Low Carbohydrate Weight Loss : Ketogenic Diets - The Evidence

By James Collier BSc (Hons) - Consultant in Nutrition and Moderator of Dietetics.co.uk

People are always looking for a quick fix to lose weight and, as health professionals, we are frequently questioned about new weight loss diets on the scene. Low carbohydrate, high protein or ketogenic diets are one of these 'faddy' diets. They are not a new concept, but, with huge media coverage from the 'success' of stars like Geri Halliwell and Jennifer Aniston, many more people are following these regimens.

Low carbohydrate, high protein or ketogenic diets seem to be all around us! They have been 'fashionable' on and off for years. Many of your patients, colleagues or even you may have tried such programmes, so it is important that you, as a health professional, are aware of the evidence. Do they 'work'? Are they healthy? Are there more efficient ways of controlling your weight?

Collectively, these regimens are known as 'low carb' or 'ketogenic' diets and have many variants. One of the better known is the Atkins diet, around since the 1970s. Others include Radiant Health (Professor Brian Peskin), the Carbohydrate Addicts Diet (Dr Richard Heller and Dr Rachel Heller), the Diet Cure (Julia Ross), Protein Power (Dr Michael Eades and Dr Mar Eades), Eat Fat, Get Thin (Barry Groves), and many more.

The Claims
These diets claim to alter the way we metabolise food, and their advocates argue that obesity is a consequence of dietary carbohydrate rather than dietary fat or total calories. It is claimed that excess dietary carbohydrate raises insulin levels which, in turn, causes more body fat deposition. The theory is that a higher protein in relation to carbohydrate intake lowers insulin levels and causes weight loss. These diets are also said to help food cravings and help fill you up as protein foods promote satiety. Other health claims are that they improve lipid profiles and benefit hypertension.

The Scientific Evidence
There is absolutely no evidence, though, that weight loss can be achieved in any way, other than by creating a negative energy balance, i.e. weight is lost when we burn up more than we eat. There is also considerable evidence that hyperinsulinaemia, hyperglycaemia, hyperlipidaemia and hypertension are closely linked to obesity, and that weight loss by any means improves these health parameters (1).

There is evidence that calorie deficient low carbohydrate, high protein diets do produce a more rapid and greater initial weight loss compared to isoenergetic more moderate carbohydrate diets, with the associated risk factor benefits (2,3). Unfortunately, the initial high rate of weight loss is not only due to reduction in energy stores, but primarily due to changes in fluid balance. The reduction in muscle glycogen (the storage form of carbohydrate) stores is accompanied by a reduction in water; for every 1g glycogen metabolised, 3g of water is lost.

Also, on extremely low levels of dietary carbohydrate (i.e. less than 50-100g per day; 10-20% total energy intake) high levels of ketones are produced. Ketones are used as a fuel by the brain and heart muscle during times of low food intake and help keep us alive. The presence of ketones means increased sodium and potassium and associated water loss (more weight loss), bad breath, taste changes and nausea, which obviously may lead to reduced food intake.

Ketogenic diets are very limited in variety, and variety in the diet is one of the fundamental healthy eating guidelines in order to achieve a balanced diet and optimal nutrition. They fill you up more than on diets of comparable energy intakes with a higher carbohydrate : protein ratio. Protein rich diets also increase thermogenesis, and therefore 24-hour energy expenditure more than with similar energy higher carbohydrate intakes (4). Whether this persists longer term remains untested.

As a consequence of all the above in ketogenic diets, there is greater potential for a negative energy balance, hence consequential weight loss. Weight loss is therefore not due to raised serum insulin levels promoting body fat deposition. There may also be a consequential reduction in lean body mass.

Potential Problems
Low carbohydrate diets are, by definition, high in protein and fat. As less fibrous starches are consumed, there is a reduction in dietary fibre intake too, as well as antioxidant vitamins and minerals. These factors signify increased risk of cardiovascular diseases, cancers, renal disease and bowel disorders. Other problems are the negative consequences of the ketotic state discussed above.

Nutritional intakes of people on low carbohydrate, high protein diets have been shown to be low in calcium content, as are the intakes of people following more orthodox weight reducing regimens. This is a result of general low food intake. However, low carbohydrate, high protein diets may also lead to increased calcium excretion and reduced bone mineral content (5). There is insufficient evidence of the direct effect of the low calcium content of low carbohydrate, high protein diets increasing risk of osteoporosis later in life, but it is very likely. The group most likely to follow ketogenic diets are women 15-30years old, the main bone building years where a good calcium intake is crucial.

In Summary
Low carbohydrate, high protein diets are no 'magic' answer for weight loss, and certainly do not follow the well-evidenced healthy-eating guidelines. They also have the potential to cause nutrition related problems to health.

However, there are no studies comparing the short-term use of such diets in relation to health problems, so there may well be benefits of short-term use outweighing any potential risk for certain obese people who really struggle to lose weight by more traditional means (6). There is no evidence describing what 'short term' actually means. Further research is therefore required. Also the extent of physiological adaptation to these regimens is unclear.

There is very good evidence though, that manipulation of low fat, moderate protein and moderate carbohydrate is more promising in respect of efficient and consistent weight reduction (6). This, of course, is improved further with the inclusion of an associated increase in physical activity.

Whilst many people do claim some good initial weight loss with low carbohydrate, high protein diets, this rate of loss slows down after a few weeks. Most people also claim to put most, if not all, the weight back on (and sometimes more) when they cease the 'diet'. Therefore defeating the object of following such a stringent plan. Many may follow such regimens for a special occasion or seasonal preference, which could lead to significant weight cycling, identified in its own right as a risk factor for cardiovascular disease.

Low carbohydrate, high protein diets do not fit in with healthy eating guidelines, they are unbalanced and difficult to fit in with lifestyles and family meals. They also take no account of the psychological influences of why people over eat, crucial in any weight reduction programme.

Ketogenic diets are not recommended for pregnant or lactating women, children and adolescents, for kidney or bowel disease patients or, indeed, for general use. However, with more research, there is the potential for short-term weight reduction in high-risk obese patients.

Key Points

  • Some evidence for greater initial weight loss compared to equivalent hypocaloric, higher carbohydrate diets
  • Main difference in weight loss is from fluid from reduced glycogen stores and the ketotic state
  • Improvements in health parameters is associated with the weight loss and not directly from the diet itself
  • Potential for increased risk of a number of nutrition related diseases
  • Insufficient research exists and further randomised, controlled cohort studies are required in order for specific recommendations to be made

1. British Nutrition Foundation (1999). Obesity. The report of the British Nutrition Foundation Task Force. Blackwell Science
2. Skov AR et al (1999). Randomised trial on protein Vs carbohydrate in ad libitum fat reduced diet for the treatment of obesity. Int J Obesity 23(5): 528-536
3. Hwalla et al (1999). High protein Vs high carbohydrate hypoenergetic diet for the treatment of obese hyperinsulinaemic subjects. Int J Obesity 23(11): 1202-1206
4. Mikkelson et al (2000). Effect of fat reduced diets on 24-h energy expenditure: comparisons between animal protein, vegetable protein and carbohydrate. Am J Clin Nutr 72(5): 1135-1141
5. Willi et al (1998). The effects of a high protein, low fat, ketogenic diet on adolescents with morbid obesity: Body composition, blood chemistries and sleep abnormalities. Paediatrics 101(11): 61-67
6. Robson (2001). High protein, low carbohydrate diets for weight loss: clarifying the evidence. Dietetics Today 36(9): 13-15