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Low-fat diet

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There’s nothing quite like diets to get people arguing. Calorie controlled, low-fat, Atkins, glycaemic index … they all bring out the worst in bigotry and charlatanism. There must be some truth somewhere, and you’d think science would find the answers, but recent experience is that a report of a study can generate apparently diametrically opposed press coverage: ‘Low-fat diets are better’ said the BBC while the Daily Mail headline said ‘Low-carb is best’ [1, 2].

Actually it was mainly the headlines that were confusing in this case as the actual results from the study were well reported in the articles but the issue with interpreting the results is how well they translate to the real-world setting.

The study involved a small number of obese subjects who followed a short (6-day) baseline diet of 2700 calories per day followed by the experimental 5-day diet where the calorific intake was reduced by a third either by reducing fat or by reducing carbohydrates. Subjects exercised daily. After a washout period of 2-4 weeks, the process was repeated with the other diet [3].

NHS Choices, which keeps a close eye on press coverage of medical research, explains that this was a well-designed study but because of the small number of subjects (19) and the short timeframe the results weren’t convincing enough to settle the debate [4].

In obese subjects fat loss is more important than weight loss and the prevailing view has been that fat loss is not possible without reducing the intake of carbohydrates [e.g. 5]. Hall’s study has shown that this is not the case and that fat loss is possible without reduction in carbohydrate intake.

Hall and his colleagues point out in the paper the difficulties of conducting randomised controlled trials of prescribed diets. For that reason Hall’s research involved subjects being admitted to the metabolic unit at the NIH Clinical Center in Bethesda, US where they stayed for the duration of the research with careful monitoring of all exercise and food intake – conditions that are neither practical nor cost-effective in outpatient studies.

The study showed that

  • Both diets led to weight loss
  • The low-carbohydrate diet affected the metabolism more than low-fat diet. Insulin levels dropped which increased fat burning and lowered carbohydrate burning
  • The low-fat diet led to more fat loss (measured as the difference between the fat taken in and the amount of fat burned) but no significant changes in fat oxidation or insulin secretion
  • Using the normal measure of % body fat, there was no change between the groups

As a result, the researchers were able to definitively reject the hypothesis [5] that carbohydrate restriction is required for body fat loss.

However, despite the clear results, Hall and colleagues stress that the opportunities to translate the results to real-world weight-loss diets for obesity are limited because the experimental model depended on strict control of food intake, which is unrealistic outside the clinic environment

It seems that the old truism remains: the best diet is the one that you can stick to!

Not all fat is equal

Our understanding of diet and how we store and process food is constantly changing. It was only in 2009 that studies showed the importance of brown adipose tissue (brown fat) in humans [6].

Brown fat is interesting because of its ability to burn food directly to produce heat; it can produce 300 times more heat per gram than any other tissue in the body but is also believed to release hormones that help to regulate the metabolism of glucose and fatty acids [see 7].

In general, slimmer people have more brown fat while obese people have less. The natural question to ask is whether the brown fat can be increased, enhanced or somehow made more active. The answer is that it can. Exposure to cold is known to activate the brown fat cells and recent studies suggest that capsaicin, the component of chilli peppers that provides the heat, stimulates brown fat in the same way [8]. Other pharmaceutical interventions to enhance the performance of brown fat could be in the pipeline, including mirabegron, which was developed originally for overactive bladder but has been found to stimulate receptors on the surface of brown fat cells [9].

Why do we need carbohydrates anyway?

From work recently published in the Quarterly Review of Biology it looks as though carbohydrates could have played a critical role in human evolution.

Our brains are bigger than those of our nearest relatives in the animal world, and they use about a quarter of the calories we consume. Karen Hardy and her colleagues outline the persuasive hypothesis that man’s brain did not develop as a result of moving from a plant-based to a meat-based diet. They propose instead that human brains were only able to evolve and grow as a result of including concentrated starch from plant food in the diet to meet the substantial increased metabolic demands of an enlarged brain – and the development of cooking would have to be a component of that development.

The evidence for the use of fire to cook food is – at present – primarily archaeological, but Karen Hardy and her colleagues have been examining the gene that produces the enzyme amylase that is essential to breaking down starch. Their hypothesis is that there must have been genetic selection for higher levels of amylase secretion at the same time as the processes of cooking were evolving. Better understanding of the amylase gene and ancient DNA data will therefore be important in the further refinement of the hypothesis [10].

So the proponents of the ‘Paleolithic diet’ are going to have to add back some starch!

 

If you would like to comment on any of the issues raised by this article, particularly from your own experience or insight, Healthcare-Arena would welcome your views.

References

  1. BBC News website. Low-fat diets ‘better than cutting carbs’ for weight loss [Internet]. BBC News. [cited 2015 Sep 4]. Available from: http://www.bbc.co.uk/news/health-33905745
  2. Daily Mail. Low-carb diets ‘are best for weight loss but forsaking fat is BETTER’ | Daily Mail Online [Internet]. [cited 2015 Sep 4]. Available from: http://www.dailymail.co.uk/health/article-3196738/STILL-great-diet-debate-rages-Low-carb-best-weight-loss-forsaking-fat-BETTER-health-latest-study-reveals.html
  3. Hall KD, Bemis T, Brychta R, Chen KY, Courville A, Crayner EJ, et al. Calorie for Calorie, Dietary Fat Restriction Results in More Body Fat Loss than Carbohydrate Restriction in People with Obesity. Cell Metab. 2015 Sep 1;22(3):427–36.
  4. NHS Choices. Low-fat diet ‘better’ than low-carb diet for getting rid of body fat – Health News – NHS Choices [Internet]. 2015 [cited 2015 Sep 4]. Available from: http://www.nhs.uk/news/2015/08August/Pages/Low-fat-diet-better-than-low-carb-diet.aspx
  5. Taubes G. Why we get fat and what to do about it. New York: Alfred A. Knopf; 2011.
  6. van Marken Lichtenbelt WD, Vanhommerig JW, Smulders NM, Drossaerts JMAFL, Kemerink GJ, Bouvy ND, et al. Cold-activated brown adipose tissue in healthy men. N Engl J Med. 2009 Apr 9;360(15):1500–8.
  7. Lambert C. Grow fat, get thin? We put brown fat to the test | New Scientist [Internet]. [cited 2015 Sep 4]. Available from: https://www.newscientist.com/article/dn27265-grow-fat-get-thin-we-put-brown-fat-to-the-test/
  8. Yoneshiro T, Aita S, Matsushita M, Kayahara T, Kameya T, Kawai Y, et al. Recruited brown adipose tissue as an antiobesity agent in humans. Journal of Clinical Investigation. 2013 Aug 1;123(8):3404–8.
  9. Cypess AM, Weiner LS, Roberts-Toler C, Elía EF, Kessler SH, Kahn PA, et al. Activation of Human Brown Adipose Tissue by a β3-Adrenergic Receptor Agonist. Cell Metabolism. 2015 Jan 6;21(1):33–8.
  10. Hardy K, Brand-Miller J, Brown KD, Thomas MG, Copeland L. The Importance of Dietary Carbohydrate in Human Evolution. The Quarterly Review of Biology. 2015 Sep 1;90(3):251–68.

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