How to Advise on Time-Restricted Eating and Circadian Eating: Chrononutrition Research

Inside: Time-restricted eating and circadian eating are two major approaches to chrononutrition, the study of how the timing of eating – including its alignment with body circadian rhythms – affects health. Amidst endless new headlines, dietitians need to know how to talk about potential effects on inflammation and metabolic syndrome. Here, we explore what research shows and what factors to check when new studies gain attention.

 

Picture this:

It’s game night, and the excitement is palpable as your teammate hurriedly sketches on a pad, desperately trying to convey some mysterious image. With seconds ticking away, you shout one guess after another, convinced each time that you know what it is – confident in your deduction skills.

These moments of trying to quickly draw conclusions in games – whether it’s Cranium (one of my personal favorites), Pictionary, or Charades – can lead to contagious laughing and fun. But when it comes to prematurely drawing conclusions about eating patterns based on incomplete or inconsistent research studies, it’s not so funny.

 

 

 

This research review explores the impact of two dietary approaches —time-restricted eating (TRE) and eating in alignment with circadian rhythms (sometimes called circadian eating or circadian rhythm fasting) — on chronic inflammation and metabolic syndrome.

Understanding the science behind chrononutrition allows dietitians to help people make informed choices about diet beyond specific food selections. Do these approaches have real potential to reverse the effects of metabolic syndrome and chronic inflammation?

 

What Is Chrononutrition? Time-Restricted Eating vs. Circadian Eating

Chrononutrition is a growing field that investigates how the timing of food consumption and its alignment with the body’s internal clocks — circadian rhythms — can affect health. Chrononutrition recognizes that metabolism and many physiological processes follow a 24-hour cycle, influenced by light exposure and other environmental cues.
Early studies of chrononutrition often focus on whether it offers benefits for weight loss. In more recent years, research is exploring a wider range of potential effects.

The big question: by aligning people’s eating patterns with their circadian rhythm, can they optimize metabolic health?

Here are two popular approaches to chrononutrition:
>> Time-restricted eating focuses on the number of hours each day in which eating can occur and the number of hours spent fasting. Condensing the eating window allows the body more time in a fasting state.
>> Circadian rhythm eating focuses on when fasting and eating occur, aiming to align eating patterns with the body’s natural circadian rhythm. This means consuming most of the day’s calories during daylight hours and fasting during the evening and night.

What is Time-Restricted Eating?

Time-restricted eating is one approach within the broad category of intermittent fasting. Other major types of intermittent fasting assign entire days as either fasting (with a maximum of 500 to 1000 calories a day) or usual eating habits. Some people alternate between fasting and non-fasting days. Others use 5:2 fasting (2 days of fasting per week).

In contrast, time-restricted eating consistently limits the time window in which eating occurs every day.

•  Time-restricted eating (TRE) may limit the time window for eating to anywhere from 4 to 12 hours, though 8 to 12 hours seems to be most common.
• This leaves the rest of the day for fasting. The fasting window should include drinking plenty of water. Drinks without calories, such as black coffee and unsweetened tea can also be included during the 12 to 20 hours designated for fasting.
• Even if someone is aiming for weight loss, time-restricted eating does not include counting calories or limiting any particular foods or food groups.

 

Most of the research on TRE involves its typical effect of reduced calorie consumption as an alternative strategy to promote weight loss. Although you may see media stories suggesting that TRE’s impact on weight involves some sort of change in metabolism, that’s not the conclusion that most research supports. Mounting evidence from observational studies and controlled intervention trials suggests that TRE leads people to spontaneously reduce their usual calorie consumption by anywhere from 350 to 500 calories a day simply by limiting the window in which eating can occur.

• Less to monitor: For people who get frustrated and overwhelmed by the challenge of tracking calories or monitoring food portions, TRE may be simpler to integrate into lifestyle for longer-lasting reduction of calorie consumption.
• Modest weight loss can occur: In short-term intervention studies, TRE tends to produce a 2% to 4% weight loss in four to sixteen weeks for people who have overweight or obesity.
  🔹Clinical significance of this weight loss is unclear. This is less than the five to ten percent weight loss that’s considered clinically meaningful. In other approaches to weight reduction, most loss occurs by six months. An important question is whether studies extending TRE to six months would produce greater weight loss.
  🔹Time for eating in these studies ranged from 4 to 10 hours, with no apparent difference in weight loss.
  🔹The proportion of body fat versus lean tissue lost does not seem to differ from what’s seen in weight loss achieved via other strategies for reducing calorie consumption.
• Questionable as a better way to lose weight: Since TRE tends to reduce calorie consumption, a key question is whether it offers any physiological advantage for weight loss over reducing calorie consumption without time constraints on eating.
  🔹An analysis combining results of multiple randomized controlled trials found that 2 to 4 months of adding TRE to intentional calorie reduction resulted in about 4 pounds more weight loss (with less than 2 pounds greater fat loss).
  🔹While that was statistically significant, if TRE is a burden for someone, is this clinically meaningful?

What is Circadian Eating?

Rhythmic changes in light and darkness each day are the main cue for the “master clock” in the hypothalamus of the brain – called the suprachiasmatic nucleus (SCN). This master clock triggers hormones, neurotransmitters, and gene expression to help regulate metabolic processes and behavior like the sleep-wake cycle.

Circadian rhythms are run by not only this master clock, but also by circadian clocks in tissues throughout the body. Light and darkness are the main cues for the master clock in the brain. Yet eating is the dominant cue for peripheral clocks elsewhere in the body, such as the liver, pancreas, gut, muscle, and adipose tissue.

When the master clock and the peripheral clocks are out of sync with each other, it’s as if the brain is in one time zone and other key body organs in another. And this misalignment has been shown in animal and human studies to reduce the effectiveness of normal body systems that regulate blood pressure, blood sugar, and other metabolic processes.

Circadian rhythms eating – or circadian fasting, depending on how you look at it – involves aligning eating time with daylight and fasting time with night.

 

 

Time-Restricted Eating: Role in Managing Chronic Inflammation and Reversing Metabolic Syndrome?

Time-restricted eating benefits could include help managing chronic inflammation and reversing metabolic syndrome. Chronic inflammation is a key driver of many chronic diseases and it promotes metabolic syndrome, which is characterized by a cluster of conditions such as high blood pressure, elevated triglyceride levels, and insulin resistance.

Obesity is associated with chronic low-grade inflammation, and studies have shown that weight loss tends to reduce biomarkers that measure inflammation. Time-restricted eating (TRE) often leads people to unconsciously reduce calorie consumption and lose weight. This alone could reduce inflammation and the insulin resistance that is a key driver of metabolic syndrome.

Other potential mechanisms, based mainly on animal studies, through which time-restricted eating might reduce inflammation and oxidative stress — and thus also help reverse metabolic syndrome – include potential to:

• Enable the body’s “metabolic switch” – when an extended fast prompts a switch from using glucose mobilized from liver glycogen stores to using fatty acids and ketones for fuel
• Reduce insulin resistance and shift glucose metabolism to less gluconeogenesis and more glycogen synthesis. More research is still needed to understand the apparent changes in multiple organs that lead to increased insulin sensitivity.
• Enhance autophagy, the process during a time of fasting when the body eliminates damaged proteins and cell components, which helps preserve normal cell function and protect against oxidative stress. But does this make a difference over time compared to other ways of shifting calorie balance? That’s not clear.
• Provide the digestive system time to rest and recover. Night is when gut mucus secretion and cell repair ramp up to maintain gut integrity, improving gut wall barrier function. Moreover, some rodent studies show that in TRE, microbiota species change, with a decrease in those that promote obesity and an increase in species that protect against obesity.

Evidence on Time-Restricted Eating and Inflammation

In mice, time-restricted feeding protects against hyperinsulinemia, fat build-up in the liver, and inflammation. But evidence is not consistent in showing such results in humans. And when it does show anti-inflammatory benefits, it’s not clear in whom or when this effect occurs.

One analysis combined outcomes of multiple clinical studies comparing time-restricted eating to usual diet in a general adult population — not restricted to people with obesity, risk factors, or elevated markers of inflammation at baseline.

• Pooled results showed no significant decrease in CRP or IL-6, two common markers of inflammation.
• In the combined analysis, two other markers of inflammation (TNF-alpha and leptin) did decrease, although many of the individual studies showed no change.
• Leptin only decreased in people with overweight or obesity, not in people with a normal BMI.

Another analysis combined results of four human trials that included only people with overweight or obesity. The trials compared people assigned to an eating window that ranged from 4 to 10 hours/day to people in control groups with no restrictions on eating time or calorie consumption. None of the markers of inflammation (CRP, TNF-alpha, or IL-6) showed any change, even with up to a 5% weight loss.

 

Research on this question is still very limited.

• Large RCTs focused specifically on how time-restricted eating affects inflammatory markers are needed.
• Results then need to be analyzed in ways that account for differences in how TRE was performed and in characteristics of people in the studies.

 

Research Findings on Time-Restricted Eating and Metabolic Syndrome

Beyond effects on weight and waist size, time-restricted eating may affect different components of metabolic syndrome – including blood pressure, blood triglycerides, blood sugar – to different degrees.

Blood pressure –

One groundbreaking study involved 19 people with metabolic syndrome whose normal eating pattern occurred over a window of at least 14 hours each day. In this 12-week intervention, they reduced the time in which they ate to a 10-hour window.

• With an unintentional 8% drop in calorie consumption, participants averaged a 3% weight loss (average about 7 pounds).
• Blood pressure dropped about 5 mm Hg/6 mm Hg. This drop is consistent with results from similar weight loss in other studies. It’s unclear whether blood pressure changed due to weight loss or to the shorter eating window itself.

A meta-analysis that combined the results of 10 intervention trials involving adults with metabolic syndrome, overweight, or obesity — but without hypertension — found that systolic blood pressure decreased by about 4 mm Hg with time-restricted eating. In studies that lasted at least 12 weeks, diastolic blood pressure was also reduced.

• Effects on blood pressure varied, which may be due to individual differences. But it’s also likely due to wide variation in the length of the studies and the time-restricted eating protocol.
• Drop in blood pressure was strongly related to amount of weight lost during the intervention, so it’s hard to identify the influence of time-restricted eating itself.
• This association with weight loss is consistent with results of another meta-analysis of studies that compared time-restricted eating to calorie-restricted diets without time restrictions on eating time. In that analysis, blood pressure dropped in both interventions, with no difference based on time-restriction.

Blood triglycerides and dyslipidemia –

In the intervention trial by Wilkinson and colleagues that reduced participants’ eating window from 14 or more hours to 10 hours, triglyceride levels did not decrease. However, non-HDL cholesterol levels dropped significantly. That biomarker includes cholesterol in both LDL and triglyceride-carrying VLDL particles, and it’s strongly related to long-term risk of cardiovascular disease.

Elevated blood triglycerides often occur in people with overweight or obesity, especially with excess fat around the waist. Although response to weight loss varies among individuals, overall, it’s one of the best ways to lower triglycerides. An analysis of studies comparing interventions with time-restricted eating to those with calorie-restriction but no time restriction on eating found no difference in triglyceride reduction.

Blood sugar –

Since time-restricted eating is theorized to reduce insulin resistance, some scientists suggest that it could be helpful to keep blood sugar in a healthy range.

Outcomes from the Wilkinson and colleagues’ intervention trial don’t provide a clear answer.

• Among participants who had elevated fasting blood sugar or A1C level at the start of the study, A1C levels were reduced when eating hours were reduced to a 10-hour window.
• However, a measure of insulin resistance (HOMA-IR) showed no change.

Results of a meta-analysis of seven intervention studies that compared time-restricted eating to calorie-restricted diets that didn’t restrict time of eating found just the opposite.

• Time-restricted eating did not change fasting blood sugar levels or insulin resistance.
• The shorter eating window showed a trend for reduced fasting insulin levels (not statistically significant, so it could occur by chance). However, similar decreases occurred when participants reduced calorie intake without time limitations.
• Participants in the trials on time-restricted eating all had overweight or obesity, but they did not necessarily have metabolic syndrome or prediabetes. The study doesn’t show any separate analysis of blood sugar or insulin effects limited to those with insulin resistance. So, a lack of change in these studies does not mean that these health measures wouldn’t be reduced in those with baseline elevations.

 

Circadian Fasting for Chronic Inflammation and Metabolic Syndrome

Circadian-based eating and fasting focus on aligning eating times with circadian rhythms. By aligning eating with times of optimal levels of hormones like melatonin and greatest sensitivity to effects of insulin, it offers potential to manage metabolic syndrome and reduce chronic inflammation.

• Glucose tolerance and insulin sensitivity are generally greater in the morning than the evening. Why? Incretin hormones, like glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP), that speed up the pancreas response of secreting insulin and help limit blood sugar rise after eating are amplified in the early part of the day.
• Appetite-regulating hormone levels may make it easier to avoid unwanted weight gain when nighttime eating is limited. For example, in one highly controlled trial, people were given identical amounts of the same food. Late eating decreased levels of leptin (a hormone that triggers feelings of satiety) and increased the ratio of the active form of ghrelin (the “hunger hormone”) to leptin, especially during waking hours. Not surprisingly, participants reported more hunger in the late eating conditions. This influence on hunger could have important indirect effects since excess body fat (especially visceral fat deep in the abdomen) promotes inflammation and metabolic syndrome.
• Eating that aligns with circadian rhythms may support a more health-promoting gut microbiome. Eating times that match or don’t match circadian rhythms can change which microbes flourish in the gut, based on rodent and limited human studies. This changes gut microbiota production of healthful short-chain fatty acids and unhealthful compounds that promote free radical production and oxidative stress. Eating aligned with circadian rhythms may also help re-establish normal cycles of cell and hormone signaling in the gut. Limited evidence so far suggests this could lead to changes in gene expression that can either improve insulin sensitivity or increase risk of insulin resistance.

Exploring Research on Circadian Eating and Chronic Inflammation

Observational studies and controlled intervention trials show that relationships of circadian eating and fasting – including when time-restricted eating occurs — with markers of inflammation are not as clear-cut as some sources may suggest.

For example, despite these mechanisms through which circadian rhythm fasting could reduce inflammation, in a small crossover controlled feeding trial of men with prediabetes, even though average level of CRP (a biomarker of inflammation) was elevated at baseline, inflammatory biomarkers did not change.

Does evening eating promote inflammation?

• Each 10% increase in the percentage of daily calorie consumption that occurred after 5:00 pm was associated with a 3% increase in a biomarker of inflammation (CRP) among a large group of U.S. women. – In fact, a longer nighttime fast was only associated with lower CRP among women who consumed less than 30% of their calories after 5:00 pm.
• Clinically meaningful? Although these associations are statistically significant, the full range of CRP levels among women in this study is not provided, and it appears that most are within a normal range. Current research has not shown a difference in health risks related to variations within the normal range of CRP values.

Does delaying morning eating promote inflammation?

• Contrary to expectations about a longer nightly fast as an anti-inflammatory strategy, men and women with the longest nightly fasts were more likely to have elevated levels of CRP. A closer look suggests that this may reflect the association of a later time of a first meal with higher CRP levels.

Do individual circumstances influence relationships between circadian eating and inflammation?

• In a study of colorectal cancer survivors followed for 2 years after treatment, a longer eating window (shorter evening fast) was associated with reduced fatigue and inflammation and with improved sleep quality.
• Studies of people with existing cardiovascular disease are limited. But they suggest the possibility that in the failing heart, eating later in the day may be beneficial by helping to prevent fasting-related stress.

Research Results on Circadian Eating to Reverse Metabolic Syndrome

Improved insulin sensitivity? Maybe.

In a crossover controlled feeding trial, eight men with prediabetes showed improved insulin sensitivity during 5 weeks of early time-restricted feeding (eating in a 6-hour window with dinner before 3:00 pm) compared to when they ate over a 12-hour window. Food was provided, ensuring equal total calories in each condition.

Lower Blood Pressure? Maybe.

A review of clinical trials of time-restricted eating found that studies with eating periods that started and ended earlier in the day showed clinically significant blood-pressure lowering. However, time-restricted eating studies as a whole showed inconsistent effects on blood pressure.

 

Practical Implications: Best Advice on Chrononutrition

Both time-restricted eating and circadian eating offer science-backed ways that they could potentially reduce chronic inflammation and improve metabolic syndrome. However, research still has far to go before it can clarify inconsistencies among studies, including the key elements of chrononutrition strategies that bring desired cardiovascular health and other benefits and the people who are most likely to benefit from these strategies.

What Gets Overlooked about Time-Restricted Eating

Diet quality may not improve: Intervention trials of TRE show that although people reduce calorie consumption, their food and drink choices generally remain the same. Diet quality scores – including consumption of added sugar, sodium, and dietary fiber, nutrients, and protective phytochemicals – do not tend to improve with TRE.

When the eating window occurs may matter: Apparent benefits of aligning eating with circadian rhythms lead to important questions for time-restricted eating. In studies on time-restricted eating, consider how timing of the window for eating may have influenced results or led to inconsistencies among studies… and what that means for individual clients.

 

Questions about How Circadian Eating Fits Individual Needs

Research is still needed to better understand these contradictory circumstances:

• Health conditions: People who have trouble consuming adequate calories and nutrients or who may be at risk from extended overnight fasts may benefit from an evening snack. People with a history of disordered eating may also be at risk from attempting strict eating rules.
• Night work: People who work at night often eat mainly at night. Preliminary studies suggest that they might benefit from redistributing eating to mainly daylight hours despite their work timing. More research is needed.
• Chronotype: People differ in preferences for lifestyle timing. Although this is not really a simple either/or, it’s often characterized as whether people consider themselves “larks” (wake early and perform best early in the day) or “owls” (prefer to stay up late, wake later, and perform best later in the day). Should people who identify as “owls” try to adapt their schedules in contradiction to what feels natural to them?

What Dietitians Can Explain about Chrononutrition

When it comes to implementing chrononutrition principles into daily lives, it’s important to combine conclusions based on the overall picture of best-available research and individual health priorities and lifestyles.

Currently, among U.S. adults:

• 35% have an eating window of 13 hours or more each day.
• 59% consume calories after 9:00 pm
• The greatest proportion of daily calories are consumed at dinner.

In short, time-restricted eating and circadian-based eating and fasting both pose significant change for most people.

 

Time-restricted eating and circadian rhythm fasting are not designed for the purpose of reducing calorie consumption, although for many people they do. Research does not yet show whether these approaches to eating offer greatest benefit for people who have overweight or obesity that is likely contributing to chronic inflammation or metabolic syndrome – or putting them at risk of these conditions. It’s possible that people who have “metabolic obesity with normal weight” – and others — might benefit if there are no contraindications.

👉 Time-restricted eating shows potential as one option that’s effective to limit calorie consumption and lose weight… though not necessarily any more effective than other ways.

👉 For people who don’t have health conditions or medical treatments that make an overnight fast of 12-14 hours a risk, it’s possible that it could lower elevated blood pressure and improve insulin resistance-related risk factors.

👉 Timing may matter. The same reduction in the number of hours in which eating occurs may have more benefits on overall health if the window starts in the morning and includes less night and evening eating.

 

 

Evidence is not strong or consistent enough to make time-restricted eating or circadian eating a key recommendation….

But it’s strong enough to help people considering whether it’s an option they want to try out.

 

 

1) Talk with people about nutrient-dense food and drink choices that will work for them in a revised eating schedule that they’re considering.

• Remind them that timing won’t outweigh the effects of diet quality.
• Do they need fresh ideas about food choices that keep hunger satiated or that can be prepared quickly?
• How will they ensure that physical activity doesn’t get omitted with a lifestyle shift?

2) Talk about implications for quality of life with time-restricted eating or circadian eating.

• Some people find it easier to stick with a shorter time window for eating than with limiting portion sizes or other ways of reducing excess calories.
• Short windows for eating may more easily fit into many people’s lives with a pattern that skips breakfast, has a first meal around noon, and includes an evening dinner. – But a later eating pattern may be associated with greater hunger throughout the day. And some studies show that blood sugar response to food is better controlled in the morning than later in the day.
• Acknowledge the value of food as a source of connection with other people and of pleasure. Fit with preferred lifestyle is important.

3) Discuss small steps that might be feasible. Research has not shown a consistent minimum threshold for how much someone needs to shorten the timeframe of eating for a potential benefit.

• For individuals whose eating pattern is similar to the average American statistics, it might be helpful to gradually reduce the eating window by one or two hours, stopping to evaluate effects and lifestyle fit at each point before making further changes.
• Plan when you’ll check results and decide whether to make further changes or focus on how to make an intended change relatively consistent.
• Emphasize that until further research shows otherwise, there’s room to experiment with different approaches to chrononutrition to find what works best for each individual’s health, lifestyle, and values.