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Most diets work, says Dr Rachel Brown of the department of human nutrition at University of Otago.
''If you want to lose weight for a wedding, go on keto and you will, but by your honeymoon you'll be putting it back on.''
That is the problem with so many trendy diets. They rebound in the long term, she says.
''The problem with what we call 'fad diets' is you do lose weight, but it's not sustained weight loss. Studies have shown 90% or more of people who go on these fad diets regain it within one or two years because they go back to their old eating and exercise patterns.
''At the end of the day, all these diets work primarily because they are reducing your calories, often by cutting out whole food groups.''
Ketogenic diets, and similar ones such as the Atkins, restrict carbohydrates. Paleo does as well but it has the advantage of recommending whole, unprocessed foods, although it avoids grains and pulses. Paleo claims to follow what Stone Age people ate, although if you look into it that's quite controversial. You can go on a whole-food diet without going paleo, she says.
A ketogenic diet promotes ketosis, which occurs when you have a very low intake of carbohydrates.
''Your brain uses blood sugar and so when you don't have a lot of carb stores and intake of carbohydrate for your brain to run on, it will start making what we call ketone bodies. These ketone bodies can act as a fuel for the brain and other things. But to me, any changes you have in metabolism are far outweighed by the reduction in calories because you are eating fewer calories,'' she explained.
A ketogenic diet is hard to follow, especially if you want to eat out or snack. You could go for nuts, but if you wanted fish and chips, you could have the fish but not the batter or chips.
Because these diets restrict a major food group, carbohydrates, they include a lot of meat and saturated fat, so are neither heart nor environmentally friendly, she said.
''Most nutritional scientists worth their weight would say that saturated fat is not a healthy fat to be predominantly in your diet, though you have to have some.''
The confusion comes when all fats and all carbohydrates are lumped together as bad without realising that there are healthy fats, such as unsaturated ones, as well as unhealthy saturated fats, and healthy carbohydrates from vegetables and whole grains, as well as unhealthy, refined carbohydrates, such as sugar and white flour.
The important thing if you want to lose weight long-term is to find a diet that you can enjoy and follow for a long time, and that usually needs lifestyle changes, she says.
Realising people could lose weight on any diet as long as they adhered to it, they looked at what support strategies could be put in place. Their study looked at intermittent fasting and paleo and Mediterranean diets, the subjects being people who were really motivated to lose weight, she said.
''Everybody was gung-ho to start with. They did lose weight for that first part. But then again with paleo, because you are cutting out major food groups and it is a little more strict, people did not adhere to it for long.''
The intermittent fasting diet - two days of very low calories and five days of normal eating - worked well for some people who didn't like reducing every day and it suited their lifestyle, Dr Brown said.
''It might be a bit more sustainable because you're not cutting out food groups and you can swap your low days, so if you're going out for dinner you can have your low day another time. But when you followed them for a year, people started to regain weight again.''
Fasting didn't suit others at all because they hated going that low, and some compensated and ate more on the normal days.
The Mediterranean diet was more of a lifestyle change. It's not very easy to follow closely in New Zealand, although as we globalise our eating patterns things are changing, she said.
Although it is not actually a weight-loss diet, people do lose weight on it and it's heart-healthy. It's also a relatively high in good fats because it contains olive oil and nuts, but not a lot of meat.
''They have lovely bread they dip in olive oil to eat. A few years ago we would never have done that. We put margarine or butter on our bread. It's also quite plant based, a little bit of red meat, some fish, but very plant based,'' she said.
A plant-based diet, a current trend especially among younger people, is environmentally and animal friendly, and can be very healthy and lead to weight loss if done carefully. However, if it includes a lot of baked goods instead of vegetables, pulses, whole grains and fruit, it can be quite unhealthy.
So to lose weight and keep it off, people need to find a way of sustainably reducing their calories in the long term, she says.
''The hardest thing with weight loss is as soon as you start to lose weight you need less energy or calories to fuel yourself because you are smaller. That's the Catch-22 situation.''
US studies had looked at people who successfully lost weight and kept it off for 10 years and found they had a moderate but not high-fat diet that was somewhat low in calories and high in fruit and vegetables.
Also they didn't watch as much television and they did some exercise. It was a whole lifestyle change, she said.
''I guess putting the onus on the individual is the problem because we are humans and we find change hard, so probably changing the environment is what's important.
''If you make an environment where it's easy to cycle or walk and you aren't going to get run over, and where good food is readily available, is easy to get hold of and reasonably priced, that's what will work.''
Nutrition and public health experts are now saying that years of trying to change what people eat and how they exercise has not worked as two-thirds of the population still weigh more than they should.
Now people realise if we are going to seriously deal with the obesity epidemic it has to be as a result of government policy and changing the environment, she says.
''That's where you'll get the most benefit, but it's also the hardest thing to do, politically. Some governments' philosophy is not that way at all, while others are a little bit more like that, but they still will come up against the food industry.''
She points out that when Cuba went through a financial crisis in the early 1990s, when they didn't get aid from the Soviet Union, the population lost on average 5.5kg because food and oil were restricted. People ate less and walked or biked.
''That shows you - not that I'm saying we need to do that - but if you do change the environment you can get change in people's weight.''