Calorie restriction

The three major macronutrients: protein, fat and carbohydrate provide different amounts of energy to the body for every gram eaten. Protein yields 17 KJ/g, fat yields 37 KJ/g while carbohydrate yields 16 KJ/g.

Due to these differences in energy content, arguments have raged concerning how to structure an optimal diet for weight loss – should it be a low fat high carbohydrate diet since fat provides so many calories or is a high protein diet a better strategy?

However, when considerations of overall energy balance are brought into play and diets made up of quite varied mixtures of macronutrients are compared on a strict basis of energy equivalence (eg different diets all providing equivalent energy deficit) then the evidence clearly shows that all diets compared in this way promote similar weight loss.

Total calorie intake and weight loss

In one large study, 8112 overweight adults were randomised to one of four different diets: high carbohydrate, increased protein, high protein or a high fat diet.

After two years following these diets and maintaining a similar energy restriction (deficit of 3.2 MJ/day) the average weight loss remained similar regardless of the composition of each diet.

This study suggests that it’s the energy deficit not the diet composition that controls the amount of weight loss.

Advantages of calorie restriction

Recent studies in ageing research in many different animals conclude that eating fewer calories results in a longer, healthier life. Further studies are now needed in humans, although research is made difficult by the long length of human life.

Importantly, any reduced energy diet needs to have complete nutritional adequacy (with high nutrient density) since any missing essential nutrients will be detrimental to overall metabolic health or have other ill effects such as contributing to the deterioration of bone structure.

When counting calorie is too difficult

Many people follow diets without precisely counting calories every day. It is just too demanding and difficult. If they abandon a strict calorie-counting regime, their intake is said to become ad libitum, that is, they eat as they please while trying to restrict food intake.

Without imposition of a defined energy deficit to make different diets have equal impact, many other factors can now freely influence food intake: appetite, physical activity level, food energy density, food palatability, portion size, social context during eating, food availability and also the composition of the diet chosen (protein, fat and dietary fibre content and the quality and glycaemic impact of carbohydrates).

Under ad libitum conditions some studies have shown high protein diets to have advantages. The optimal ad libitum diet may however vary for different population groups.

High protein diets may have good satiating and weight loss effects in healthy people but have adverse effects in people with deteriorating kidney function (eg people with type 2 diabetes).

People with impaired glycaemia may do better on a Mediterranean diet with lower carbohydrate content than on a low fat diet where carbohydrate is of poor quality.

Compromise fasting solutions

If daily energy restriction is too difficult and ad libitum intake is too relaxed can a third way be devised? Interest is growing in the alternative approach provided by intermittent fasting or alternate day fasting.

In 2013, Michael Mosley detailed a popular 5:2 intermittent fasting plan in which participants restrict energy intake to 25% of normal level (fast) for two days each week and then continue to eat normally on the other five days of the week. Studies in animals suggest that alternate day fasting provides the greater degree of energy restriction and more profound metabolic effects.

Results from initial trials in overweight people suggest that alternate day fasting for 8 to 12 weeks can result in a 5-6% reduction in body weight with reduction of visceral fat mass, reduced heart disease risk and preservation of lean tissue.

One recent study in obese people randomised to either an alternate day fasting diet or to a diet with a daily energy deficit of 1.7MJ/day found similar weight loss with both approaches (around 8kg drop in weight) that weight regain was also similar during 24 weeks of unsupervised follow-up.

Recently a further study also indicated that alternate day fasting results in similar weight loss to a diet with a daily energy deficit.

Are there long term consequences?

Although alternate day fasting may be effective for weight loss, questions have been raised however, about its safety including whether fasting increases gastrointestinal disturbance, affects sleep, and impacts adversely on blood sugar levels or whether it might promote disordered eating behaviors or poor body image perception.

A recent study has countered some of these concerns, with the finding that when obese patients followed an alternate day fasting diet for a period of 8 weeks, the diet reduced levels of depression and binge eating although fear of fatness remained unchanged.

Adverse events reported by a few participants in this study included dizziness, constipation, water retention, weakness and bad breath.