Intermittent fasting: is it better to fast in the evening or in the morning in order to lose weight?
If you practice intermittent fasting without seeing any benefits on the scale, it may be because you're not skipping the right meal. This is what recent studies suggest.
The benefits of intermittent fasting
Intermittent fasting has similar effects to calorie restriction, but it is easier to follow in the long run. Its health benefits are numerous and a 2019 article in The New England Journal of Medicine explains why intermittent fasting can be a key component of a healthy lifestyle thanks to its beneficial effects.
What the study says
The results of this recent scientific review (whose author Mark Mattson has been studying fasting for 25 years) add to those previously obtained on the health benefits of intermittent fasting.
A series of studies conducted on animals, but also on humans shows that intermittent fasting promotes cellular health, probably by triggering an adaptation to periods of food scarcity, a kind of metabolic "switching". This phenomenon occurs when cells deplete their reserves of quickly accessible sugar-based fuel and begin to convert fat into energy (in the form of ketone bodies) in accordance with a slower metabolic process. This change improves blood sugar regulation, resistance to oxidative stress and decreases inflammation. Ketone bodies are not just a source of energy during fasting, but they also have important effects on metabolism.
Intermittent fasting is also beneficial for cardiovascular health, as it helps improve cardiovascular risk factors: it reduces blood pressure, blood lipid levels and resting heart rate.
There is also evidence to suggest that intermittent fasting may alter risk factors associated with obesity and diabetes. A study of 100 overweight women showed that those who fasted with the 5:2 method, lost the same weight as women who switched to a low-calorie diet, but they performed better for improving insulin sensitivity and reducing belly fat.
Intermittent fasting also plays a beneficial role against cancer: in animals, it reduces the spontaneous occurrence of tumor cells and slows the growth of established tumors while increasing sensitivity to treatments. A few clinical trials in cancer patients have been conducted or are ongoing. The first trials show that in men with prostate cancer, calorie restriction is well tolerated and does not cause any side effects. A few case studies of people with glioblastoma suggest that intermittent fasting may suppress tumor growth and prolong survival.
Intermittent fasting could also improve physical performance: in mice of the same weight, those who fast every other day have better endurance in running than those who have unlimited access to food. Fasting 16 hours a day has also allowed young men to lose fat while maintaining muscle mass.
More recently, studies have found that intermittent fasting may have beneficial effects on brain health. A multicenter clinical trial of 220 adults who were calorie-restricted for two years showed that participants achieved an improvement in their memory, as assessed by several cognitive tests. Other studies have reached the same results. Preclinical studies also suggest that fasting every other day may delay the onset and progression of the disease process of Alzheimer's disease in animal models. Fasting increases resistance to neuronal stress. However, clinical trials on patients with or at risk are lacking.
Finally, fasting can be beneficial before undergoing surgery: it reduces tissue damage and inflammation and improves surgical outcomes. Two weeks of preoperative calorie restriction improves outcomes in patients undergoing gastric surgery.
How to do intermittent fasting?
Several studies have already shown the beneficial effects that changing meal times can have on weight loss, in particular by increasing the duration of nighttime fasting and eating a little earlier in the evening and a little later in the morning.
Intermittent fasting is not so much a question of meal composition, but rather of calorie distribution.
What I mean by that is that the content of the plate is not the only food parameter to have an impact on health and weight, nor is the amount of food ingested: meal times also matter. Indeed, our circadian rhythm (regulation of the body over a 24-hour cycle by wakefulness and sleep) implies that certain types of nutrients are metabolized differently during the daily cycle. This circadian rhythm influences the production of hormones. However, the latter plays a significant role in the storage and destocking of fats, whether they are related to carbohydrate metabolism (insulin), stress (cortisol) or biological sex hormones (testosterone, estrogen).
Taking into account the effects of the circadian cycle on nutritional metabolism in the case of fasting seems to be relevant especially if fasting is undertaken to lose weight.
If eating breakfast seems the most beneficial in the case of intermittent fasting, for those who prefer to skip it, it is better to distribute your meals of the day as follows:
Lunch between noon and 1 p.m.
Small snack in the afternoon around 4 p.m.
Dinner between 7 p.m. and 8 p.m.
Skip breakfast or skip dinner?
In a study published in PLOS ONE[1], researchers compared the effects of two forms of intermittent fasting on 6 healthy people aged 51 to 63 (4 men and 2 postmenopausal women), in particular on the oxidation of lipids and carbohydrates (their use as a source of energy), as well as their storage.
The diet was the same (same number of calories), the only difference being the time of food intake. Participants tested each program, with just over 2 days break between the two sessions:
Program 1: Breakfast meal session
Breakfast (700 kcal) between 8am and 9am
Lunch 12:30 and 13:30
Dinner between 17:45 and 18:45
Program 2: Evening snack session
Lunch between 12:30 and 13:30
Dinner between 17:45 and 18:45
Snacks (700 kcal) between 10 pm and 11 pm
Results: Despite similar energy intake and expenditure, the researchers recorded differences in carbohydrate and fat metabolism between the two programs. Taking a snack in the evening thus favored the oxidation of carbohydrates to the detriment of lipids with an average of 15 g less fat burned compared to program 1.
Indeed, taking a snack in the evening would maintain glycogen stores (a form of carbohydrate storage) in the liver causing a reduction in the need for lipid oxidation during overnight fasting.
In the long term, such a meal pattern could explain greater fat storage and therefore weight gain, despite the expected benefits of the 14-hour fasting window.
Coffee, tea, herbal tea… can we drink during intermittent fasting?
For short fasts, it is possible to consume only water. As for tea and coffee, there are two beliefs. One advises against them because they contain a stimulant compound that overworks the liver. The other belief is that those beverages make fasting more bearable. The main thing is to avoid consuming calories, so if you decide to drink tea or coffee, you need to avoid adding sugar and milk, and avoid fruit juices.
Intermittent fasting: what to eat during feeding periods?
Normal feeding periods should not compensate for fasting periods. Similarly, fasting will be less effective if it only serves to compensate for excess. It is therefore advisable to maintain a balanced diet rich in fibre foods, healthy fats and proteins with minimally processed foods, added sugars, refined carbohydrates, and ultra-processed products which are generally full of empty calories.
Is intermittent fasting a diet?
Fasting should not be considered a diet strictly aimed at weight loss but rather a healthy lifestyle practice in the same way as a sports activity. This is because fasting can easily be part of your routine several times a week over long periods, then it will become a habit.
What kind of weight loss can one expect with intermittent fasting? Why doesn't intermittent fasting work for everyone?
During intermittent fasting, the temptation to eat more in the "allowed" periods and to reduce physical activity, is strong which greatly influences the ability to lose weight.
Intermittent fasting consists of alternating periods of eating with periods of food intake. This way of eating allows you to lose weight without counting the calorie composition of meals, provided you respect a few rules.
One way to follow intermittent fasting is to adopt the "5:2 diet": reduce calories two days a week, for example by consuming 25% of the usual intake on those days, and eat normally on the other five days. But some people fail to lose weight with this diet. Why?
What the study shows?
14 healthy men participated in an English study published in the journal Appetite[2]. The objective was to find out how people who had to reduce their caloric intake over a day reacted the days before and after fasting, regarding their diet and physical activity.
The study consisted of two sets of experiments, each three days long:
The group experiment: On the first day, the researchers explained to the participants that the next day they should reduce their calorie intake to about 700 calories. The next day, the volunteers followed the calorie restriction as planned and the next day they had access to an all-you-can-eat breakfast buffet. The researchers recorded what the men ate before and after the calorie restriction. Physical activity and appetite were also evaluated,
The control experiment: participants were asked to eat 2,800 calories a day over three days. The same readings on diet, activity, and appetite were conducted.
Results: When they knew there would be calorie restriction the next day, participants ate 6% more calories the day before. The day after fasting, they increased their intake of breakfast by 14%. Yet, compared to the control experiment, the participants were no hungrier. They anticipated or compensated for their fast rather than responding to their hunger.
In addition, participants decreased their physical activity by 11% the day before fasting and by 18% on that day. Even low-intensity everyday activities, such as washing dishes, were reduced!
This study suggests that, during intermittent fasting, there is a risk of "cheating", eating more before and after fasting, and reducing physical activity, which could cancel out the benefits of fasting and decrease the chances of weight loss. A previous study[3] had also shown that people who skipped breakfast could "catch up" on other meals and tended to be less active: in this case, too, the benefits associated with cutting a meal could be lost...
David Clayton, one of the authors of the study, concludes: "Intermittent fasting is not a miracle diet, but some people can benefit from its flexibility, and with a few minor adjustments, it could be even more effective."
In practice
This study warns about the psychological mechanisms at work that may destroy the efforts made during intermittent fasting. For a diet to work, you often have to burn more calories than you take in. In the context of intermittent fasting, this means that periods of normal eating should not compensate for periods of fasting, otherwise there will be no decrease in intake. Similarly, physical activity should not be reduced, otherwise fewer calories will be burned.
There are several easy ways to practice intermittent fasting:
by following the 5:2 diet as explained above,
skipping a meal, for example, ie breakfast or evening dinner,
fasting 24 hours over the course of a week.
Generally speaking, exercise should be encouraged during intermittent fasting if the goal is to lose weight.
[1] Kelly KP, McGuinness OP, Buchowski M, Hughey JJ, Chen H, Powers J, et al. (2020) Eating breakfast and avoiding late-evening snacking sustains lipid oxidation. PLoS Biol 18(2): e3000622. https://doi.org/10.1371/journal.pbio.3000622
[2]James R, James LJ, Clayton DJ. Anticipation of 24 h severe energy restriction increases energy intake and reduces physical activity energy expenditure in the prior 24 h, in healthy males. Appetite. 2020 Sep 1;152:104719. doi: 10.1016/j.appet.2020.104719. Epub 2020 Apr 26. PMID: 32348797, https://pubmed.ncbi.nlm.nih.gov/32348797
[3] James A Betts, Judith D Richardson, Enhad A Chowdhury, Geoffrey D Holman, Kostas Tsintzas, Dylan Thompson, The causal role of breakfast in energy balance and health: a randomized controlled trial in lean adults, The American Journal of Clinical Nutrition, Volume 100, Issue 2, August 2014, Pages 539–547, https://doi.org/10.3945/ajcn.114.083402
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