We’re into the annual national purge, binning the biscuits and embracing the latest fad diet in a bid to lose the weight that crept on over Christmas. We’ve been here before. Like last year, when we almost returned our body to prime shape, only to undo it all on holiday by eating too many tacos in Tulum.

What is the solution to healthy, long-term weight loss? Dr Michael Greger, author of How Not To Diet, has spent years researching the answer. In the 570-page book, he has mined scientific research and data to reveal the most effective ways to lose weight. To help give you a headstart in 2020, we have cherry picked his most salient pieces of advice.

Netflix documentary The Game Changers might have courted controversy over some of its claims, but Dr Greger found that a diet low on meat, dairy, eggs and processed food was one of the most successful ways to achieve long-term weight loss. “A single meal high in animal protein can nearly double the level of stress hormone cortisol in the blood within a half hour of consumption,” he says. Chronic high cortisol levels increase obesity risk and predict cardiovascular death. “Switch to a lower-protein diet centred around bread, fruits and vegetables and cortisol levels drop about a quarter within 10 days.” Studies have also found a plant-based diet easier to adhere to perhaps because, as one study showed, “participants felt so good eating healthfully, they were losing weight without any calorie counting or portion control”.

The most studied form of intermittent fasting is alternate-day fasting, where you limit yourself to eat only every other day. “The greatest rate of breakdown and burning of fat over a three-day fast happens between the hours of 18 and 24 of the 72-hour period,” says Dr Greger. In an eight-week study, where obese subjects were restricted to 500 calories every other day, they reported very little hunger after two weeks on the diet and they each lost about 12lb. A note of caution: in a one-year study of three diet groups – alternate-day fasting, daily restricted calories and a control group of regular eating – those who fasted were found to have a 10 per cent higher rate of LDL cholesterol, a prime risk factor for heart disease. If that doesn’t put you off, “I would advise you to have your cholesterol monitored to make sure it comes down with your weight,” says Dr Greger.

A study found that overweight and obese men and women who drank two cups of water before each meal lost nearly 5lb more body fat in 12 weeks than those in the control group. A similar trial found that about one in four in the water group lost 5 per cent of their body weight compared with one in 20 in the control group. Water, especially when it’s ice-cold, helps to “rev up” the noradrenaline nerves, which gives you a metabolic boost. “Never drink more than three cups in an hour,” warns Dr Greger, “since that starts to exceed the amount of fluid our kidneys can handle.”

The next time a colleague evangelises about the low-carb, high fat ketogenic diet, tell them about this research. A 2012 study by America’s Nutrition Science Initiative found that while keto dieters lost weight according to the scales, “their rates of body fat loss slowed by more than half, so most of what they were losing was water”, says Dr Greger. In another experiment, dieters achieved 68 per cent more daily fat loss when they cut down on fat instead of carbs. “A meta-analysis of 32 controlled feeding studies swapping fat and carbs found the same thing,” says Dr Greger. “Less fat in the mouth means less fat on the hips, even when taking in the same number of calories.”

Unlike the inflammation you get from, say, a splinter, “chronic inflammation is persistent, systemic and non-specific and it appears to perpetuate disease”, says Dr Greger. The biggest driver of this inflammation, which can be gauged with a blood test, is what we eat. A study found that people who eat more pro-inflammatory foods, the worst of which are saturated and trans-fat, had a higher annual weight gain and those on the most inflammatory diets had a 32 per cent increased risk of becoming overweight or obese in an eight-year period. Pro-inflammatory foods damage the “appetite-regulating apparatus in our brains that can lead to, and sustain, obesity”, says Dr Greger. Replace pro-inflammatory foods such as cakes, cheese, ice cream and burgers with anti-inflammatory legumes, herbs, vegetables and fruit.

How Not To Diet: The Groundbreaking Science Of Healthy, Permanent Weight Loss by Dr Michael Greger, published by Bluebird, is out now