How to lose belly fat in 7 weeks

Groundbreaking research has found that combining time-restricted eating with HIIT workouts can have dramatic results for your waistline. Peta Bee explains

January 2 2023, The Times



Professor John Hawley has spent four decades observing how people try and fail — often repeatedly — to lose weight. A researcher and director of the Centre for Exercise and Nutrition at the Australian Catholic University who has published more than 300 scientific papers on diet and exercise, Hawley says he frequently comes across people who are disillusioned having tried a dozen or more diets, “only to find themselves back where they started”.

His mission has been to find a method that works — a plan that is almost effortlessly simple but that brings results in terms of inches lost and health gained. It is the elusive golden carrot of the diet industry, but in findings he describes as “the most promising” of his career, Hawley has moved closer to realising it.

In new research he has discovered that a potent combo of two on-trend powerhouses of the wellness world — time-restricted eating (TRE) and high-intensity interval training (HIIT) — is better at blasting fat than any approach he has come across previously. Adhere to the rules and, in as little as seven weeks, you will shift weight, lose body fat and belly fat, and shed inches from your waistline faster than with many diet regimens — and at double the rate you would by employing either of these approaches alone.

Independently, both TRE (which involves consuming all of your daily calories within a predefined eating window) and HIIT (which involves brief, high-effort activity interspersed with set recovery breathers) have been shown in studies to reduce internal visceral fat, high levels of which are associated with insulin resistance and type 2 diabetes, as well as to lower blood pressure, boost mood and enhance sleep. They have each become success stories in their own right: research shows that, in terms of weight loss and health gains, the time-deprived and exercise-reluctant can get as good a return from doing the short, intense HIIT workouts as from lengthy cardio sessions, while TRE is a more stress-free route to shedding pounds than more fashionable but less sustainable diets.

What intrigued Hawley is whether the two are more powerful when performed together. Working with Trine Moholdt, a research scientist and head of the exercise, cardiometabolic health and reproduction research group at the Norwegian University of Science and Technology in Trondheim, and her colleagues, he set about investigating the synergistic effects of the combo with a group of 131 overweight women in their twenties and thirties. “I didn’t invent either TRE or HIIT, but we were the first to put them together in a research study,” Hawley says. The overriding goal was to provide a plan that would be “as easy as humanly possible” to maintain. “Some of our subjects had previously done 9 to 13 different diets [and] they didn’t work,” Hawley says. “We wanted to find something that did.”

For the study the women were purposely given as much free rein as possible. Over seven weeks they were asked to follow one of four different interventions — one group did TRE, consuming all of their calories within a ten-hour window, another group participated in three supervised and lab-based HIIT sessions per week, a third group did a TRE and HIIT combo, and the fourth group of women acted as the control group. With those trying TRE there were no rules about what, or how, they should eat, no carb cutting or calorie counting, no messing with macronutrients to increase or decrease fat or protein consumption, no shakes, shots or supplements prescribed, making it the antithesis of strict and restrictive diets. “We didn’t suggest any changes concerning food choices or amounts of foods people should consume,” Moholdt says. “No foods were banned or encouraged, so the choice of what to eat was made entirely by the participants in the trial.”

Their results, published in the journal Cell Metabolism, were a surprise even to the research team. So effective was the TRE-HIIT combo that it led to a twofold greater reduction in body fat and the risky visceral fat stored around the organs compared with TRE or HIIT performed in isolation. Those who followed it also had better long-term blood sugar control compared to those in the control group and increased their cardio fitness. Encouragingly, people had no problem sticking to the rules. “Adherence was incredibly high,” Hawley says. “In the TRE-only and TRE-HIIT groups participants adhered to a ten-hour eating window for an average 6.2 and 6.1 days a week respectively.” Even when they relaxed the window rules for one day during each week, usually at weekends, the pounds fell off.

Why it works is intriguing. By its nature TRE tends to lead to fewer calories consumed inadvertently each day just because you can’t fit them in. When you are restricted to eating within a defined window, that late-night chocolate, bag of crisps or glass of wine — what Hawley refers to as the dietary “biggies” — are automatically off the agenda. It is, he says, these sorts of “subtle changes and omissions that can make a big difference over time”. In the study participants who practised TRE reduced their average daily energy intake by approximately 10 per cent without even thinking about calories. But it’s about more than that.

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While it is often confused with intermittent fasting (IF), which advocates periods — days or hours — of strict calorie restriction, TRE, says Hawley, is “definitely not the same thing”. Where TRE wins is in playing to our natural circadian rhythm, or body clock, which regulates everything from metabolism to gut health. “The regular pattern of eating with TRE helps to get our biological circadian rhythms back in sync,” Hawley says. “Intermittent fasting doesn’t do that and, in fact, can disrupt circadian rhythm.” Some researchers have shown that, when that happens, hunger and satiety hormones are thrown into disarray, not knowing when to expect the next meal. It can lead to snacking and eventually to an overconsumption of daily calories that leads to weight gain.

With HIIT added to the mix, the metabolic and weight-loss outcomes are unmatched by existing diets. It’s not even that the HIIT workouts were colossal calorie-burners — each participant expended only an average total of 3,350 calories a week in completing them. But, as Moholdt says, the intense activity sessions led to “whole-body adaptations such as the remodelling of the cardiovascular system and skeletal muscle tissue” that are unlikely to be seen with TRE alone. “Exercise training imparts greater whole-body and tissue-specific metabolic health benefits than any current diet intervention,” she says. Combined, though, TRE and HIIT proved more potent than the sum of their parts. In particular, says Hawley, levels of HbA1c, a clinical measure for blood sugar, improved only with the dual approach. “That was surprising,” he says. “We thought there would be positive changes in HbA1c with TRE and HIIT on their own, which proves there are synergistic effects when they are done together.”

Ultimately the combo works for weight loss because it is as effortless a strategy as you are going to get if you want results. “The real appeal for people is that TRE is a less tedious method for losing weight than daily calorie counting,” Moholdt says. “And HIIT is tolerable, time-efficient and safe for even previously sedentary individuals.” Their next step is to find out if the women who took part in the trial continued with the approach without the support of the scientists when they are invited back for follow-up tests two years after the original study ended. Hawley believes the TRE-HIIT combo is “the brightest thing that we’ve had on the horizon” in terms of weight loss and health. Here’s how to get started:

How to do TRE

Decide on your own eating window

For the purpose of the trial TRE was loosely defined as consuming all daily calories within a ten-hour time window. When that window should be set is your choice. Some people naturally prefer to postpone their first meal of the day until 11am or noon, others to consume breakfast when they wake up. Much depends on the inner workings of your body clock, so decide what works for you. “Our only recommendation was that they should not consume food or snacks after 7pm each day,” Hawley says. “We know that when you bring in your evening meal that little bit earlier, your glucose concentrations during the night are much lower.”

Don’t obsess about calories, fat and carbs

The beauty of the TRE-HIIT combination is that the stress of thinking about what to eat is removed. There are no rules about how much of what food to consume or avoid and, in the study, participants barely changed the ratio of fats, carbs and proteins they consumed. In theory this appeals because you can eat what you like — chocolate, crisps and a glass of wine are permitted, provided you consume them within your window. In reality you may find that your eating habits change for the better simply because weight is falling off and you feel healthier.

Don’t consume any calories outside your window

Be strict with yourself and consume nothing but black coffee, tea without milk or water outside your eating window. “In our study participants could consume drinks outside the TRE window as long as they didn’t contain any calories,” Moholdt says. “Milk and sugar in your drinks count as calories, so they are out.” And that sneaky extra glass of wine or even hot chocolate later in the evening is also a no-no.

Eat lots of fibre

Although Hawley and Moholdt did not specify which foods to eat, there are known strategies that might help to cement your progress. In their trial hunger was noticeable in the first week of switching to a ten-hour eating window — although it was not an issue later on — so selecting nourishing foods that will fill you up is particularly important at the outset. “Plenty of nuts, seeds and lots of fresh fruit and vegetables contain fibre that is filling and nourishing for the gut microbiome,” says Alex Ruani, researcher in nutrition science at University College London and chief science educator at the Health Sciences Academy. “Fibre aids digestion and actually helps to reduce overall calorie absorption by the body compared with sugary processed foods from which calories are rapidly absorbed.”

And a handful of almonds a day

Eating berries, cherries and apples three times a week was shown to help overweight people shed pounds in a University of East Anglia study involving 124,000 people over 24 years that was published in the BMJ. And a recent trial from the University of South Australia found that nibbling on a handful — 30 to 50 grams — of almonds every day could keep extra weight at bay. “We found that people who ate almonds experienced changes in their appetite-regulating hormones, and that these may have contributed to reduced food intake by 300 calories daily,” the study author Dr Sharayah Carter reported in the European Journal of Nutrition.

How to do HIIT

Aim for three 30-minute HIIT sessions a week

In the recent trial participants attended three laboratory-based HIIT sessions each week, performing a short warm-up before getting down to the hard work as follows:

● Two of the sessions comprised four repetitions of four minutes of hard running — aiming for 90 per cent of your maximal heart rate or a rate at which you are breathing hard — on a treadmill. (You can also do this outside, or up and down your own stairs at home.) Recovery after each effort bout was a three-minute active recovery at 60 to 70 per cent maximum heart rate, so a slow run or fast walk.

● The third session comprised 10 repetitions of 60-second bursts of effort at the same intensity, also on a treadmill, followed by 60 seconds of low-intensity activity such as walking.

● A 3 to 5-minute cooldown of moderate to light activity — slow jogging or walking — followed the final tough repetition of each session.

If anyone had knee pain or other problems that prevented them from running they performed the same training on a stationary bike or rowing machine instead, Moholdt says. And if they struggled to manage this amount initially, their workload was reduced by removing a repetition or two. The total weekly exercise time of trial participants was approximately 110 minutes per week — an average of just 36 minutes per workout. However, you don’t need scientists in attendance to replicate these sessions yourself. “Running upstairs can be a HIIT session, as can sprinting hard on an indoor bike or outdoors on a football pitch,” Moholdt says. “Be creative and inventive to make it work for you.”

Progress the HIIT workouts as you get fitter

Progression is important for continued results. The HIIT workload was gradually increased in sessions over the seven-week trial to account for improvements in cardiorespiratory fitness. This could mean working harder or adding another repetition if you weren’t able to manage the full quota to start with. “There are other ways to progress with HIIT sessions as you become accustomed to doing them,” Moholdt says. “You could do them more frequently or add more workouts during one session.” Don’t push on with something you find intolerable. “It is fundamental to find a type of exercise that you can continue doing,” she says. “If you enjoy something you are more likely to stick with it — don’t run on a treadmill if you hate it, but choose some other way to do HIIT.”

Remain as active as possible on other days

Aside from the HIIT workouts, no additional physical activity guidance was given in the trial. However, Moholdt says that remaining active as much as possible will always be beneficial to health. As you get used to doing HIIT, she suggests adding variety. “I think that you should combine HIIT with other types of exercise, such as moderate intensity endurance training — a jog or cycle, for example — and strength training with or without weights on a couple of days a week as you get fitter,” she says.

Invest in a heart-rate monitor

Strictly speaking you don’t need to spend anything on exercise equipment (other than a decent pair of trainers) but Moholdt says that investing in a heart-rate monitor might help you to assess the intensity of your HIIT sessions. For the trial she and her team assessed exercise intensity at every session using Polar heart-rate monitors with the aim being to complete the efforts at 90 per cent maximal heart rate. “At a basic level you can assume you are making the right amount of effort if you are breathing heavily and are not able to talk in full sentences during the activity burst,” Moholdt says. “If you do use a heart-rate monitor, it is important that you have a good estimate of your heart-rate maximum — using a general calculation such as 220 minus age is not good enough since the variation between people is huge.” For more accuracy, invest in a tracker with a heart-rate monitor such as those in the Polar range (from £169.50;, which will guide you through your individual heart-rate zones.

Work out in the afternoon or evening

Most participants in the study did their HIIT workouts between 7am and 4pm and Moholdt says the important thing is “to try and fit it in at a time of day when you feel most energised”. That may be early morning or later in the day, although previous studies by Moholdt have shown that aerobic exercise training, including HIIT, performed in the early evening reduced overnight blood sugar concentrations. “We saw some signs of better health gains in overweight or obese men who exercised in the evening in a short-term exercise study,” she says. “But any time of day is always better than none.”

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