Intermittent Fasting: Which is More Effective for Weight Loss—Morning or Evening Fasting?
If you’re practicing intermittent fasting but not seeing results on the scale, the timing of your fasting periods might be the issue. Recent studies suggest that choosing the right time to fast can make a significant difference.
The benefits of intermittent fasting
Intermittent fasting offers similar benefits to calorie restriction but is often easier to sustain. According to a 2019 article in The New England Journal of Medicine, intermittent fasting can be a key component of a healthy lifestyle due to its numerous health benefits.
What the Studies Say
The latest scientific review, led by Mark Mattson who has studied fasting for 25 years, builds on previous findings about the health benefits of intermittent fasting.
A series of studies on both animals and humans shows that intermittent fasting promotes cellular health by likely triggering an adaptation to periods of food scarcity, known as metabolic "switching." This process occurs when cells deplete their readily accessible sugar-based fuel and start converting fat into energy in the form of ketone bodies, a slower metabolic process. This switch improves blood sugar regulation, enhances resistance to oxidative stress, and reduces inflammation. Ketone bodies serve as an energy source during fasting and have important effects on metabolism.
Intermittent fasting also benefits cardiovascular health by improving various risk factors: it lowers blood pressure, reduces blood lipid levels, and decreases resting heart rate.
There is evidence to suggest that intermittent fasting can positively affect risk factors associated with obesity and diabetes. For example, a study involving 100 overweight women found that those following the 5:2 fasting protocol lost as much weight as those on a low-calorie diet but showed better improvements in insulin sensitivity and reduced belly fat.
Additionally, intermittent fasting may have anti-cancer properties. In animal studies, it reduces the spontaneous occurrence of tumor cells and slows the growth of existing tumors while increasing sensitivity to treatments. Early clinical trials in cancer patients indicate that calorie restriction is well-tolerated and does not cause significant side effects. Some case studies suggest that intermittent fasting may suppress tumor growth and extend survival in glioblastoma patients.
Intermittent fasting can also enhance physical performance. In studies, mice that fasted every other day had better running endurance than those with unlimited food access. Young men who fasted 16 hours a day were able to lose fat while maintaining muscle mass.
More recently, research has shown that intermittent fasting may benefit brain health. A multicenter clinical trial involving 220 adults who practiced calorie restriction for two years found improvements in memory based on cognitive tests. Preclinical studies also suggest that fasting every other day might delay the onset and progression of Alzheimer's disease in animal models by increasing resistance to neuronal stress. However, more clinical trials are needed in patients at risk of or with Alzheimer’s disease.
Finally, fasting may improve surgical outcomes by reducing tissue damage and inflammation. Two weeks of preoperative calorie restriction have been shown to improve outcomes in patients undergoing gastric surgery.
How to Practice Intermittent Fasting?
Multiple studies have highlighted the benefits of adjusting meal times for weight loss, especially by extending nighttime fasting and eating earlier in the evening and later in the morning.
Intermittent fasting focuses more on the timing of calorie intake rather than the composition of meals. This means that the timing of your meals can significantly impact your health and weight, alongside the actual content of your diet. Our circadian rhythm—our body's 24-hour cycle of wakefulness and sleep—affects how we metabolize nutrients at different times of the day. This rhythm influences hormone production, which plays a crucial role in fat storage and release, whether it's related to carbohydrate metabolism (insulin), stress (cortisol), or sex hormones (testosterone, oestrogen).
Considering the effects of the circadian cycle on nutritional metabolism is especially relevant for those fasting to lose weight.
If breakfast seems beneficial for intermittent fasting, those who prefer to skip it should consider the following meal distribution:
Lunch between noon and 1 p.m.
A small snack around 4 p.m.
Dinner between 7 p.m. and 8 p.m.
Skipping Breakfast or Dinner?
A study published in PLOS ONE compared the effects of two intermittent fasting schedules on six healthy individuals aged 51 to 63 (four men and two postmenopausal women), focusing on lipid and carbohydrate oxidation (their use as energy sources) and storage.
Participants followed each program with the same calorie intake, differing only in meal timing, with a break of just over two days between sessions:
Program 1: Breakfast Meal Session
Breakfast (700 kcal) between 8 a.m. and 9 a.m.
Lunch between 12:30 p.m. and 1:30 p.m.
Dinner between 5:45 p.m. and 6:45 p.m.
Program 2: Evening Snack Session
Lunch between 12:30 p.m. and 1:30 p.m.
Dinner between 5:45 p.m. and 6:45 p.m.
Snack (700 kcal) between 10 p.m. and 11 p.m.
Results: Despite similar energy intake and expenditure, there were differences in carbohydrate and fat metabolism between the two programs. The evening snack session favored carbohydrate oxidation over lipid oxidation, resulting in an average of 15 grams less fat burned compared to the breakfast meal session. This pattern suggests that evening snacking maintains liver glycogen stores, reducing the need for lipid oxidation during overnight fasting. Over time, this could lead to greater fat storage and weight gain, despite the 14-hour fasting window.
What Can You Drink During Intermittent Fasting?
For short fasts, it's best to consume only water. Opinions vary on tea and coffee. Some advise against them due to their stimulant compounds, which can overwork the liver. Others believe these beverages make fasting more bearable. The key is to avoid consuming calories, so if you choose to drink tea or coffee, do so without adding sugar or milk. Fruit juices should also be avoided.
What to Eat During Feeding Periods
Feeding periods should not compensate for fasting periods. Fasting will be less effective if it's used to offset overeating. Aim to maintain a balanced diet rich in fibre, healthy fats, and proteins, while minimizing processed foods, added sugars, refined carbohydrates, and ultra-processed products, which are typically high in empty calories. Eat to satiety but do not overeat.
Is Intermittent Fasting a Diet?
Intermittent fasting should not be viewed strictly as a diet aimed at weight loss, but rather as a healthy lifestyle practice, much like regular physical activity. Since it can be seamlessly integrated into your routine several times a week over long periods, it can become a sustainable habit.
What Kind of Weight Loss Can One Expect with Intermittent Fasting, and Why Doesn’t Intermittent Fasting Work for Everyone?
The effectiveness of intermittent fasting can vary significantly between individuals. One major challenge is the temptation to overeat during the "allowed" eating periods and to reduce physical activity, both of which can hinder weight loss efforts.
Intermittent fasting involves alternating periods of eating with periods of fasting. This approach allows for weight loss without meticulously counting calories, provided certain guidelines are followed.
One popular method is the "5:2 diet," where you reduce calorie intake to about 25% of usual levels for two days a week while eating normally on the other five days. However, some people still struggle to lose weight with this method. Why?
What the Study Shows
An English study published in the journal *Appetite* involved 14 healthy men. The objective was to observe how participants adjusted their diet and physical activity before and after a day of calorie restriction.
The study consisted of two sets of experiments, each lasting three days:
Group Experiment: On the first day, participants were informed that they would need to reduce their calorie intake to about 700 calories the next day. On the day of calorie restriction, they followed the plan. On the third day, they had access to an all-you-can-eat breakfast buffet. Researchers recorded the participants' food intake before and after the calorie restriction, as well as their physical activity and appetite.
Control Experiment: Participants were asked to consume 2,800 calories per day over three days. Similar measurements of diet, activity, and appetite were taken.
Results: When participants anticipated calorie restriction, they consumed 6% more calories the day before and increased their breakfast intake by 14% the day after fasting. Interestingly, their hunger levels did not increase compared to the control experiment; they ate more in anticipation or compensation for the fasting rather than in response to hunger.
Moreover, physical activity decreased by 11% the day before fasting and by 18% on the fasting day. Even low-intensity activities, such as washing dishes, were reduced.
This study suggests that during intermittent fasting, there is a risk of "cheating" by eating more before and after fasting and reducing physical activity, which can negate the benefits of fasting and hinder weight loss. A previous study had also shown that people who skipped breakfast tended to compensate by eating more later and being less active, thus diminishing the benefits of skipping a meal.
David Clayton, one of the study's authors, concludes: "Intermittent fasting is not a miracle diet, but some people can benefit from its flexibility. With a few minor adjustments, it could be even more effective."
In Practice
This study highlights the psychological mechanisms that can undermine the effectiveness of intermittent fasting. For any diet to work, you need to burn more calories than you consume. In the context of intermittent fasting, this means that your eating periods should not compensate for your fasting periods; otherwise, there will be no net reduction in calorie intake. Likewise, reducing physical activity will result in burning fewer calories, which can negate the benefits of fasting.
There are several easy ways to practice intermittent fasting:
Following the 5:2 diet, where you reduce calorie intake to 25% of usual levels for two days a week and eat normally on the other five days.
Skipping a meal, such as breakfast or dinner.
Fasting for 24 hours once a week.
In general, incorporating exercise into your routine is highly encouraged during intermittent fasting, especially if your goal is to lose weight. Regular physical activity can help maximize the benefits of fasting by increasing the number of calories burned.
[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