How safe is intermittent fasting beyond 12 hours?

Greetings. For sometime I have been stuck at a weight of 94 kg, with a goal of reaching and maintaining 80 kg (my ideal weight).

I follow a low carb, low fat diet with 1 balanced main meal at lunch and 4-5 small light heathy meals throughout day, all of which helps me to effectively maintain my blood sugar levels consistent and within the desired range. I also exercise regularly.

I have been reading a lot about the effectiveness of intermittent fasting for weight loss, and I am considering trying it to help burn my stored calories and bringing my weight down. I am however worried about hypos and its dangers. I find that whenever I do not have my light snacks every 2-3 hours, my levels drop and I start experiencing symptoms.

I want to try fasting but how can I do so safely? Also, some people believe in 3 total meals a day for weight loss, but multiple small meals throughout the day was what I was told by medical practitioners when I was first diagnosed (2 years ago with Type 2). So what’s your take? Thanks for sharing.

MissMuffett said:

Low carb and low fat isn’t a good idea. How low carb are you going? The idea is you have to replace the carbs with healthy fat to keep you from feeling hungry and so you don’t go into a low calorie mode as this will mess your hormones and metabolism up, (saturated fat doesn’t raise your BG and it doesn’t cause heart attacks, which is what we were told back in the 70s and 80s). Don’t consume vegetable (seed) oils they’re classed as toxic and will cause clogged arteries but healthy fats found on beef, lamb, pork etc. use butter, olive and avocado oils.
I increased my fat intake and make sure I eat around 150g protein a day and I’ve lost about 24kg. I finish eating about 6:30pm and apart from coffee with cream in the morning, I eat a proper meal at 12 noon so you could class that about a 17 hour fast! I don’t count the coffee as it doesn’t spike my BG like eating would.

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Hello MissMuffett. Thanks. I truly appreciate this advice.

Regarding how low my low-carb diet is, I try to stay away from all quick-release starches, and have slow-release starches in moderation (such as long-grain parboiled rice or brown rice, or brown pasta, all cooked al dente) once a day for lunch. I try to stick to the 50/25/25 rule for lunch (50% veg, 25% protein & 25% slow-release starch). I also eat Low Gi multigrain whole-wheat breads in a sandwich, usually once a day. My in-between snacks are mainly nuts, fruits (dry & fresh), and plain yoghurts. I also eat All Bran flakes and steel-cuts oats, dry, without milk, as a nibbler.

Regarding my low-fat diet, I do not shun away from animal fats (which I love, especially the charred fats on a grilled steak, chicken or chop), but I do avoid all fried foods and processed fat products, like processed meats, margarine, etc.

Yes, seed-oils I avoid as much as possible, which I mainly use to thinly base the pot when cooking stews or curries (it is just a pity that all the healthier substitutes are so expensive, so I use olive oil mainly for salads and grills). And yes, I love all the good fats from nuts, fish and avocados, and I eat full cream dairy products, like milk and white cheese. So I guess, my diet is not really low-fat in totality.

Thanks. I love your idea of an extended morning without eating, and since you included a coffee with cream (which is my weakness too), I am all in with that idea. I however exercise in the morning and take 1 metformin 500g tablet at 8 am and another at 8 pm. So I was debating to not take my morning metformin tablet when I do the extended fasting (from 20:00 to 13:00 (17 hours).

PS: I normally have a light meal at 7 pm, and a small fresh fruit at 08:30 pm, then nothing until 7:30 am the next day (that’s an 11-hour fast on my standard day).

Please advise, when I do extend my fasting to 17 hours, how often should I do this (once a week, once in 2 weeks?), and is it a good idea to drop my morning metformin tablet when I do?

Thanks again for your advice. It’s great being able to discuss these things with people who’ve ‘been there and done that’.

HSSS said:

My personal take and experience suggests the advice to eat multiple meals is flawed for me. Each time you eat you raise blood glucose to some extent and insulin to deal with that glucose. Both of which can be counterproductive to type 2 who are as a group usually insulin resistant. (Raised insulin can contribute to increasing insulin resistance, resulting in ever decreasing circles of more and more difficult management). The idea that multiple meals “balances” levels really just means you get a consistent (higher) level than if you pigged out on the wrong stuff because you were hungry. When I eat fewer but nutrionally dense meals my levels also remain level (and lower) the rest of the time and without spikes following said meals. I also don’t keep spiking my insulin by initiating it so often, so am working towards reducing my insulin resistance. Insulin also acts to inhibit body fat burning encourage body fat storage. Not what I want.

What kind of levels are you seeing? Between meals, before and after? What levels are you at when you feel “hypo” symptoms? What symptoms? Are you quite sure it’s not just plain old fashion hunger or because you are accustomed to constant grazing?

Metformin isn’t associated with hypos the way other medications are. Actual, noticeably below 4mmol, hypos are quite unlikely if that is the only medication unless other, rarer, factors are involved. Non diabetics can go a little below 4 if they are hungry or exerting themselves for instance. The human body self corrects when it sees this happen8ng. Type 2 tend to overcorrect and do this very well as part of their condition. In fact metformin is designed to help limit this overcorrect, but not stop the safety mechanism altogether. Much is made of diabetes and hypos but they are caused by the treatment of the diabetes not the diabetes itself when medications and glucose are not aligned for various reasons.

It’s not unusual to have hypo like symptoms at higher levels that true hypo levels if that still represents a level lower than your body is used to. It’s called a false hypo despite feeling quite unpleasant. They go away when you reeducate your body to what the new healthier normal is and it stops panicking in response to a delayed meal and less high glucose levels.

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Hello HSSS. Thank you. Your analysis and advice is truly an eye-opener. The science behind diabetes management is more complicated than I realized. I am also realizing that doctor advice is not always good advice. For this very reason, this group is of enormous help to us ‘Newbies’, as people like you and MissMuffet, have the experience factor most doctors do not. Thank you for this Forum.

Please see my diet in my reply to MissMuffet. On average, I have a light breakfast (usually dry All Bran flakes or a healthy sandwich, and coffee), when a good size lunch, and a light supper (a sandwich or soup or salad). I then have a fruit, nuts, or yoghurt in between. I could however reduce the size of my lunch, which is normally a full plate; but when I do, I then become hungry later and eat ‘wrong’ things to curb my hunger.

Regarding my levels, when I used to check at bedtime, I was averaging between 6.5 and 8 mmol/L, but now I only check in the morning, before eating, which averages between 6.5 and 7.5 mmol/L. My last HbA1c test was around 7.1 mmol/L, which I haven’t checked this year (I will check in Dec).

When I was testing 3 times a day, I hardly dropped below 5 (which only happened after a 5 km walk) and I hardly exceeded 10 (which only happened in the evening after the odd party). My highest since I got my BG under control, was 11.3 mmol/L, and that was on New Years Eve ().

I don’t take my tester to work, so I haven’t checked my reading when I feel the symptoms, which for me, are - 1) blurred vision, 2) light headedness, 3) slight disorientation, 4) headaches, and, 5) loss of energy (like a slow-puncher on my energy-levels). I usually experience these symptoms whenever I miss my mid-morning snack or when I go beyond my normal lunch time. This happens when I am stuck in meetings and can’t step out to eat.

However, I believe I can do an extended 17-hour fast, once in a while, maybe on a Saturday when I am not at work. But how often should I try this extended period of fasting?

I hear you on the issue of insulin and how it contributes to stored calories / fat. I recently view many of Dr Jason Fung’s videos (thanks to this Forum and the thread - “What is insulin resistance”), and that’s when I started to consider intermittent fasting, which then led me to make this Post in fear of hypos.

What’s your take on me skipping my morning dose of metformin when I do my planned 17-hour fast? PS: I usually take 500g at 8 am and 500g at 8 pm. I also exercise for 1-hour between 6-7 am. I usually have my light breakfast just before 8 am.

Thanks again for your great advice and for this great Forum. It’s a pleasure hearing from everyone.

HSSS said:

We can’t advise on whether to stop your medication. Just explain how it works. Metformin is sometimes advised to be taken with food to avoid stomach upsets that may occur on empty stomachs. It doesn’t affect everyone that way and some taken it without food just fine, some all at the same time once a day, some at unevenly spaced intervals. Have you specifically been advised to take it only with food and 12 hrs apart or is that simply your preference?

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Thanks again HSSS. It’s my choice to take my meds 12 hours apart, give or take 1 or 2 hours. It was really an assumption that it should be 12 hours apart, as the medical instruction was 500g twice per day. I was taking one other tablet during the first 4-6 months, but stopped when my readings were getting too low.

I was also told to take my metformin after eating something (anything small was ok), so I usually take it after breakfast and after my evening light meal.

I hear you regarding advice on meds (which I appreciate). Thanks again for your advice and the valued guidance.

HSSS said:

Wow that’s a lot of carbs to become glucose in the digestion process. For me (and for many others in this forum and elsewhere in diabetes discussions) when tested against a meter it makes little difference if any if those carbs that turn to glucose come from brown or whole grain sources compared to white. Fruits other than berries have a lot of sugar and any kind of cereal is 3/4 carbs and totally avoided. Carbs make me more hungry and inclined to want to eat far more frequently than focusing on proteins and fats. I assume due to the rollercoaster of glucose and insulin they create and the hunger hormones stimulated as a result.

I understand some drs and organisations still advise this way of eating but increasingly many are turning away from this to advising fewer carbs from grains and starchy foods as well as less sugar. They are also adding a lot more nuance to which fats are beneficial and which are harmful. It’s a bit of a lottery which advice an individual gets now as a result.

So in what respect are you low fat? Or is it more selective which fats. If you reduce glucose causing carbs you need to replace that energy with another source or become hungry.


I’d be better off putting the meals together slightly earlier and choosing more veg rather than extra fruit.


I’d say whatever suits you. Lots of people do it different ways. Some do it daily, others occasionally, others still randomly. Your lifestyle and any other health concerns might influence what’s best for you.

Perhaps start gently by adjusting meals an hour every so often. Or by merging the snacks into just 3 meals as a first step. Or by adjusting the carb balance. Too much too soon is overwhelming for some and may cause some adjustment discomforts. Others like to dive in finding that more to their suiting.

Importantly are you testing to see what the food choices and timings are doing to your body and what levels you are actually getting independently of symptoms. Without doing that is harder to assess how changes might affect that?

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Thanks HSSS. I will take all the advice received from everyone, and as you say, I will gradually test this and that approach to see what works best for me. I will certainly consume less starch, even what I assumed was better starch (slow-release starches). I like the idea of combining my mid-snacks into single meals, to reduce the number of times I eat per day (as advised by Dr. Jason Fung).

Moving forward, I will aim for 3 meals a day plus the occasional extended fasting (during which time I will eat 2 meals), and see. I believe I now have a plan on how to rid myself of the 14 kg weight I need to shed off.

I am also realizing now, that my average BG of 7.1 mmol/L, is a safe range for me not to worry about excessively low levels during intermittent fasting. I will just monitor my readings as I test each variant to my usual program.

I also now have a better understanding on the use and effects of metformin. Thank you everyone, for all the great advice. Have an awesome day.

KennyA said:

I can certainly fast for much more than 24 hours by accident without much fuss and with no preparation. I have an extremely low carb (~20g/day) regime and I have no idea how much protein and fat I'm eating, it's not limited at all. I would normally eat one meal a day in the evening and maybe have a bit of cheese or ham/salami/sausage around 1 or 2pm. I've been doing that for nearly five years now with no ill effect. I had my annual a week or so back and HbA1c is 39, all bloods normal.

I would guess (based on my own experience) that your snacking on carb is keeping your BG levels higher than they might be, and the lack of fat in your diet is not triggering satiety, so you're staying hungry. The food you describe eating above is certainly pretty high in carb - rice, pasta, all-bran, fruit, bread etc. Dried fruits are very sugary. Have you an estimate of how much carb you're actually eating a day? many people (I'm one) find that GI is useless as a concept in practice.

You say "my levels drop" but don't say from what to what. You can get a thing called a "false hypo" when BG levels start to fall from a relatively high level - that's the body (in particular the liver) responding to a drop from what has become its "normal" but elevated BG level. For most non-diabetic people, the liver does a good job of providing only as much glucose to the body as is actually needed.

A genuine hypo would see your BG below 4mmol/l and for most T2s not on active glucose-lowering medication it's not something we would normally experience. I have once, and it's not something I ever want to repeat. But in normal day to day life I run quite happily with levels in mid-fours to low fives.

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Thank you KennyA. I didn’t realise that we can go 24 hours without eating. You also eat next to nothing, in comparison to my daily consumption.

From what I am starting to appreciate from all the replies and received advice from everyone, is that:

1. I am eating too much food. Before diabetes, I was always a big eater and my love for food, has been a weakness I am still fighting to overcome. I need to eat less. I will soon start fasting, to also allow my stomach to shrink, as maybe my years of big meals is still a factor. After fasting, I should start feeling full, sooner.

2. I have been fearing low BG levels because of all the warning messages we Newbies read and see, to a point where I figured that I rather have readings above 7 mmol/L than readings below 6 mmol/L, but now I realize that readings below 4 mmol/L, are rare for most T2’s. So I will now no longer fear hypos as I used to, but at the same time, I will be cautious and will test my readings when I venture into unchartered territories, like when I do long periods of fasting.

3. My starch intake is way too much. I was convinced that slow-release starches were not bad for us, and I paid too much trust in Low Gi products, assuming Low Gi meant good to eat. I have made a conscience decision to stop my 50/25/25 rule, and to try avoid all starches as much as possible.

4. I eat too many times in a day. I will slowly move towards 3 meals a day, and during extended fasting, 2 meals. I will do this by cutting out snacking in between my 3 meals.

5. I do not fast. I will start slowly by slipping breakfast and eating a light snack mid-morning. I will also try a 16+ hour fasting once every 2 weeks. While doing so, I will also monitor my BG readings.

6. I do not consume enough good fats and animal fats. I will try to increase my intake of fats as much as possible.

Thanks again to everyone. All the advice received to my thread, has been amazing and an enormous eye opener for me. I believe that I will now achieve my 80 kg Goal. I have more questions, but I will save it was another day. Thank you. Salani kahle (stay well).

Chris24Main said:

@LiSwati - I can only offer my own experience (based on some time of using a CGM to really understand my own responses to various foods etc)

I was convinced (also after reading some Jason Fung, as it happens) that IF was going to be the most effective route for me, alongside cutting out all starches, sugars and seed oil. For what it's worth, the history of what leads us to think in terms of "slow release starches and whole grains" is relatively new and not based on great evidence - it's much easier to simply think; "is it starchy ? then no".

I also knew that metformin should not be taken on an empty stomach, so it was going to be difficult to combine a 36 hour fast with metformin. So, my plan was to monitor the hell out of everything, prep my wife with all the details, and basically prepare a case for my GP that the fasting was better than the meds. I'm absolutely not suggesting you do that, but my thinking was that I was trying to empty my liver of stored energy periodically. Metformin stops (or reduces the livers capability for doing) that.

You can easily enough come up with a routine that matches your meds with meals though - 36 hours is not the only way to go, and there is plenty of sense in dipping your toe in.

Getting out of the habit of snacking may be just as effective; IF is really about living in a way that keeps your insulin levels low, and regular snacking very much is not going to achieve that.
Ultimately - try stuff, test and repeat - only you can decide what works best for you.

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Thank you Chris.