Very low calorie diets: what NICE allows and why

Eating under 800 calories a day is not an aggressive diet. In UK guidance it is a clinical intervention with defined indications, a maximum duration and required supervision — closer to a short course of treatment than to a lifestyle choice.

Understanding why makes the rules seem less arbitrary.

The definitions

NICE uses two terms ^[1]:

Both are normally delivered as total or partial meal replacement, because hitting those intakes with ordinary food while remaining nutritionally complete is close to impossible.

What NICE permits

The recommendations are unusually prescriptive ^[1]:

Low-energy diets (800–1,200 kcal) — consider only as part of a multicomponent strategy with long-term support, within a specialist overweight and obesity management service or another long-term condition service, for people living with obesity, or with overweight and type 2 diabetes.

Very-low-energy diets (under 800 kcal) — consider only within a specialist service, for people living with obesity who have a clinically assessed need to lose weight rapidly — for example to make surgery safer or possible.

Neither should be used as a long-term strategy.

Both must be nutritionally complete, last no more than 12 weeks, and include ongoing clinical supervision with access to a registered dietitian or nutritionist, plus advice on reintroducing a wider range of foods.

The risks NICE asks to be explained

Before starting, guidance says the following must be discussed ^[1]:

• Weight cycling and weight regain — and that regain is likely, and is not a personal failure
• That this is not a standalone long-term strategy
• For very-low-energy diets specifically: constipation, fatigue and hair loss
• An assessment and counselling offer for possible eating disorders or other mental health issues
• A review of all medicines

That last item is the one most likely to matter medically. Blood pressure medicines, diabetes medicines — particularly insulin and sulfonylureas — and some others need dose changes as intake drops, sometimes within days.

The hair loss point deserves its own mention, because it surprises people badly. Rapid weight loss commonly triggers telogen effluvium, which starts around three months after the trigger and is self-limiting. We cover it in hair loss after weight loss.

Other risks worth knowing

Gallstones. Rapid weight loss increases the risk of gallstone formation, sometimes requiring surgery.

Gout. Rapid loss can precipitate an attack in susceptible people.

Muscle loss. Expected with rapid loss; reduced but not eliminated by adequate protein and resistance training.

Cold, tiredness, poor concentration. Common and usually tolerable, but a real consideration if you drive for a living or operate machinery.

Where they genuinely shine

Type 2 diabetes. The DiRECT trial used total diet replacement and produced 46% remission at 12 months. At five years, remission had fallen to about 13% in the extension group against 5% in controls — durability is much lower than the headline ^[3].

The NHS has built a national programme on this. The Type 2 Diabetes Path to Remission Programme provides 12 weeks of total diet replacement at 800–900 kcal, structured reintroduction, and a year of support, free, for adults aged 18–65 with type 2 diabetes diagnosed in the last six years ^[2].

If you have type 2 diabetes and are considering an 800 calorie diet, ask your GP about that programme before buying anything. It is the supervised version, and it is free.

If you are going to do it anyway

Some people will, and pretending otherwise helps nobody.

• Use a nutritionally complete product designed for the purpose, not a homemade 800 calorie plan
• Tell your GP, specifically so medicines can be reviewed
• Drink plenty and address constipation early
• Keep protein at the upper end of what the product provides, and do resistance work twice a week
• Set an end date — 12 weeks maximum — and write the reintroduction plan before you start
• Stop and seek advice for chest pain, palpitations, fainting, severe abdominal pain, or persistent vomiting

When to speak to a clinician

Speak to a clinician before starting, and urgently if problems arise, if you:

• take insulin, a sulfonylurea, or blood pressure medicines
• have type 1 diabetes — these diets are not appropriate
• are pregnant or breastfeeding
• have a history of an eating disorder
• have gallstones, gout, kidney or liver disease, or a history of cardiac arrhythmia
• develop severe upper abdominal pain, fainting, palpitations or persistent vomiting while on one

This article is for general information and isn’t a substitute for advice from your own clinician. If you feel unwell, contact your GP or NHS 111. In an emergency, call 999.