I did a similar very low calorie diet (Lighter Life, 700 kcal/day for the women, a couple of hundred calories/1 extra shake for the men) over a decade ago. My sister, her husband, and my brother and his wife did it at the same time. Over a period of 5 months we all lost vast amounts of weight, between 6 and 7 stone each.
My sister in law, who is only 5 foot tall eventually stopped losing weight, despite not cheating and drinking 2-3 litres of water every day. The consultant told her this was normal, and she would eventually shift those last few pounds if she kept at it. After at least a month of not losing any weight she did manage to very slowly reach her target.
The shakes and meal bars were supposed to contain all the nutrients needed to cover an adult's daily requirements. Yet somehow, around the 4th month my sister and I both started losing our hair, great clumps clogging the shower drain, and our fingernails cracked and splintered.
After reaching our target weights we had to follow a very strict readjustment diet. Only non starchy vegetables and fruit for the first weeks, then gradually adding a potato the next week and a slice of bread the week after that until we were all eating very healthy diets. The two men managed to stay normal weight for 2-3 years before their fat returned, but my sister in law started getting podgy in the space of a few months, and a year later she was fatter than she had been when she started dieting, despite eating small portions of healthy food and doing an active job. My sister and I also regained the weight, though we are taller (5'7" and 5'5") so we could put away more calories before we started regaining the weight. 3 years after ending the diet I had regained all the lost weight plus an extra one and a half stone on top, my sister and sister in law were the same.
It took longer for the blokes to regain the weight, which may be because their metabolisms were less damaged (their hair didn't fall out) and because they have more muscles and less psychological hang ups around food and nurture. It was around 5 years before they were back to their starting weight.
That was the third time I lost a significant amount of weight by restricted eating. I lost 5 stone in my teens, regained enough to lose 4 stone in my thirties, and then 6.5 stone in my fifties. I also joined weight watchers and slimming world between those large weight losses, and struggled like Sisyphus to lose the same 15 pounds over and over again. I was always strong and active, could walk all day in the forest, dig my allotment and heave 60L sacks of compost around with ease.
During lock down I reached my highest ever weight, (BMI 59.5) and was crippled by a bout of rheumatoid arthritis that filled all my joints with gravel, and made merely walking to the toilet and back to my bed or chair a symphony of agonising pain. I heard a podcast about how chronic inflammation is tied to the gut microbiome, and that eating 6 small portions of fermented foods daily had reduced inflammation markers in the blood of test subjects by around 20%.
So I ordered kimchi, kombucha, sauerkraut and kefir, and made sure to have a bit of each one every day.
I made other changes to my lifestyle after listening to scientists on the Zoe podcast talking about the latest developments in food science and gut microbes.
I keep track of everything I eat in a myfitnesspal diary, and have very gradually (since February 2022) reduced my daily calorie intake from around 3600 kcals/day to today's 2300 kcals/day. So far I have lost 69kg (10 stone 12lbs?). Without ever feeling like I'm dieting or deprived. In fact I'm falling asleep before midnight after decades of tossing and turning till 3 and 4 in the morning, my raised blood pressure is now normal, and my diabetes is in remission (Time in range 92% over the past 90 days with no hypos).
I concentrate on eating a wide variety of plants (normally around 70/week), at least 6 portions of fermented food/day, nuts, berries, olive oil, avocados, herbs, dark chocolate, all the good stuff. Since reading Ultra Processed People I have taken to reading the ingredients list before buying treats, avoiding the vegetarian options that contain methyl cellulose, gums and emulsifiers, and baked goods made with mono and diglycerides of fatty acids, modified starches and sugars and artificial sweeteners. I've even started eating fish again after being vegetarian/vegan since the early 80s.
I fast for between 16 and 18 hours every night, to give the gut microbes that come on shift after the food has passed through a chance to proliferate, mend the gut lining and excrete all those short chain fatty acids that are needed for a healthy immune system, good sleep and mental equilibrium. I even pamper the little buggers by sprinkling inulin powder on my kefir and defrosted berries, since I know they thrive on fructooligosaccharides.
I'm 66 years old, 5' 5" tall, disabled (can walk fewer than 20m with two sticks, so not clocking up loads of steps each day) and just lost 2.1 kg over the past four weeks eating an average of 2300 kcals/day. It doesn't feel like I am dieting or denying myself anything, on the contrary it feels as if I am pampering myself with optimal nutrition.
I don't normally eat after 4 or 5 in the afternoon, and have breakfast between 10 and 11 in the morning. Sometimes in the evenings I am tempted to eat the kind of stuff I used to binge on, but it is very easy to tell myself I can have it "after breakfast tomorrow" and then, having eaten a very satisfying breakfast I've completely lost interest in whatever it was that was so alluring the night before.
All the other times when I've lost weight I've had to deny myself the foods I wanted to eat, forcing myself to eat "healthy" stuff that I didn't much enjoy. Constantly thinking about weight loss, what I'm not allowed to eat, castigating myself for being greedy, and using enormous amounts of will power to resist the incessant temptation. This time I wasn't even trying to lose weight, I just wanted to lower the excruciating levels of chronic inflammation in my joints. I added the fermented foods, make a point of eating nuts and healthy oils every day, it seems I'm the kind of person who finds adding extra foods a lot easier than having a big list of foods that must be restricted.
The weight loss was totally unexpected. Google tells me a woman my age only needs 1600 kcals/day for weight maintenance. The myfitnesspal website tells me every day that I will have gained weight in five weeks time if I keep eating so much, yet when I weigh myself (every 4 weeks) I always seem to have lost a bit more.
I expect the weight loss will slow down as my BMI gets closer to normal for my height, but my metabolism still hasn't cottoned on to the fact that I have been losing weight steadily for 2 and a half years. Previously when I've restricted calories it has quickly gone into starvation mode, making me feel permanently ravenous and turning every available calorie into fat. This time around feels like I've tricked it by going stealth. I did try restricting my calories to 2000/day last year, I felt hungry all the time but consoled myself that I was going to see a much bigger weight loss the next time I climbed on the scales. Turned out I only lost 0.4kg after 4 hungry weeks, so then I went back to eating what I wanted (which averages out between 2150 and 2300/day, and went back to losing an average of 2-3 kg every four weeks.
My advice to anyone desperate to lose weight is to forget about the calories and concentrate on eating 30 different plants/week, dodging UPF ingredients, eat 6 small portions of fermented food every day, make sure you get plenty of fibre including inulin (start with half a teaspoon and gradually increase as you get more beneficial microbes in your gut that proliferate once it is reliably available as food), and give your guts a complete rest from calories for at least 14 hours of every day. Most overweight people who do the Zoe project lose weight, and everyone who improves their gut microbiome feels happier, sleeps better, lowers their chronic inflammation levels and improves their immune defence system.
You don't actually have to fork out hundreds of pounds to take part in the Zoe project, all their food science podcasts are available free online.