What You Should Know About the 800-Calorie Diet — And Who Should Avoid It
A new diet plan is making headlines for its extremely low calorie target—just 800 a day, less than what you’d get from a cheeseburger and fries. Supporters claim it leads to rapid weight loss and may even help reverse type 2 diabetes. But the plan has stirred debate because it’s as concerning as it is appealing. Before you consider trying it, it’s important to understand where it came from, who it was meant for, and why health experts warn it isn’t safe for everyone.
The Origins Of Eating So Little
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The 800-calorie diet has roots in medical research, and in some cases, it’s still used under strict supervision. But outside of a clinical setting, the risks can stack up.
Very low-calorie diets, sometimes abbreviated as VLCDs, have been around since the 1970s. Doctors developed them to help patients with severe obesity lose weight rapidly when health concerns like diabetes or high blood pressure require speedy intervention. These programs typically cap intake at around 800 calories a day, often through meal replacement shakes that are fortified with nutrients so patients don’t completely miss out on essentials like protein, vitamins, and minerals.
This approach isn’t intended for the general public. It was designed for patients with a body mass index (BMI) of 30 or higher, or for those needing to drop weight before surgery. And even then, it was always paired with close medical supervision.
Over time, the science behind these diets inspired spin-offs, like Dr. Michael Mosley’s Fast 800, which combines intermittent fasting with a Mediterranean-style diet. That version gained traction because it’s more flexible and feels less clinical, but the backbone of it still involves cutting down to 800 calories a day for weeks at a time.
Why People Turn To It
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The appeal is in the rapid results. Studies linked to plans like the Fast 800 show participants losing as much as 20 pounds in about eight weeks, often with improved blood sugar levels and lower blood pressure.
Research like the DiRECT trial in the UK even found that some people with type 2 diabetes saw their condition go into remission after several months of carefully monitored restriction. For those carrying excess abdominal fat or battling insulin resistance, the results are tempting.
Some reports suggest added perks, like improved insulin sensitivity and reduced fat around the liver and pancreas. Others point out that short bursts of calorie restriction can encourage the body to switch into a mild form of ketosis, where fat is used more efficiently for energy.
And while not everyone feels it, many describe a phase after the first two weeks when hunger fades and energy levels stabilize, which makes sticking to the diet feel more manageable.
The Red Flags You Can’t Ignore
For every success story, there are warnings. Health organizations like the NHS make it clear: diets this restrictive should only be followed for a maximum of 12 weeks and only if recommended by a doctor. Dropping to 800 calories without professional guidance can leave you short on critical nutrients and energy. That means a higher chance of dizziness, headaches, constipation, hair thinning, and even more serious issues like weakened bone density.
Rapid weight loss also increases the risk of gallstones, especially in women. And while fortified shakes can cover some nutritional bases, relying on them for weeks can feel monotonous and may affect your relationship with food after you stop.
Trainers and dietitians also warn about rebound weight gain, as the body tends to fight back by slowing metabolism and ramping up hunger hormones once the restriction ends. You may lose fast, but the pounds can often creep back on with extra.
Who It’s Really For
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The 800-calorie remains a medical tool for specific groups: people with obesity-related complications, patients preparing for surgery, or individuals with type 2 diabetes being monitored in a structured program.
Outside of those situations, experts stress it’s not appropriate, especially for teenagers, older adults, anyone pregnant or breastfeeding, and people with a history of eating disorders. It also doesn’t mix well with endurance exercise, since the body simply can’t fuel itself adequately at that level.
For the average person looking to slim down safely, national guidelines recommend far higher intakes — around 1,400 calories a day for women and 1,900 for men—which can be adjusted based on activity.