I thought the sardine diet would reset my health – the only result was constipation

A lot of my algorithm is health information – and the weirder the claims, the more I am drawn to them. The more I click, the faster they come and the stranger they get.

Recently, I noticed one trend spreading like no other: the sardine diet, during which you eat nothing but sardines – tinned or fresh – for a week. You can eat them with extra virgin olive oil, lemon, zero-calorie chilli sauce, salt and pepper, but that’s it. Black coffee with sweetener is okay, water with a spritz of lime is allowed, and coconut water for minerals and electrolytes is heavily recommended.

I’m suddenly getting 10 different reels a day on Instagram, all with a heavy dose of hyperbole. Apparently, the diet will heal, act as “a metabolic re-setter”, make you “superhuman”, even “open up your pineal gland”. There are also claims it is “the fastest route to ketosis” – when your body burns fat for energy instead of glucose – and that it results in “improved heart health”, “improved thyroid function”, “better bone health” “antiaging for skin” and “fast inflammation reduction”.

Normally, I can sniff out snake oil, but I know enough about omegas and high DHA doses to know there is some science underpinning this. I’ve been part of several ADHD clinical trials, and while those studies were over six months and not five days, I’ve seen first-hand the impact omegas can have on the body and brain. So the idea that you could somehow reset your body in five days with the internet’s latest superfood intrigued me.

There are, of course, other considerations with an experiment like this: mainly, the fact that any extreme diet should come with alarm bells attached. But, in the name of “research”, I decide to crack on. The day before I start, I speak to my friend, nutritionist Rosemary Ferguson, whose overall perspective is that, while an influx of omegas could help with some of what’s being suggested online, it’s unlikely that five days alone will trigger anything dramatic. But also warned that any extreme diet with such a low calorie intake (about 650-800 calories a day) would be a dangerous zone to be in for five days.

Rose tells me to get bloods done before and after the experiment and to look at markers like beta-hydroxybutyrate (BHB), which measures ketosis, fasting glucose and insulin levels, inflammation markers, Omega-3 to Omega-6 ratios, Vitamin D and selenium levels.

The plan is simple: one set of bloods before the fast and one set afterwards. So off I go to get my blood drawn and buy keto sticks to pee on, which will tell me if my body has actually entered ketosis.

Day One 

I wake up enthusiastic. With my broken arm, anything that might reduce inflammation and help build my bones back up feels like a lifeline. Plus, if I can lose a bit of weight and relieve some pressure on my wrist, that’s a bonus.

I’ve been sent 15 cans of different flavours by the high-end Tinned Fish Company, which makes this feel less like punishment and more like some weird fish advent calendar.

I open the first; smoked sardines with salt. They’re absolutely delicious, and I eat them exactly as they are. By the end of day one, I’m relieved, excited and surprisingly optimistic. I feel full, satisfied and annoyingly virtuous. Unsurprisingly though, I’m not in ketosis yet.

Day Two 

I wake up still enthused about the day ahead. The quality of the tins is helping: every meal feels like another little fishy surprise. I add lemon, sea salt and, in some cases, chilli sauce. I also start drinking raw coconut water, to hit the sweet spot I suddenly miss.

My first (pre-sardine diet) blood results come in. I call Dr. Nadja Auerbach MBBS BSC, who runs me through my results. Generally, my health markers look pretty good. The only marker that really stands out is my LDL cholesterol (fair enough; I am fundamentally a glutton) but it’s still within a healthy range.

My ferritin levels are also elevated, which can happen with inflammation and is apparently pretty normal when you’ve got a broken bone. Dr. Nadja also said it would be a pointless exercise, but I’m still going to plough on. Plus, I’m still feeling pretty good – but there are no signs of ketosis yet and I’m starting to get slightly bored of sardines.

Day Three 

I wake up feeling a little less excited. I have my first tin and it’s fine. But then I forget to bring sardines with me to my meeting in Soho. I scan menus everywhere and can’t find any; I think fresh sardines must be slightly out of season.

Eventually I end up in sushi restaurant Taro and order tuna and tuna belly sashimi instead, four pieces of each, before discovering miso soup is apparently allowed, along with seaweed, soy and wasabi. It is an absolute breath of fresh air. Raw fish is less fishy than canned sardines and I’m back in the room. And yes, I am aware I’m drifting away from “the sardine diet”, and I do feel guilty. Especially because a lot of the supposed health magic comes from eating the bones and getting all their calcium and minerals.

On the way home, I manage to find fresh sardines, and we grill them for dinner with really good olive oil; the difference between fresh and canned is just a whole world of deliciousness. The char of the grill adds a smokiness that changes everything and just the act of cooking something improves my mood dramatically. I keep up the bone broth, coconut water, lemon water and black coffee, but I’m definitely feeling more emotionally fragile around food.

Day Four 

I wake up and instantly know something has changed: just the thought of sardines for breakfast makes me gag. And then, in some cruel bit of sod’s law, the first tin I open is absolutely gnarly. There’s part of the fish’s face still attached, visible gills, and this thick extended spine running through the middle of it.

I take one bite and crunch through huge bones and weird flappy cartilage bits and immediately start retching. I manage about a third of the tin before giving up. I end up in such a bad mood that I barely eat all day. Instead, I panic-dose myself with DHA oils, krill oil and salmon oils instead, like some sort of desperate omega maniac.

By dinner I cave and eat a piece of salmon with crispy skin just to get through the evening. I know I’m cheating. The only upside is that I’m finally in full-blown ketosis and I’ve lost about half a kilo, although frankly I feel like I deserve more after the psychological warfare I’ve been through.

Day Five 

Today I’m off to the BAFTAs. I also realise I haven’t actually been to the loo all week. Even though I’m slightly lighter and properly in ketosis now, I feel incredibly bloated. The fact I haven’t gone to the loo becomes a full-blown obsession. I get ready at the Corinthia London and while the dress technically fits, it’s not exactly the image I was going for. I look pregnant.

I eat a tin of sardines in the morning and even bring another tin with me to eat before the red carpet, but I just cannot get it down. My friend orders a prawn cocktail and I steal some with a bit of avocado, which feels wildly rebellious at this point.

I get home after the BAFTAs, my arm is absolutely killing me, and somehow I still force down two more tins of sardines before falling asleep praying for the next day to arrive. The whole thing has been thoroughly miserable.

It’s ruined something that should’ve been really fun and I don’t even feel or look good for it.

The aftermath 

On day six I weigh myself. I haven’t even lost a kilo. Over the next two days, once I reintroduce fibre and my digestion finally restarts, I lose about 3kg almost immediately, which tells me most of this was constipation and water retention rather than some miraculous metabolic transformation.

Two days later my final blood results come in. And… almost nothing has changed. My general health markers are basically identical. The only thing that’s meaningfully shifted is my blood sugar, which was expected. Even my omega markers barely move, which I suspect is because I already take so many omega supplements and DHA oils that this experiment barely touched the sides. Mostly, I feel like I’ve lost time, money and a genuine love of sardines.

We’re living in a strange new landscape, where increasingly vulnerable people are getting dietary advice from an endless loop of engagement farming and wellness hysteria disguised as health optimisation. Just because AI or your algorithm becomes fixated on something does not mean we should trust it, no matter how convincing the evidence might appear.

Eating oily fish is genuinely important for human health, but once or twice a week, not as part of some bizarre internet endurance challenge. It was objectively stupid – but now I’ve done it, hopefully you don’t have to.