Hi I’m interested to hear of any endurance athletes with experience of the FAST 800 diet. Have you experienced headaches, muscle pains and fatigue? How do you time your meals? How long can you do 800 per day, do you 5:2 or 2:5? How is training going, what do you do when you race? etc..

I’m a triathlete, training an average of 8hrs a week. Having turned 50 I have found weight slower to shift after the winter increase so I started on 2 weeks of 5:2 in May ’19 then after completing a long ride event (>100 miles) shifted to the FAST 800. I’m on day 8 of this and thinking to do 14 days. It has been hard training on 800 calories (in one day running 7 miles and swimming 2.5k) so I think I’ll go back to the 5:2 after day 14 as I’ll find it more sustainable – taking fast days as non training days. However, in just 8 days I have lost a stunning 11 lbs and am finding each FAST day is getting easier. (Now just under 200 lbs – BMI 25.4 so medically just overweight).

I’d appreciate hearing about your experiences. Thank you.

Hi. I’ve been doing the BSD since mid-Jan and have lost 2.5 stone to date – and training for a triathlon. Been OK so far, but I’ve not done the two days of 800 calories – just kept the eating level with 7 ‘normal’ days (only using recipes from the book). Not encountered too many issues so far with training, but on occasion I do just run out of energy – particularly as I am now stepping up my training ahead of an event. I’ve found that making sure I eat 30/45 mins before I train so I have the energy and then have a (BSD friendly) snack as soon as I get back from training. I am still losing 2/3 pounds a week doing that. Well done on your weight loss so far and good luck with the training!

I would recommend that you look into the work of Stephen Phinney, on low carb endurance sports. – He is an expert on the topic and has a useful book which will explain more.

I have a physically active lifestyle but am not training for events. – When you move to low carb from high carb you can go through a period known as carb flu, and you can suffer from cramps as a result of low electrolytes. Our bodies store fluid when on a high carb diet, when you go low carb the kidneys release that fluid but with it release electrolytes. A little additional salt each day is usually enough to replace the missing salts.

If you get your carbs low enough and get your body used to using fat as a fuel, your body simply switches over to using your fat stores. Which is what it should do, so the number of calories you eat is only relevant in so much as the difference between calories burnt and calories eaten is drawn from your fat stores. Remember a lb of fat equals 3500 calories.

ladynellie1972 – Thats an amazing weight loss – bravo! I note your experience with eating prior to training. So far I’ve been missing breakfasts and eating midday and early evening, training has been hard. This week I’m going to try and schedule training after eating to manage energy levels better. Today I ate a breakfast & reduced the lunchtime calories accordingly. Energy levels were better in the day and after tea I ran a 10k drills & tempo session. It was harder hitting the paces than usual but I did manage it. Tomorrow am my schedule is weights in the gym – but this week it will be after a breakfast….

JGwen – Thank you, I have ordered “The Art and Science of Low Carbohydrate Performance” by Stephen Phinney. It looks very promising from the Amazon reviews.
I certainly experienced Carb Flu, though I didn’t expect the headaches to go on so long. I think today, day 8 is the first day when they’ve not been an issue. Likewise I wasn’t ready for the aching muscles, I’ve been drinking loads of water and every third litre using a HIGH5 electrolyte tab.
I’d like to think my body is adjusting. Tonights 10k session started slow and heavy but once I got going I did hit my tempo paces but it was definitely harder than usual to do so. I am amazed to find after a week, I can still train on just 800 (low carb) calories a day. Clearly lots of fat lbs waiting to be converted to calories! I like the equation 1lb = 3,500 cals, I’ll not be so quick to replenish calories used in exercise in the future.
Really looking forward to seeing what Stephen Phinney has to say about it.