The Obesity Epidemic (part 6): The Effects of Diet

The previous blog in this series discussed some of the genetics surrounding obesity.  More than 20 genes have been discovered that may be involved.  

I’m sure there are more.  

The next issue I’d like to examine is the effects of diet on weight loss.  But interestingly, there is evidence that the type of diet one has is influenced to some degree by the types of genes you have.

First, there are generally four types of diets: low fat, low carbohydrate, low calorie, and very low calorie.

The low-fat diet, as the name implies, reduces the percentage of fat in one’s diet. Concurrently, calories are reduced because energy-dense fat is reduced.

The low carbohydrate diet is higher in protein and fat, since carbohydrates are reduced. These include the Atkins and Protein Power diets; the new Paleo diet would also fit in this category.  

Low calorie diets are what their name suggests—they typically produce a deficit of 500-1,000 calories/day. The DASH diet and Weight Watchers are low calorie diets.

Finally, the very low-calorie diet is a near-starvation diet, providing 200-800 calories/day.  

Well, all of these diets will cause you to lose weight.  An interesting question is, which is better?

Guess what—there is NO scientific evidence that one diet is better than the other.  They all cause weight loss—and perhaps weight loss is observed because they all result in a reduction in calories—a conclusion reached by almost all the studies.  Let me repeat: there is no evidence that having high or low carbohydrates, high or low protein, or high or low fat confers a diet that has any more superiority over another for weight loss.

Such was a 2012 report of 424 men and women who had BMI’s of over 30, tracking fat as well as muscle loss.  Tracking muscle loss makes this study different from most weight studies, who only look at waist circumference and/or total weight.   Further, this study looked at total fat, visceral fat, subcutaneous abdominal fat, as well as hepatic fat, using X-ray analysis.

Simply, visceral fat is “deep fat” that wraps around organs, resulting in a large belly and waist, and is considered the most dangerous of the types of fat.  Subcutaneous abdominal fat is fat found directly under the skin, and is typically measured by a “pinch test.” Hepatic fat is fat leading to “fatty liver disease” wherein fat accumulates in liver cells.

This 2012 paper looked at the effects of four weight loss diets that all had reduced calories (the following percentages are amount of energy provided by the nutrients): 1.) low fat (20% fat), average protein (15% protein), high carbohydrate (65%); 2.) low fat (20% fat), high protein (25%), high carbohydrate (55%); 3.) high fat (40% fat), average protein (25% protein), low carbohydrates (35%).

They found that after 6 months on the diets participants lost an average of 9.3 pounds of total fat and 4.6 pounds of muscle.  Of the fat, 5.0 pounds were abdominal fat, 3.3 pounds were visceral fat.  There was no significant difference for hepatic fat.  Further, as stated above, there was no significant difference between the diets.  Finally, after two years, participants gained back 40% of their weight loss. There was also apparent “cheating” on the diets, as, for example, urinary nitrogen levels were the same between the low and the high protein groups (it is expected that as protein in the diet is increased, then there is more excretion of nitrogen).

In recent years there has been a lot of interest shown in high protein diets.  A 2013 paper reported a meta-analysis of 15 studies involving 1,990 male and female subjects, all obese, and lasting for a year or more. The dietary protocol of all studies was high or low protein, low fat, variable carbohydrates, and 11 studies had energy restriction and 4 did not.

What did they find? No effects of either high or low protein diets on weight, waist circumference, or fat mass. Nor were there effects on total cholesterol, high density lipoprotein (HDL) ,though HDL was near statistical significance, and total triglycerides, or blood pressure.  However, high protein diets DID result in statistically lower fasting insulin levels. And finally, for those of you on high protein diets (like myself sometimes!), there was no difference in renal (kidney) function.

HOWEVER, all may not be so simple—which seems to be a general conclusion in dietary studies. A 2013 meta-analysis of “all-cause mortality” which looked at 17 studies covering 272,216 people, the largest such analysis to date, concluded that low carbohydrate diets had a statistically significant increase in mortality, and there was no effect one way or the other on cardiovascular disease. The authors conclude:

“Given the facts that low-carbohydrate diets are likely unsafe and that calorie restriction has been demonstrated to be effective in weight loss regardless of nutritional composition…it would be prudent not to recommend low-carbohydrate diets for the time being.”

Now, to turn to the point I introduced at the beginning of this blog: the types of genes you have may influence the effectiveness of weight loss diets as well as where on your body you lose weight.

As mentioned in the Obesity-5 blog, some FTO genes have been associated with obesity.  Actually, I glossed over the point that there are different kinds of FTO genes—some are associated with obesity and some are not.  Of relevance here is that one of these genes is known by scientists as rs1558902.   Let’s call it gene “A” for this discussion.  Further, let’s call the normal FTO gene (not causing obesity), “T.”

In a study published in 2012, 742 obese adults were classified as having the “A” gene or “T” gene.  They next randomly divided these individuals into four diets: 20% fat, 15% protein, 65% carbohydrate; 20%, 25%, 55%; 40%, 15%, 45%; and 40%, 25%, 35%.    Interestingly, the found that HIGH PROTEIN diets showed the most impact on weight loss parameters amongst those that that the “A” gene.   So in other words, if you only had the “T” gene you did not respond to a high protein diet.  

This constitutes proof that your response to a particular diet depends on the types of genes you have—or to say it differently, people that do not respond to high vs. low protein diets may do so for innate reasons.

I can only assume that given the large number of genes that influence obesity, many other such interactions would be found—leading to the conclusion that not one diet fits all people.  

But, the question is NOT just which diet causes weight loss, but HOW sustainable is the diet? That is, is the diet one that folks will stay on for years? The rest of their life?  How about other life-style changes that are incorporated in to some diets, such as exercise—is it sustainable?  A rather shocking observation in all of the diet studies that I looked at is that the drop out rates were discouraging. The 2012 study above had 20% drop out rates; the 2013 meta-analysis of 15 studies had dropout rates ranging from 8% to 55%, with most around 30%.  Plus, there is evidence of dietary cheating in some studies—which must raise questions about some conclusions.

In summary, there is no scientific consensus as to which diet is best for weight loss.  The ONLY consensus (so far) is that calorie reduction has the greatest impact, and even that may depend on they types of genes you have.  

And now I’m done with Obesity!! On to other subjects.  

Useful References:

http://www.cmaj.ca/content/174/1/56.full
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3041737/
http://www.nejm.org/doi/full/10.1056/NEJMoa0804748#t=articleBackground
http://ajcn.nutrition.org/content/95/3/614.long
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3636027/    
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2646098/