How to lose 20 kilos: how long it takes and where to start

Can you lose 20 kilos in a month?

No, and it is not a matter of willpower. With very severe restriction, the scales drop substantially in a few weeks. That is what sachet plans, prolonged fasts and single-food plans achieve. But what is lost is primarily water and muscle mass, and it returns as soon as you eat normally.

And it has a price. When restriction is very severe, the body responds with an obsession with food and constant urges to snack. Blood tests and the reserves of calcium, magnesium, iron and vitamin B12 can be altered, with the tiredness and weakness that brings.

Losing muscle also lowers energy expenditure, and each week is harder than the last. Preserving it depends on eating enough protein and doing strength exercise, as a review on muscle during weight loss summarises [3].

There is another consequence that is almost never mentioned: very strict plans are linked to eating disorders. That is why we do not suggest any one-month plan. And that is why the catalogue of rapid weight-loss diets is renewed every year without any of them reaching the fourth week.

How long does it take to lose 20 kilos?

Around ten months, at half a kilo a week. The guidelines of the Spanish Society for the Study of Obesity (SEEDO) consider it advisable to lose between 0.5 and 1 kg a week [1]. At that pace, twenty kilos takes between five and ten months. The less weight there is to lose, the slower the pace tends to be and the longer the timeframe.

Three things worth knowing before you start counting weeks.

• You always lose more in the first week. Almost all of it is water, so it does not set the pace of the following ones.
• The first kilos are the easiest, because motivation is high. The ones in the middle are what decide the outcome.
• Going slowly has advantages. Gradual loss reduces fat, limits muscle loss and curbs the drop in expenditure that accompanies rapid weight loss.

If you have considerably less to lose, start with the weight loss diet guide or the one for losing 10 kilos or, if you have about fifteen to lose, the one for losing 15 kilos. See “Vegetarian diet for weight loss”.

First, a medical assessment

With twenty kilos to lose, the first step is a medical assessment, before any plan. It is not about asking permission to eat better. With this figure it is worth checking first your blood pressure, blood sugar, liver, thyroid, sleep and the medication you take. If any of that is behind the weight, or will change when it goes down, the order matters. This guide is not designed for pregnancy, breastfeeding, under-18s or anyone who needs to gain weight. Nor for people with chronic kidney disease, or who have or have had an eating disorder. Diabetes treated with insulin or with medicines that can lower blood sugar too much is also excluded. If you have coeliac disease, a diagnosed allergy or heart or liver failure, follow your specific plan. In the postpartum period, avoid restrictive diets.

It is what the obesity guidelines set out, the Spanish one [4] and the British one [5]. Both describe the approach as work on several components at once (nutrition, physical activity and the behaviour that sustains the first two), coordinated between professionals.

It is also worth saying where our boundary lies. A team of dietitian-nutritionists specialising in weight loss works on nutrition, habits and body composition, with psychology and endocrinology alongside when needed. Diagnosing, ordering tests and prescribing medication is for the doctor.

What the plate looks like and how to organise meals throughout the day

The plate idea is the only thing you need to learn, and it works for lunch and dinner. It is about proportions, so there is nothing to weigh.

• Half a plate of vegetables, raw or cooked. This fills you up with little energy and dictates how much of the rest you eat.
• A quarter of the plate for protein. Lean meat, fish, seafood, eggs, pulses, soya and derivatives.
• A quarter of the plate for carbohydrates, preferably wholegrain, such as brown rice or wholegrain wheat, pulses, quinoa or oats.

Carbohydrates are reduced, but they stay. They are in the three main meals, in a smaller portion than usual. Diets that cut them out are harder to sustain and last less time.

A change that pays off more than it seems is moving from refined grain to whole grain. It is in line with the Spanish recommendations, which advise eating more vegetables and fruit and less meat, especially red and processed meat [6].

To get bread right, read the label. Only bread made exclusively with wholemeal flour can be called “wholemeal bread” or “100% wholemeal bread”, according to the bread quality standard [7]. If it says “made with wholemeal flour”, the percentage goes next to it, and it can be low.

The rest of the day is organised around two things. Four or five meals, every three or four hours, and a dinner you can repeat without getting bored. If dinner is a struggle, there are nine dinner ideas that fit this plate.

Does fasting work for losing 20 kilos?

After the timeframe, it is what most people search for, and the answer is less exciting than it promises. Fasting is another way of eating less, with no shortcut. We neither recommend nor prohibit it. Anyone who loses weight through fasting does so because of the deficit, not the clock. And anyone who does not change what they eat during their eating hours does not lose weight.

The longer the fast, the more it resembles the very severe restriction in the first section, with the same consequences for muscle. And it is not for everyone. Fasting is not suitable if you are under 18, are pregnant, are breastfeeding or have or have had an eating disorder. Nor if you have diabetes treated with insulin or with medicines that can lower blood sugar too much.

Over ten months, what matters is finding a way of eating that you can keep up without reaching dinner with hunger that has built up.

What about medicines: Ozempic, Wegovy, Mounjaro?

Semaglutide (Ozempic, Wegovy) and tirzepatide (Mounjaro) exist. Ozempic is authorised for type 2 diabetes; Wegovy and Mounjaro, for weight management. Saxenda also exists for weight management. A doctor assesses and prescribes these medicines according to their summary of product characteristics. We do not prescribe treatments or give doses.

We look after your nutrition during and after treatment: making sure you eat enough protein and preserve muscle during treatment, and that when it ends you have habits that maintain your weight. If you are at that point, there is nutritional support with GLP-1 and what we explain about the rebound effect after Ozempic and Wegovy.

The trial figures for semaglutide and tirzepatide (how much was lost, at which dose and over how many weeks) are in how much weight people lose with Ozempic, Wegovy or Mounjaro. They are group averages, and your doctor assesses your case.

What mistakes do we see in consultations?

Ten months include the Castanyada, Christmas, Easter and quite a few family meals. In consultation we see that what takes you off track is usually the month after each celebration.

• Arriving at the celebration without a plan. Decide beforehand what you will eat and in what portion. Neither isolate yourself nor stuff yourself.
• Cutting the amount when you stall. First check whether you spread your meals throughout the day and whether a food group is missing. One is usually missing, especially when anxiety has appeared.
• Getting bored of the dish and giving it up. Change the way you cook it. The same pumpkin can be a puréed soup, a baked pizza base, cooked en papillote with mushrooms or a bed for sea bass.
• Letting one bad day turn into three. A day that gets out of hand does not erase two months. What matters is the “since I’ve already messed up” attitude over the following days.

If what lies behind it is how you feel about food, that is worked on with psychology.