"Horrible" - is how Niklas Ekstedt sums things up when asked how he's fared during the pandemic.

"It's so difficult, because I'm used to travelling all the time and meeting people from all over the world... and now I'm just stuck," admits the Swedish chef, 42.

There are some upsides to being stuck at home though, Ekstedt tells me on the phone from Stockholm, where he's on the way to pick up his eight-year-old son from school and it's already gone dark at two in the afternoon.

"I'm really now sure that I've met the love of my life," he says of wife Katarina. "You know we haven't had one fight during this time and we spent all this time together. I made the right choice there."

Not that it's non-stop matrimonial bliss in the Ekstedt household, however. "My wife gets crazy when she opens the refrigerator and it's filled with cans that smell, she's like, 'What is this?!'"

With more time on his hands, the chef and author has recently got very into fermenting (which is why the family fridge is overflowing with vegetable-filled receptacles), inspired by what he learned while working on his latest book, Happy Food For Life, alongside journalist Henrik Ennart.

The follow-up to bestselling Happy Food, once again it focuses on how what we eat can affect how we feel, combining the latest research with health-boosting recipes.

"The research within that field has [moved on] so dramatically quickly and so many new things have come up. So we thought we would make an update and make it a little more modern," Ekstedt explains.

Now well-versed in the importance of nourishing one's gut bacteria with a varied diet and fermented foods, the cook says he "wasn't aware at all" before he teamed up with Ennart.

"I knew nothing, it's been a very personal journey," he reveals, explaining he's had to pay more attention to his diet as he's got older.

"When you were in your 20s, you could just consume - it just vanished - but now you quickly gain weight and it's more difficult to get rid of it.

"Also you feel more depressed, at least I feel more affected by bad food. I really need to eat healthy food to stay alert and be ready for battle."

So what's his key advice for staying fit in mind and body?

"I think the secret to a healthy lifestyle is to eat a lot of different things. Broaden your variety of foods when you shop, so don't get stuck on just one thing, and also you discover new things that you might find delicious."

Ekstedt and Ennart believe a largely plant-based diet, using whole ingredients rather than processed foods, is generally best - but they don't say you have to cut out meat altogether, unless that's your preference.

In a bid to choose more plant-based meals, Ekstedt originally started eating more salad, but quickly realised the leafy dishes didn't give him enough energy.

"Less salad and more food," was what he was craving. "I needed to complement my meal with some something warm. So soup is really good. Also use a lot of spice when you eat plant-based, like chili sauce, so you feel alive when you eat it."

Variety is key, but the veggies don't have to be fresh from the market every day.

"We have this almost obsessive thing about everything's supposed to be fresh," Ekstedt says, "and it's gone to the level that it's almost not good for us, because we forgot the way to can and preserve and ferment foods."

And while a surplus of sugar and fats isn't advisable, there's no reason you can't indulge in a bit of what you fancy every now and again. In fact, Ekstedt says it's OK to "go crazy" when you treat yourself, as long as it's not too often.

"That's something we really apply here at home with the family, so once a week we do this, like, massive dessert or chocolate party!"

With the chef's Stockholm restaurant, called Ekstedt, closed for two months in the summer, he says he relished having more time at home with his sons, aged eight and 12, but was sad not to be able to travel to the UK for a holiday as planned.

"I'd like to go to Cornwall. We were planning a trip there this summer but we never went. I'm usually busy when I'm over there [in the UK], it's my second home."

Apart from seeing his parents for the first time in months, Ekstedt says travelling is what he's most looking forward to when the pandemic is over.

"I think it's just the mental planning itself. Going online and Googling a travel destination or hotel and knowing you can actually go there will be mentally releasing."

In the meantime, he's enjoying evangelising about the mood-boosting powers of a plant and grain-heavy diet and isn't afraid to speak out about the dangers of junk food - even when faced with criticism.

"Before, I just cooked delicious food at restaurants and got awards and attention and went on TV and stuff. And then suddenly when you cook something and you tell people that this can actually make you feel better, this could increase your longevity, people get really interested, but also you get criticised for it," he says.

"It's kind of like our war. If it was tobacco companies in the Sixties and Seventies, we're now fighting the food companies."

Ultimately, Ekstedt is committed to teaching people that a healthy diet doesn't have to be bland or boring.

"I think we need to highlight these things in a very positive way, in a very delicious way."

Happy Food For Life by Niklas Ekstedt and Henrik Ennart, photography by David Loftus, is published by Bloomsbury Publishing, priced £22. Available now (bloomsbury.com).