no one makes a nickel from encouraging people to fast, including the food manufacturers who often sponsor mainstream nutrition information sources. The science behind the benefit of at least a modest program of intermittent fasting is becoming quite clear.)

But not every fasting diet plan comes with good instructions. While there’s evidence that fasting done right can lead to sustained weight loss, some people find it challenging to stick to the eating plan, while others might fail to see results.

As it turns out, a big part of both these issues may stem from a common mistake which, fortunately, is also one of the easiest to remedy. It comes down to what you’re eating, and there’s a simple takeaway: Go easy on carbohydrates.

You can think of carbs as easy energy. They’re what our bodies turn to first for fuel. When you’re consuming a lot of carbohydrates, your body never needs to turn to its backup energy stores; that is, fat. The result is that your fat stores never get touched, even if you’re exercising vigorously and often.

And in some cases, eating a lot of carbohydrates can drive you to eat even more than you actually need. This results from the dreaded “sugar crash” that filling up on easy energy can lead to, and the typical outcome is a gnawing hunger that leads to more eating. But with some simple tips, like tweaking the mix of carbohydrates, fats and protein you’re consuming, you can avoid crashing and burning when fasting.

Carbohydrates in the body

Chart of the 3 main macronutrients with food examples
It's a rare whole food that contains only one of these macronutrients. Most contain all three to varying degrees. The whole foods that are mostly made of protein and fat are the ones most likely to help curb hunger and promote successful intermittent fasting.

When our bodies need energy, they can get it from three main sources, or macronutrients: carbohydrates, proteins and fat. Carbohydrates, and especially simple carbohydrates like sugar, are quick and dirty because they’re quickly broken down in the digestive system and pumped into the bloodstream for use. Fats and proteins take longer to deliver power to our cells, but they’re more long-lasting energy sources. Because our bodies have different energy needs at different times, they rely on these sources interchangeably.

Inside our digestive systems, carbohydrates get turned into glucose, a type of sugar that our cells can use for energy (Holesh et al., 2021). We typically have some glucose circulating in our blood, ready to be used by cells when they need a boost. But our cells can’t just scoop that glucose up and use it by themselves — they need the hormone insulin to bring it through their cell walls first. Glucose that isn’t used by cells gets turned into glycogen, which is stored in our muscles and livers for later use. Once glycogen storage reaches the body’s capacity, it is stored as fat.

Modern diets tend to contain far more carbohydrates than anyone needs. Our relatively sedentary lifestyles mean we don’t need huge amounts of this easy energy as we go about our days. In fact, metabolically, carbohydrate is the only nonessential macronutrient. One cannot survive on a diet that contains no protein or fat, but one can live indefinitely on a no-carb diet, as hunting-based societies throughout human history often did.

When we eat a lot of carbohydrates, our bodies respond by releasing a surge of insulin from the pancreas (Holesh et al., 2021). Together, the resulting glucose and insulin deliver energy to our cells, and we often feel that as a boost of vigor after a carb-heavy meal. But just as quickly as it comes, that boost can disappear as our insulin spikes and our bodies clear out the glucose. Because most carbohydrates are quickly converted in the body to sugars, the term “sugar crash” is appropriate here. The scientific term is “postprandial hypoglycemia,” meaning a sudden drop in blood sugar levels following a meal.

Call it what you will, low blood sugar is not fun. Symptoms include low energy levels, shakiness, anxiety, sweating, irritability, confusion, mental fog and, most acute of all, powerful hunger.

How fasting works

Fasting diets rely on restricting your daily eating to a narrow “window,” which can range from four to 12 hours, depending on the diet (Cho et al., 2019). Other fasting diets involve what’s called alternate-day fasting, which consist of one, or several, days of normal eating followed by a day or more of restricted eating.

Clock showing intermittent fasting principle
Intermittent fasting takes many forms. A common, simple one is to fast for 16 hours – say, from 8 pm to noon the next day – and eat within the noon to 8 pm “window.” 

Recent studies comparing participants who fasted to those who didn’t showed increased weight loss as a result of fasting, though not every fasting protocol seems to deliver the same results. Scientists say more study is needed to better understand the best way to fast for weight loss.

(Read more: Take Big Breaks From Food?)

So-called “keto” diets use periods of fasting, as well as a very low-carb diet, to nudge your body into a state known as ketosis. During ketosis, our bodies react to a lack of dietary energy by drawing on stored triglycerides (fats) for energy. It can be difficult to get to true ketosis, and attempting to can lead to unpleasant side effects like headaches and low energy levels, especially at first. Some researchers also suggest that long-term keto diets can lead to nutritional deficiencies and might be difficult to keep up with (Giroux, 2019). But other studies show that keto diets done properly are safe and do lead to weight loss (Bueno et al., 2013).

Fasting diets, including variations like the keto diet, rely on reducing dietary energy intake to convince our bodies to use the energy they already have stored (Izumida et al., 2013). When our bodies get low on glucose, or its stored form, glycogen, they begin burning fat. The goal is to use up stored fats and ultimately shed pounds.

For any diet that relies on restricting dietary energy in some way (which is most of them), high amounts of carbohydrates will likely make it harder to see results. That’s even more relevant for fasting and ketosis diets. Not only does the excess energy mean you’re unlikely to burn stored fat, the effects of carbohydrates on your hunger and energy levels make it doubly hard to stick to a diet plan.

The Bottom Line

To even out your appetite and make it easier to follow a diet, try easing up on carbohydrate-heavy meals and include more fats and protein in your diet — we’d recommend healthy options. Wild Alaskan Sockeye Salmon, of course, but branch out and explore Wild Salmon Jerky Strips or Pacific Spot Prawns.

Contrary to the popular assumption, many fats can actually help us as we diet. As an alternate, longer-lasting source of energy, fats don’t lead to the quick glucose binge and subsequent blood sugar crash that carbohydrates do. That means you’ll feel full for longer, and find it much easier to resist going back for a midnight snack. (Read more: When Is the Best Time to Eat?). Protein, which is typically used to build muscle but can also be a source of energy, also helps you feel full for longer.

Regardless of whether you’re trying to lose weight or not, no one wants to get hit with a carbohydrate-fueled sugar crash after a meal. Load up on more durable sources of energy to keep yourself going, and satiated, all day long – including, if you choose, a few hours of fasting.

 

Citations:

Altuntaş Y. Postprandial Reactive Hypoglycemia. Sisli Etfal Hastan Tip Bul. 2019;53(3):215-220.  doi:10.14744/SEMB.2019.59455

Bueno NB, de Melo IS, de Oliveira SL, da Rocha Ataide T. Very-low-carbohydrate ketogenic diet v. low-fat diet for long-term weight loss: a meta-analysis of randomised controlled trials. Br J Nutr. 2013 Oct;110(7):1178-87. doi: 10.1017/S0007114513000548. Epub 2013 May 7. PMID: 23651522

Cho Y, Hong N, Kim KW, Cho SJ, Lee M, Lee YH, Lee YH, Kang ES, Cha BS, Lee BW. The Effectiveness of Intermittent Fasting to Reduce Body Mass Index and Glucose Metabolism: A Systematic Review and Meta-Analysis. J Clin Med. 2019 Oct 9;8(10):1645. doi: 10.3390/jcm8101645. PMID: 31601019; PMCID: PMC6832593.

Giroux, NF. The Keto Diet and Long-Term Weight Loss: Is it a Safe Option? 2020. Inquiries Journal, 12(10). http://www.inquiriesjournal.com/a?id=1807

Holesh JE, Aslam S, Martin A. Physiology, Carbohydrates. [Updated 2021 Jul 26]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2021 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK459280/

Izumida Y, Yahagi N, Takeuchi Y et al. Glycogen shortage during fasting triggers liver–brain–adipose neurocircuitry to facilitate fat utilization. Nat Commun 4, 2316 (2013). https://doi.org/10.1038/ncomms3316