Losing a lot of weight: where to start

When the scales show 30 or 40 kg more than you would like, the road looks very long. You do not need to plan all of it from the first day. You need a first goal you can reach, a pace that does not wear you out, and an idea of when your doctor should be involved too.

The first goal: 5% to 10% of your weight

The US guidelines on obesity (AHA/ACC/TOS, 2013) suggest an initial goal of losing 5% to 10% of your weight within 6 months. They also say that steady loss of even 3% to 5% already brings health benefits, and that greater loss brings greater benefits.

When you reach the first goal, you set the next one. That way ‘a lot of weight’ breaks into pieces you can see happening, instead of one number that seems out of reach.

How fast

The UK's National Health Service (NHS) suggests steady weight loss of 0.5 to 1 kg a week. When you start from a high weight, the same change in what you eat usually leads to a larger loss in kilos, according to a model published in the Lancet in 2011. The same model shows that it also takes longer for weight to level off.

Even so, in these articles we keep an upper limit of 1 kg a week for everyone. The main reason is gallstones:

• An analysis of 5 studies found that the risk rises sharply when weight loss exceeds 1.5 kg a week.
• In a Swedish programme with 3,320 pairs of people, those who followed a 500 kcal a day diet for 3 months had, over a year, more than three times as many gallstone cases needing hospital care as those on a 1,200 to 1,500 kcal diet: 152 against 44 per 10,000 people a year.

Both are observational studies, so the figures are of low certainty. The tool in the article on the rate of weight loss shows what the pace means in kilos for your own weight.

The first weeks

The first week will show more. When you eat less, your body uses glycogen, the carbohydrate it has stored in the muscles and liver. Glycogen is stored together with three to four parts water, and when it goes, the water goes too. If you see 2 or 3 kg less in the first week, do not expect it to carry on at the same pace.

A few changes that last. Start with what you repeat every day, such as the oil in your food and the afternoon snack. The Greek National Dietary Guidelines suggest measuring olive oil with a spoon or a measuring dispenser rather than pouring it straight from the bottle. We explain how many calories you need to start with in the article on daily calories.

Activity from where you are now. NICE suggests building up gradually, with goals you agree on, starting with everyday activities such as brisk walking and always in line with your fitness. The programmes in the US guidelines aim for at least 150 minutes a week, about 30 minutes on most days, which is also what the Greek Guidelines ask for. If you walk for 10 minutes today, 15 is already progress.

Weighing once a week. In randomised trials, weighing every week made no significant difference compared with weighing every day. Weigh yourself under the same conditions, in the morning before you eat.

Not too low in calories

With a lot of weight to lose, it is tempting to cut your food sharply. The US guidelines suggest a deficit of 500 to 750 kcal a day, or 1,200 to 1,500 kcal for women and 1,500 to 1,800 for men. They accept diets under 800 kcal only in limited cases, with medical supervision, because of the fast rate and possible complications.

NICE (2025) is just as strict. It suggests diets of 800 to 1,200 kcal only within specialist services with long-term support. Under 800 kcal, only when there is a medical reason for rapid weight loss, for up to 12 weeks and with clinical supervision.

When to talk to a doctor first

• If you have diabetes, high blood pressure or sleep apnoea, or a condition that makes weight loss harder. The NHS suggests asking your doctor for help in these cases.
• If you take medicines for a long-term condition, such as diabetes or blood pressure. When your eating changes a lot, the dose may need changing, and only your doctor can decide that. NICE asks for medicines to be reviewed before any diet under 1,200 kcal.
• If your body mass index (BMI) is 40 or more, or 35 to 39.9 with a serious condition that would improve with weight loss. NICE then suggests referral to a specialist team, to assess whether surgery is also an option. You do not have to choose it, but it is worth knowing that it exists.

Who should not start a weight-loss diet on their own

In some cases a diet with a calorie deficit needs a doctor from the start, or should not be followed at all:

• in pregnancy, where NICE says that deliberate weight loss is not recommended because of possible effects on the baby, and during breastfeeding without advice from a doctor or dietitian,
• under 18,
• if your BMI is under 18.5,
• if you have or have had an eating disorder,
• if you have type 1 diabetes or take insulin, sulphonylureas or SGLT2 inhibitors,
• if you have chronic kidney disease stage 3 or above, or a kidney transplant,
• if you have heart failure or cirrhosis, are having cancer treatment or had bariatric surgery in the last 2 years.

This does not rule out losing weight. The programme should be planned by your doctor, often together with a dietitian.

What to expect in the long term

In an analysis of 80 trials, diets with fewer calories, alone or together with medicines, gave 5 to 8.5 kg on average in the first 6 months, after which weight levelled off. At 4 years, participants still weighed 3 to 6 kg less on average. An analysis of 29 US studies with at least 2 years of follow-up found that people who had lost 20 kg or more kept off more weight than those who had lost less than 10. At 5 years, the average participant still weighed more than 3 kg less.

The weight often comes back, partly or completely. In a trial of 200 adults, those who completed the programme had regained about 71% of the weight they lost after about 3 years, whether they had lost it quickly or slowly. On diets under 1,200 kcal, NICE says that if the weight comes back, it does not mean that you or your doctor have failed. To keep it off, the US guidelines suggest 200 to 300 minutes of activity a week and weighing yourself every week.

The MesogeaFit app, coming soon, will work out the grams of each meal and will not lower your calories below a safe floor. If you like, we can let you know when it launches.