New approaches to dieting and eating: what are they and do they work?

Fad diets come and go and you may have been approached by the general public for advice about them. Rod Tucker compares two popular diets

Apple and measuring tape

Apple and measuring tape (Natalia Merzlyakova/

Source: (Natalia Merzlyakova/

In the past 20 years obesity levels have risen across the globe and become a major public health issue. The World Health Organization (WHO) states that obesity is an “abnormal or excessive accumulation of fat that might impair health”1 and defines it in terms of the body mass index. A person with a BMI greater than 25 is classed as overweight and obese when their BMI reaches or exceeds 30. In England, the prevalence of obesity in adults has risen from 13 per cent in 1993 to 24 per cent in 2011.2 Moreover, WHO estimates that 2.8 million people die annually as a consequence of being overweight or obese.1 Obesity is also considered a major risk factor for the development of cardiovascular disease, stroke, diabetes, certain cancers and musculoskeletal conditions such as osteoarthritis.

The body obtains energy in the form of calories from food. For instance, one gram of carbohydrate and protein each contain four calories whereas a gram of fat contains nine calories. Energy is required to drive the various biological systems necessary for survival and this is termed the “resting energy expenditure” and accounts for roughly 60 per cent of the body’s total energy needs. In addition, approximately 10 per cent of the total energy is necessary to digest, absorb and metabolise food and the final 30 per cent is used to facilitate any form of movement which includes bending, lifting and reaching, etc, as well as physical exercise.3

General advice on losing weight is predicated on the notion that the body is a closed thermodynamic system such that the energy of the system remains constant and can be represented by:

Excess stored energy = energy in – energy out

If energy intake exceeds energy output, the excess is stored as fat. Consequently, to reduce sur energy stores, it is necessary either to lower energy intake (eating less) or increasing energy expenditure (moving more).

Although this basic premise is relatively straightforward, unfortunately, there is a vast array of dieting options available and patients can be easily drawn towards popular diets advertised in the media or endorsed by celebrities. Two approaches that appear to be fashionable at the moment are the low carbohydrate diet and intermittent fasting. But is there a scientific rational for each approach?

This article provides an overview of the evidence to determine whether either of these dieting regimens is worth pursuing.

Low carbohydrate diets

The term “low carbohydrate” (LC) is ill-defined but generally involves limiting carbohydrate intake to between 6 and 45 per cent of total calories. For example, for someone consuming 2,000 calories/day, a moderate carbohydrate intake will be 225–300g/day (45–60 per cent of energy intake). An LC diet will be less than 130g/day (26 per cent of energy) and a very LC diet is defined as fewer than 30g/day (6 per cent of energy). At such a low level of carbohydrate intake the body is forced to burn fats as a source of fuel, giving rise to ß-hydroxybutyric acid, a ketone body which can be used as a source of fuel for any tissues that possess mitochondria, including muscles and the brain. In fact such ketogenic diets have been used in the management of epilepsy in children.4

Robert Atkins, in his book ‘Dr Atkins’ diet revolution’ (1972), popularised weight loss through carbohydrate restriction but the idea of LC diets can be traced back to 1869 when William Banting, an English undertaker, published his “letter on corpulence” in which he described how he lost 46 lbs over 12 months through avoiding “bread, butter, milk, sugar, beer, and potatoes”.5 Atkins himself developed his diet on the work of Alfred Pennington who had suggested in 1953 that unrestricted calorie diets based on fat and protein and limiting carbohydrates were a potential treatment for obesity.6

Many clinical studies comparing LC with low fat diets have shown that those assigned to LC diets experience weight loss and reductions in cardiovascular risk. Typically, patients assigned to an LC diet experience an increase in HDL cholesterol, a reduction in plasma triglycerides and a decrease in blood pressure despite an increased consumption of saturated fat. Meta-analyses of randomised LC diet trials lasting at least 12 months have confirmed these benefits. One review in 2009 concluded that “low carbohydrate/high fat diets are more effective at six months and are as effective, if not more so, as low fat diets at 12 months in reducing weight and cardiovascular disease risk at one year.”7 A more recent review (up to August 2012) came to the same conclusion.8

An LC diet has been advocated as valuable for patients with type 2 diabetes and, in a recent systematic review of dietary interventions for type 2 diabetic patients, it was found that an LC diet (among others such as a low glycaemic index and high protein diet) led to improvements in cardiovascular risk factors compared with control diets.9

Harm associated with LC diets

LC diets involve consumption of higher amounts of fat, including saturated fat, which has been associated with an increased risk of cardiovascular disease. However, a recent meta-analysis of prospective cohort studies has largely exonerated the role saturated fat has as a risk factor for cardiovascular disease.10 Concern has also been expressed that the high level of protein intake in an LC diet could adversely affect kidney function, although this does not appear to be a problem in practice. In a 24-week study in 83 obese patients consuming 30g/day of carbohydrate, it was observed that there was no significant change in creatinine and urea levels.11 Although these results are reassuring, one recently published prospective cohort study in Sweden following over 43,000 women for 15 years found that a decrease in carbohydrate intake was associated with an increase in cardiovascular disease incidence.12

Intermittent fasting

In the 1930s experiments in rats demonstrated that calorie restriction prolonged life13 and successive studies with other species have confirmed these findings although a recently published 23-year follow-up study in monkeys found that calorie restriction did not increase life-span.14 Nevertheless, there is evidence that calorie restriction of between 15 and 40 per cent in both humans and animals leads to improvements in health outcomes such as reductions in blood pressure, heart rate and improved insulin sensitivity.15,16 Calorie restriction can be difficult to maintain over time and a modification of the protocol has been the use of alternate day fasting in which participants have feast days alternating with fast days. Strictly speaking, fast days do not necessarily involve complete absence of food. Some studies allow 15 per cent calorie intake while others only allow calorie-free liquids.

In a study in 2005, 16 non-obese men and women fasted completely (apart from energy-free liquids) every other day for 22 days and lost a mean of 2.5 per cent of their body weight.17 In a more recent study, 16 obese subjects undertook a 10-week alternate fasting trial consuming 25 per cent  of their energy needs on the fast days. At the end of the trial, body weight decreased by a mean of 5.1kg.18

A variation on the alternate day fasting is the 5:2 diet first promoted in a BBC Horizon programme in 2012 and, more recently, in the book ‘Fast diet’ (2012) by Michael Mosley. Despite the popularity of the diet, there is little evidence to support it, although recently a study has explored consecutive day fasting. In the study 107 overweight or obese women were assigned to either daily calorie restriction of 25 per cent or a 75 per cent reduction on two consecutive days per week for a total of six months. At the end of the trial weight loss was not significantly different (approximately 6kg) between the two groups and there were similar improvements in other health markers such as blood pressure and insulin sensitivity.19 Intermittent fasting is potentially valuable for type 2 diabetes as demonstrated in a recent pilot study in which patients experienced reversal of their condition by calorie restriction.20

Harms associated with intermittent fasting

There are no studies that have specifically considered the potential harms due to fasting. According to NHS Choices, fasting may cause heartburn and it is not advised for type 1 diabetic patients and may lead to dehydration (unless fluid intake is maintained) and constipation.21 Further research is needed to establish the range of adverse effects incurred by fasting.


Both approaches to weight loss are supported by clinical studies and, although superficially different, in practice, both involve reducing calorie intake. For example, because protein satiates the appetite more than carbohydrate, people on LC diets reduce calorie intake.22 A good deal of evidence suggests that it is weight loss per se, irrespective of the diet used, that leads to the reductions in cardiovascular risk and health improvements observed with both diets.23,24 Yet the ultimate challenge is to maintain weight loss. In a systematic review of weight loss maintenance in obese and overweight patients, it was found that on average, interventions led to 9.5 per cent loss of body weight but, after one year, only 54 per cent of this loss was maintained.25 Likewise, whether long-term adoption of either regimen is either tenable or even harmful to health remains to be seen.


Rod Tucker is a community pharmacist in East Yorkshire


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2 Statistics on obesity, physical activity and diet — England, 2013. Health and Social care information centre. Available at: (accessed 20 January 2014).
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4 Bilsborough SA, Crowe TC. Low-carbohydrate diets: what are the potential short- and long-term health implications? Asia Pacific Journal of Clinical Nutrition 2003;12:396–404.
5 Banting WH. Letter on corpulence. 4th Edition. London: Harrison; 1869.
6 Pennington AW. Treatment of obesity with calorie unrestricted diets. Journal of Clinical Nutrition 1953;1: 343–8.
7 Hession M, Rolland C, Kulkarni U et al. Systematic review of randomized controlled trials of low-carbohydrate vs. low-fat/low-calorie diets in the management of obesity and its comorbidities. Obesity Reviews 2009; 10:36–50.
8 Bueno NB, de Melo IS, de Oliveira SL et al. Very-low-carbohydrate ketogenic diet v. low-fat diet for long-term weight loss: a meta-analysis of randomised controlled trials. British Journal of Nutrition. 2013; 110:1178–87.
9 Ajala O, English P, Pinkney J. Systematic review and meta-analysis of different dietary approaches to the management of type 2 diabetes. American Journal of Clinical Nutrition. 2013; 97:505–16.
10 Siri-Tarino PW, Sun Q, Hu FB. Meta-analysis of prospective cohort studies evaluating the association of saturated fat with cardiovascular disease.  American Journal of Clinical Nutrition. 2010;91:535–46.
11 Dashti HM, Mathew TC, Hussein T et al. Long-term effects of a ketogenic diet in obese patients. Experimental and Clinical Cardiology 2004; 9:200–5.
12 Lagiou P, Sandin S, Lof M et al. Low carbohydrate-high protein diet and incidence of cardiovascular diseases in Swedish women: prospective cohort study. BMJ 2012;344:e4026.
13 McCay CM, Crowell MF, Maynard LA. The effect of retarded growth upon the length of lifespan and upon the ultimate body size. Nutrition 1989; 5:155–71.
14 Mattison JA, Roth GS, Beasley TM et al. Impact of caloric restriction on health and survival in rhesus monkeys: the NIA study. Nature 2012; 489: 318–21.
15 Anton S, Leeuwenburgh C. Fasting or caloric restriction for healthy aging. Experimental Gerontology 2013; 10: 1003–5.
16 Varady KA, Hellerstein MK. Alternate-day fasting and chronic disease prevention: a review of human and animal trials. American Journal of Clinical Nutrition 2007; 86:7–13.
17 Heilbronn LK, Smith SR, Martin CK et al. Alternate-day fasting in nonobese subjects: effects on body weight, body composition, and energy metabolism. American Journal of Clinical Nutrition 2005; 81:69 –73.
18 Klempel MC, Bhutani S, Fitzgibbon M et al. Dietary and physical activity adaptations to alternate day modified fasting: implications for optimal weight loss. Nutrition Journal. 2010;9:35–43.
19 Harvie MN, Pegington M, Mattson MP et al. The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers: a randomised trial in young overweight women. International Journal of Obesity 2011; 35: 714–27.
20 Lim EL, Hollingsworth KG, Aribisala BS et al. Reversal of type 2 diabetes: normalisation of beta cell function in association with decreased pancreas and liver triacylglycerol. Diabetologia. 2011; 54:2506–14.
21 Fasting: health risks. NHS choices. Available at: (accessed 20 January 2014).
22 Bertenshaw EJ, Lluch A, Yeomans MR. Satiating effects of protein but not carbohydrate consumed in a between-meal beverage context. Physiology and Behaviour. 2008; 93:427–36.
23 Van Gaal LF, Wauters MA, De Leeuw IH. The beneficial effects of modest weight loss on cardiovascular risk factors. International journal of obesity and related metabolic disorders 1997;21 Suppl 1:S5–9.
24 Vidal J. Updated review on the benefits of weight loss. International journal of obesity and related metabolic disorders 2002;26 Suppl 4:S25–8.
25 Barte JMC, ter Bogt CW, Bogers RP et al. Maintenance of weight loss after lifestyle interventions for overweight and obesity, a systematic review. Obesity Review 2010; 11:899–906.

Citation: The Salvadore DOI: 10.1211/PJ.2014.11135526

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