Why ultra-processed food gets in the way of weight loss and how to put together 10-minute meals when you have no time to cook

Chapter 9 of the book I co-wrote, Eat to Succeed (a Korean-language book, 2026), opens with this line: "I'm too tired after work to face cooking. I end up ordering delivery." Many people know what they should eat but say they have no time to cook. Before long, more and more meals are crisps, instant noodles and ready meals.

Walk down the middle aisles of a supermarket and you will find them full of crisps, cereal, ready meals, tinned food and frozen food. Chapter 1 of the book describes what these foods have in common. Processing strips out vitamins, minerals and fibre and adds sugar, salt, trans fats and various additives. They are made so tasty that they stimulate the brain's reward system and lead to overeating.

The study described in the book was carried out at the US National Institutes of Health. Twenty weight-stable adults were admitted to hospital and ate an ultra-processed diet and an unprocessed diet for two weeks each. Each meal was matched for calories, carbohydrate, fat and protein, sugar, sodium and fibre, and they could eat as much as they wanted. During the ultra-processed diet they ate 508 kcal more a day, and the extra came from carbohydrate and fat. Weight went up 0.9 kg during the ultra-processed period and down 0.9 kg during the unprocessed period.

This was 20 people spending four weeks in hospital. It does not tell you how many kilograms you would lose by cutting back on processed food at home. What is worth remembering is that the same people, offered the same calories, ate more when the diet was ultra-processed.

When told to cut back on processed food, some people ask whether anything in a packet is off limits. Yet the 10-minute recipes in Chapter 9 use frozen vegetables, tinned tuna and tinned chickpeas. The book notes that frozen vegetables save washing and chopping time and lose few nutrients. For tuna, choose tins in water rather than oil and drain it before use.

So when shopping, check the label rather than whether something is packaged. See whether the ingredients have only been washed, cut and frozen, or whether sugar, salt and additives have been added.

Many people ask whether a meal can really be made in 10 minutes. The book gives three secrets.

• Prepare aheadspend two hours at the weekend cooking chicken breast in one batch and refrigerating it, washing and chopping vegetables into airtight containers, and freezing brown rice in single portions.
• Simple cookingskip complicated recipes, stick to grilling, steaming and boiling, and use the microwave.
• One bowlput rice, protein and vegetables in one bowl. Cooking is simpler and there is less washing up.

One study looked at whether planning meals ahead is linked to diet. Among 40,554 French adults, 57% planned their meals a few days ahead, and their diets were closer to nutrition guidelines and more varied. Women who planned were less likely to be overweight or obese, and men less likely to be obese. Because it was a survey at a single point in time, it cannot show that planning made people gain less weight, and the authors say no causal link can be drawn.

Many people say chicken breast is dry and salads get boring. Chapter 9 calls the idea that diet food tastes bad a prejudice. Before changing ingredients, we suggest changing how you cook them.

• Spices and herbsuse them instead of salt and sugar. Marinate chicken breast in an acid such as lemon juice or yoghurt with a little olive oil and spices such as garlic, ginger and paprika, then grill it.
• Roasting vegetablescut broccoli, carrots and peppers into bite-sized pieces, toss with 1-2 teaspoons of olive oil and roast at 200°C for 20-25 minutes. They taste sweeter than boiled.
• Less oiluse an air fryer instead of deep-frying, and cook eggs and fish in a non-stick pan with just a little oil.

After hearing that processed food is bad, some people decide to empty the fridge. Starting that way, one slip during a work dinner or a busy week can feel like everything is ruined. The book calls perfectionism the enemy of dieting.

The principle in the book is to cut back. Eat plenty of vegetables and a moderate amount of protein, and reduce carbohydrates and processed food. Sweet drinks are limited to once or twice a week. Habits that last matter more than a single meal, so the book says that if you manage 80% well, you can be flexible with the other 20%. Over three months, that is 70 days out of 90.