If you’re on a calorie-restricted diet, you might want to look away now – because scientists have discovered that losing weight will be twice as hard as you think.

Researchers from the National Institute of Diabetes and Digestive and Kidney Disease have challenged the conventional diet guidelines that advise dieters to cut 500 calories from their diet a day to see steady weight-loss.

The study claims that the advice doesn’t take metabolism levels into account, as continuous dieting and calorie counting slows the metabolism, meaning it’s harder for the body to shed weight and will eventually reach a standstill.

As a result, dieting is always going to be a struggle as cutting calories means that ultimately, it’ll take the body longer to lose weight.

"People have used this rule of thumb to predict how much weight people should lose for decades now, but it turns out to be incredibly wrong,” Dr Kevin Hall said during an annual meeting of the American Association for the Advancement of Science (AAAS), as reported by the Press Association.

"The reason it's wrong is because it doesn't account for the metabolic changes that take place when people change their diet.

"We know that if you cut the calories in somebody's diet their metabolism starts to slow down, and it slows down more and more the more weight that is lost. So eventually you'll reach a plateau.

"Some of my work has been to develop realistic mathematical models about what happens to metabolism when people start changing their diets and can we come up with some better rules and better predictions."

Researchers predicted that if a dieter slashed 100 calories from their diet a day, it would lead to a loss of just 10lbs in three years.

And 5lbs of that weight would fall off in the first year and the rest would slowly drop off over the next two years, due to a slow metabolism from constant calorie cutting.

According to NHS Choices, the recommended daily calorie amount for men is 2,500 and for women it's 2,000.

If you want to speed up your metabolism and drop those stubborn pounds, here are simple ways to do it:

Eat plenty of protein-rich foods. Research shows that around 25% of calories in a protein-rich meal may be burnt off. But make sure you choose low-fat protein foods such as lean meat, skinless chicken and low-fat dairy products.

Drink ice cold water. One calorie is burned when the body temperature is raised by one degree. This is because the stomach has to heat up cold liquid in the stomach, meaning the metabolism has to work harder and faster.

Go spicy. Adding spices to food increases the metabolic rate, as the heat from the spices keep the metabolism racing three hours after you've eaten them.

Eat breakfast. If you start the day on empty, your body goes into starvation mode, slowing the metabolism right down to preserve energy. Eating breakfast kick starts it for the day.

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• How To Stay Motivated On Your Diet

  Expert tips from <a href="http://www.slimmingworld.com/" target="_hplink">Slimming World</a> obesity expert, <strong>Dr. James Stubbs</strong> and Dr. Dr Khandee Ahnaimugan from <a href="http://www.drweightloss.com/" target="_hplink">The Weight Loss Doctor</a>.

• Get Support

  "Slimmers who join with like-minded people with similar goals and problems - whether online, in social media networks or in a local community group - are more likely to stay motivated to succeed. They benefit from sharing experiences and taking inspiration and motivation from fellow slimmers to help them lose weight. Getting support is crucial to having the tools to cope with small weight gains and stay on track without giving up," says Dr James Stubbs, obesity researcher for <a href="http://www.slimmingworld.com/" target="_hplink">Slimming World</a>.

• Don't Get Too Hung Up On Weight

  "Don't get too hung up on your weight measurements. A common cause of giving up on losing weight, is when people look at the scales and get disappointed with their progress. This all ties in with having deadlines and wanting to lose weight urgently. The scales don't always show the results of your efforts straight away. Weight also goes up and down due to other factors like hormones, hydration and your last meal. Don't read too much into one weight measurement. You should be looking at the overall trend. If you are sure that you are doing the right things (eating less and being more active) then the results will definitely come. Be patient," says Dr. Dr Khandee Ahnaimugan, from <a href="http://www.theweightlossdoctor.co.uk/" target="_hplink">The Weight Loss Doctor</a>.

• Don't Expect Perfection

  "For some reason, when people try and lose weight, they have this belief that they need to stick to their new regime perfectly. This is part of the diet mentality and it is very harmful. It means that people who have a "bad day" often feel like they have failed. And in the worst scenario it makes them want to give up. But this expectation of perfection is totally unrealistic. You should expect to have "bad days". Don't beat yourself up over them. A normal life includes days when you eat a bit more and days when you eat a bit less. The main thing to remember is that after days when you have a bit more, you need to get back on track as soon as possible," says Dr Khandee Ahnaimugan, from <a href="http://www.theweightlossdoctor.co.uk/" target="_hplink">The Weight Loss Doctor</a>.

• Do It Yourself

  "Do it for yourself: Being told to lose weight by someone else scores low as a motivator. Although being told by your GP that your health is at risk can be the shock that sets you on the weight loss road. "Setting your own target weight and losing weight for the reasons that suit you, when they suit you makes all the difference to success. Getting praise from fellow slimmers or colleagues for weight loss achievements is a great boost to help stay on track because it gives a sense of achievement, so spurring you on," says Dr James Stubbs, obesity researcher at <a href="http://www.slimmingworld.com/" target="_hplink">Slimming World</a>.

• Make It Easy For Yourself

  "Make things as easy as possible. It might not sound like a revelation, but the more unpleasant you make your weight loss programme, the less likely you will stick to it. This seems like common sense but so many people still believe in the "no pain - no gain" approach. This might work for a few people, but for most of us, we are much more likely to succeed if we make things as pain-free as possible. How do you do that? Make small changes each week that you know you can maintain. Instead of setting the bar too high and failing, if you make small changes each week, you get a track record of success behind you," says Dr. Dr Khandee Ahnaimugan from <a href="http://www.theweightlossdoctor.co.uk/" target="_hplink">The Weight Loss Doctor</a>.