Lets start with definitions.
The Oxford Dictionary online defines diet as follows:
Verb "To restrict oneself to small amounts or special kinds of food in order to lose weight".
Noun "A special course of food to which a person restricts themselves, either to lose weight or for medical reasons".
So, in the context of this blog post we'll eliminate dieting for medical reasons and focus on dietary restrictions or courses of food undertaken with the aim of losing weight.
The next question is 'how do we define if a diet has worked'?
Well, for something to have worked it must have achieved the desired result or effect. For most people dieting, this would obviously mean achieving some degree of weight loss. How much weight loss is acceptable for one person or another is open to question but fundamentally for a diet to have worked, weight loss (what ever the amount) should have occurred.
So back to the original question, 'do diets work?' or to be more specific, 'does the modification of the amount and/or type of food we eat lead to weight loss?
The simple answer is most of the time, yes.
Following any diet closely usually leads to a reduction in calorie intake and calorie restriction, ideally combined with exercise (i.e. increase in energy expenditure) will lead to a loss of body weight. And when I say any diet I mean any diet. I could put someone on an ice cream diet and they would likely lose weight as long as the calorie intake was sufficiently small! Of course as you can see from this example none of the above speaks to the health implications of a particular diet or whether weight loss occurs over an appropriate period of time or is maintained in the long term and this is the crux of the matter.
In the world of diets, weight loss is the selling point and as I've illustrated any and all diets can achieve this. This is why we've had the Atkins diet, the cabbage soup diet, the blood type diet, the paleo diet, the zone diet, high fat, low fat, high carb, low carb, high protein, the list goes on and on. It doesn't matter which approach you take you can achieve weight loss and as a large percentage of the population are overweight dieting is big business.
Unfortunately most of the diets commonly found in the best sellers list are really just quick fixes. If you follow them you'll lose weight in the short term. Most however require such significant changes to dietary habits that they are impossible to maintain and could even be detrimental to health over a long period. This means that more often than not people return to their old dietary habits and any weight lost is regained. The purveyors of fad diets don't care though because this means there's always another diet to invent and book to sell!
For a diet to really be deemed successful (to have worked) any weight lost shouldn't be regained. For this to happen dietary and exercise modifications should be such that they can be maintained in the long term. This is easier said than done and all the evidence suggests the quick fix alternatives don't lead to the desired outcome.
I wont go into great detail but here's a general summary of the evidence based recommendations for diet success;
General recommendation: Weight loss programs should target both eating and exercise behaviours as energy/diet restriction combined with physical activity (energy expenditure) increases weight loss as compared to diet alone.
Dietary restriction: A reduction of 500-1000 kcals/day should be targeted to produce 0.5-1 kg weight loss per week.
Exercise duration and energy expenditure: 150-250 minutes/week of moderate intensity physical activity should be targeted, with the evidence suggesting that >250 minutes per week is necessary to prevent weight regain. >2000 kcal a week energy expenditure should be targeted during the time spent exercising.
Types of Exercise: It's recommended that resistance and cardiovascular exercise be included in an exercise regime for weight loss.
As a side note; for long term weight management it is also important that dietary modifications are easily manageable. To this end food should be acceptable in terms of cost, ease of acquisition and ease of preparation. It shouldn't be a chore to follow a diet. Dietary changes should also be acceptable from a socio-cultural perspective.
So, going back to my original question "do diets work?" My answer is no they do not. The lions share of evidence points towards lifestyle modification along the lines described above for success. Unfortunately however there's no gimmick here, no trendy tag line, no easy fix. Losing weight and/or keeping it off is I'm afraid, a lifelong project.
Position of the American Dietetic Association: Weight Management JOURNAL OF THE ACADEMY OF NUTRITION AND DIETETICS February 2009 Volume 109, Issue 2, Pages 330–346
Appropriate Intervention Strategies for Weight Loss and Prevention of Weight Regain for Adults MEDICINE & SCIENCE IN SPORTS AND EXERCISE Dec 2001 - Volume 33-Issue 12- pp 2145-2156
Appropriate Physical Activity Intervention Strategies for Weight Loss and Prevention of Weight Regain for Adults. MEDICINE & SCIENCE IN SPORTS AND EXERCISE Feb 2009- Vol 41- issue 2- pp 459-471
Popular Diets: A Scientific Review, Freedman, King, and Kennedy. 2S OBESITY RESEARCH Vol. 9 Suppl. 1 March 2001