Cutting calories sheds weight initially, but body changes can drive regain

by Yasmine Probst, Olivia Wills, The Conversation

edited by Swati Mestri, reviewed by Andrew Zinin

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In the last few years, there's been a lot of interest in calorie-restricted diets for weight loss.

These can vary significantly, from intermittent fasting (where you restrict eating to a certain window of the day) and alternate-day fasting (where you consume limited or no calories every other day) to the Fast 800, which promotes rapid weight loss by drastically reducing calories while following a Mediterranean diet.

The basic idea might seem obvious: Reduce how much you eat, and you'll theoretically lose weight.

But are these diets healthy? And will you just put the weight back on if you stop?

What are calorie-restricted diets?

The amount of energy you need each day varies based on your age, sex, body composition, physical activity and any underlying health conditions.

Calorie-restricted diets aim to reduce a person's overall energy (calorie) intake. In certain instances, when managed carefully by health care teams, there is evidence of benefits beyond weight loss, including as an intervention before surgery.

The general aim is to eat less than the body uses, causing it to draw on stored energy—fat. Over time, using more energy than you consume can lead to weight loss.

But these diets are not meant to be followed forever. Most involve a period of restricting calories before transitioning to a less restrictive diet that helps you maintain a healthy weight long term.

Diets vary in how restrictive they are

For example, some diets recommend a moderate restriction: eating 500–600 fewer calories a day. That's like subtracting one takeout hamburger from your normal daily intake.

Very low-calorie diets (also called very low energy diets) typically recommend eating fewer than 800 calories a day.

These diets may be medically recommended—for example, when short-term rapid weight loss before surgery could reduce someone's risk of complications.

They require close monitoring and support from a dietitian to make sure people still get enough essential minerals and vitamins and don't become malnourished.

Do calorie-restricted diets work for weight loss?

The short answer is yes—at first.

Research consistently shows calorie-restricted diets can significantly reduce weight and waist circumference.

They can also improve how the body manages blood sugar and cholesterol levels, helping to reduce cardiovascular risk, particularly among people who are above a healthy weight or live with obesity or type 2 diabetes.

But much of the evidence for calorie-restricted diets comes from short-term studies lasting only 2–12 weeks.

Unfortunately, maintaining the weight loss is much more challenging than achieving it in the first place. Many people regain some or most of the weight they initially lost once the diet ends.

It's not just about willpower

Highly processed foods are cheap, convenient and everywhere around us. This makes long-term restrictive eating patterns very difficult to follow.

But there's more to it.

Losing weight also triggers a range of adaptations as our bodies try to restore the lost weight.

Basically, your body tries to defend against further weight loss. For example, as you lose weight, the hormones that regulate appetite change—often making you hungrier.

Meanwhile, you may feel less full, and as your body becomes more energy efficient, it will burn fewer calories than before.

This doesn't mean the body enters "starvation mode," as is often claimed on social media. But your weight loss slows over time, and hitting a plateau is very common. As motivation falters, some people may regain weight.

You will feel hungry

For many people, constant hunger is the main reason they don't continue with calorie-restricted diets.

Feeling hungry all the time can affect your mood, sleep and relationship with food. It can also be your body telling you something else is going on.

Constant hunger can indicate you're stressed or have an underlying health condition.

You should speak to your doctor if your appetite has increased or decreased for no apparent reason, especially if you also feel dizzy or very tired or have difficulty concentrating.

If you have these symptoms, your doctor will likely advise you to stop a calorie-restricted diet.

Should I try one?

Calorie-restricted diets may be suitable for people above a healthy weight or with obesity who wish to lose weight. For some chronic conditions, such as type 2 diabetes or arthritis, losing weight can also help manage the disease.

But they're not recommended for:

• people who are pregnant or breastfeeding
• children and teens without specialist supervision
• people with a history of eating disorders

For most people, dietitians recommend focusing on improving their overall diet and creating long-term habits, rather than simply cutting calories as aggressively as possible.

Healthy and sustainable weight loss

The Australian government shares some key tips for maintaining a healthy body weight:

• limit foods high in salt, added sugar and fat, and limit alcohol consumption
• reduce portion sizes
• find enjoyable ways to be more active
• plan meals and snacks ahead of time
• eat mindfully and pay attention to hunger and fullness cues
• stay hydrated

Dietitians also encourage strategies that maximize fullness, such as eating enough protein and including fiber-rich foods such as vegetables, legumes and whole grains.

Evidence suggests you're more likely to stick to healthy eating if you focus on overall health rather than numbers, like what's on the bathroom scale.

The bottom line

If you're considering a calorie-restricted diet, it's worth checking in with an accredited practicing dietitian first.

They can help you work out whether the approach is appropriate, ensure your nutritional needs are met, and develop a plan that supports weight loss and helps you keep the weight off long term.

Key medical concepts

Weight Loss

Clinical categories

Weight managementNutrition & Healthy eatingAllied healthFamily medicineHealthy living Provided by The Conversation Who's behind this story?

Swati Mestri

Swati Mestri holds a bachelor's degree in Electronics Engineering and has worked as a content editor since 2019. She has experience editing research documents across technology, health care, and materials science, and has a particular interest in technology and space. Full profile →

Andrew Zinin

Master's in physics with research experience. Long-time science news enthusiast. Plays key role in Science X's editorial success. Full profile →

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