This article is intended as general information and should not in any way be interpreted as specific medical advice. If you are worried about your symptoms, or they last longer than you expect, please make contact and discuss things with your medical provider.

Common initial side effects

One of the most important reasons for some of the common initial side effects when you start the programme is a degree of dehydration. There are two main reasons for this dehydration:

  1. Firstly, reducing calories will obviously reduce your food volume, but also the fluid content of your food. There may also be a reduction in the salt content of your diet, particularly, if you are used to eating processed food.
  2. Secondly, you will begin to use up your stored energy supplies, and your metabolism utilises large amounts of fluid to do this. You may notice that when you start the programme, you make more trips to the loo!

Dehydration may account for the following symptoms:

  • Headaches
  • Fatigue
  • Dry mouth
  • Feeling lightheaded
  • A decrease in blood pressure
  • Feeling cold

This is why it is really important that you anticipate this, and we encourage you to drink an extra 1-1.5 litres of fluid a day, or more if you are doing vigorous exercise or in hot weather. You should be drinking enough water to pass at least 5-6 good volumes of urine a day and you can also add a bit of salt to your food too.

Your pharmacy may suggest simple headache remedies if these are a problem. You could also add a good multivitamin as per Dr Mosley’s advice in The Fast 800 book.

Mildly low blood pressure doesn’t usually cause problems in people who are otherwise fit and well. They may feel slightly lightheaded if they get up too quickly. However, if the low blood pressure is more significant, you are on medication, or if it is due to any underlying cardiac condition, you should discuss this with your medical professional before embarking on 800 calorie fasting days.

Anecdotally some people complain of generally feeling colder when fasting, and although there is some suggestion that this is because blood flow increases to fat stores and away from your skin, there is no clear evidence.

Changes to your metabolism

Other symptoms when first starting the programme may be due to a change in the way your metabolism uses fuel and begins to burn fat for energy. These could include:

  • Bad breath
  • Fatigue
  • Headaches
  • Mood swings and irritability
  • Feeling lightheaded

The body first uses up much of the stored energy from the liver and from your muscles. Blood glucose and insulin levels start to fall, and your metabolism then begins to burn fat.

The liver converts this fat into fatty acids and then into ketone bodies which can provide all the energy you need. This is sometimes known as ketosis and, particularly happens overnight in your fasting state.

Because many of us tend to ‘graze’ we are not used to fasting and to the change in fuels. This process is sometimes referred to as ‘flicking the switch’ from burning sugars to burning fat for energy. Some people feel a bit rough at this stage and these symptoms of lethargy, headaches and irritability are sometimes known as ‘keto flu’.

The upside of this process is that within days you usually adapt to this and begin to feel better with more energy, a sharper mind and less food cravings as you are no longer experiencing sugar troughs and spikes. General advice is to drink plenty of water, and don’t combine this sudden reduction in calories with vigorous exercise, at least at first.

Add simple headache remedies if you suffer from them and remember to add a bit of salt to your food if applicable. You could also add a good multivitamin as per Dr Mosley’s advice in the book.

You may notice that you start producing a sweet fruity smell of ketones on your breath. when first starting this plan. This is normal and shows the diet is working. Brush your teeth regularly and press on. It should improve as your body adjusts to a lower carb intake.

Changes to your bowel movements

Other symptoms may be related to the digestive tract and the change in diet when you first start the programme. These may include:

  • Diarrhoea
  • Constipation

Any radical change in diet can initially affect the digestive system and may cause loose motions or even diarrhoea, particularly when increasing fibre intake. You might also find you are passing more wind and getting occasional bloating. As your gut adjusts to this new healthy way of eating, your bowel movements should too. This process can last up to a week or so, and during this time, it is extremely important to drink plenty of water as you will be losing more than normal amounts of water through your bowel movements.

Constipation is a particularly common problem for people when first starting on the plan. One of the main causes is again dehydration as discussed previously, and this happens particularly in the first few weeks on the plan, before your body adjusts. Added to this you may also be eating smaller volumes of food which may affect bowel movements.

We encourage people to drink an extra 1-1.5 litres of water when they are doing 800 calorie days, to avoid dehydration.

In addition to drinking extra fluids, adding more fibre-rich foods should help. Adding half a plateful of leafy green vegetables with meals may help to smooth things. You may also wish to consult your pharmacist about adding additional fibre or a stool softener to address this but remember both these products will require reasonable amounts of fluid to be effective.

Changes to your sleep patterns

To lose weight whilst sleepingYou may notice that your sleep patterns become disturbed at first, but this is by no means universal. The reasons for this are obviously many and varied and may be different for everybody. There are some studies to show that lowering carbohydrates in your diet can cause sleep disturbances at first. The Fast 800 is low carb but not very low carb or keto, so this may be less of a problem than other diets.

Anecdotally, some people sleep better when they start this plan and longer term many of our members report better sleep quality overall.

Snoring very often improves with weight loss and leads to less daytime drowsiness and possibly better relations with your sleep partner!


If any of the symptoms in this article are severe or last longer than a week or so, it would be wise to book an appointment with your GP – they will be able to advise appropriate treatment.