It has been suggested that variations in genetic makeup make it easier for some people to lose weight than others when it comes to certain diets. Interest in genomic testing and dietary advice based on one’s genes is increasing. And we are not talking about theories that suggest people’s blood types determine what foods they should and should not eat. Proponents of low carbohydrate diets have long argued that some people are genetically prone to gain weight, develop insulin resistance, and eventually get type 2 diabetes mellitus (DM) on a high carbohydrate diet. Gene analyses has identified genetic variations linked with how the body processes fats and carbohydrates. It has been claimed that people with genes associated with insulin resistance and type 2 DM are able to lose weight more easily by restricting dietary carbohydrates than by limiting fat intake. To test the validity of this theory, researchers at Stanford University headed by Dr. Christopher Gardner conducted a randomized control trial involving 609 overweight adults. Prior to being randomly assigned to either a healthy low-carbohydrate (HLC) or healthy low-fat (HLF) diet, the subjects underwent genetic testing and insulin testing. They were then followed for 12 months to see if certain genes or their fasting insulin or insulin response to a carbohydrate load impacted their ability to lose weight.
“We told everyone in both groups to eat as little white flour and sugar and as many higher-fiber vegetables as possible,” says lead investigator Christopher Gardner, professor of medicine at Stanford University. The low-fat group was advised to eat healthy carbohydrate-rich foods like lentils, low-fat yogurt, steel-cut oats, quinoa, fresh fruit, and beans. The low-carb group was told to eat high-quality fats like salmon, avocados, nuts, seeds, hard cheeses, and olive oil. And remarkably no one told the participants to focus on reducing calories. “If you prescribe calorie restriction, people feel deprived,” says Gardner. “We just said, ‘Eat as low as you can on fat or carbs and don’t be hungry.’”
The results: Whether they cut fat or carbs, “each group averaged about a 500-calorie reduction daily,” says Gardner. And after a year, the people in each group had lost an average of about 12 pounds. In other words, when it comes to weight loss, it doesn’t matter if you cut carbs or fat. Although the study participants weren’t told to count calories, calorie intake still mattered.
Perhaps the most surprising finding of Dr. Gardner’s study to many people will be that it really didn’t matter if people were resistant to their body’s insulin or not when they entered the study. Contrary to what has been claimed for years by promoters of low-carbohydrate diets, this new study found people who were insulin resistant and/or genetically prone to gain weight and become insulin resistant the low-fat, high-carbohydrate diet composed of mostly healthy high-carbohydrate foods like whole grains, fruits, and vegetables was just as effective or slightly more effective than the healthy high-fat diet for improving insulin sensitivity and just as effective for weight loss. And yet according to Dr. Gardner: “We assumed that insulin-resistant people would do better on a low-carb diet—as they did in some earlier studies—but they didn’t.” Maybe there was no difference in weight loss because both groups were told to eat healthy foods, he suggested. “In some older studies, when researchers told people to eat less fat, they weren’t particular about which low-fat foods to eat. Coke and white flour and sugar are low-fat” noted Dr. Gardner. What seemed to make a difference was healthy food choices regardless of their fat to carbohydrate ratio according to the researchers. Indeed, it was the study participants who ate the most vegetables and consumed the fewest processed foods, sugary drinks, and unhealthy fats that ended up losing the most weight. Professor Lennert Veerman from the School of Medicine at Griffith University in Queensland Australia suggested the study showed that there was probably no such thing as the single right diet based on a person’s genetic make-up.
Dr. Gardner also noted that — as seen in several prior studies — the results varied dramatically among subjects. “Someone lost 60 pounds, someone gained 20 pounds, and we saw everything in between,” he says. “The range, which was similar in both diet groups, was stunning.” However, neither genetic factors associated with the development of insulin resistance and type 2 DM or elevated insulin response to foods were of any value in predicting the subject’s response to the HLF or HLC diets in terms of weight change or insulin sensitivity. Subjects on both the HLC and HLF diets experienced almost the same average of about a 12 pound weight loss after one year regardless of genes, insulin levels, or diet type assignment. While there were big differences in terms of how various people responded to each type of diet, these differences were not related the percent of fat or carbohydrate calories of the diet they were assigned to follow.
My Commentary: The time has come to focus on what people eat rather than on how much they eat for long term health and weight control. It seems increasingly clear that people can lose as much weight on a healthy low-fat diet as a healthy low-carb diet. Most people find it cumbersome to count calories and fight hunger, especially over the long term. The key to long term weight control appears to be eating mostly whole foods, mostly plants, and especially those with a lower calorie density and higher fiber content. Cut out calorie-dense and nutrient-poor foods like those composed largely of refined white flour, added sugars, and refined fats and oils. Limit the tendency to eat calorie-dense snack foods and desserts that are usually loaded with refined grains, sugars, fats, and oils. Those fattening foods promote weight gain because they provide far less satiety per calorie than whole plant foods. Calorie-dense modern processed foods and large amounts of beverage calories force people to choose between chronic hunger or being chronically overweight and/or cycling between those too unpleasant states. Instead of those fattening foods and drinks, eat as many fiber-rich vegetables and whole fruits as possible. More salads and more soups loaded with vegetables, whole grains, beans, and only modest amounts of low saturated fat animal foods. Avoid beverage calories and especially those high in sugar and with little or no fiber including fruit juices. Doing so increases satiety per calorie thus helping people reduce their calorie intake without having to think about it and without being chronically hungry. This doesn’t mean that calories don’t matter, it just means that counting them has a long history of failure and all too often leads to frustration, which helps create confusion and disordered thinking about eating. This disordered thinking is what has been labelled as an eating disorder and appears to result in most cases from repeated failed attempts to keep the weight off over the long term by attempting to control how much one eats rather than focus on what one eats.
Bottom Line: Dr. Gardner’s study should inform us that it is time to shift the focus away from some imaginary optimal ratio of dietary fat, protein, and carbohydrate and instead focus dietary advice on reducing the consumption of highly processed and refined foods and beverages and saturated fat-rich animal foods and increasing the consumption of whole fruits, vegetables, whole grains, and beans supplemented perhaps with modest amounts of low-saturated fat animal foods.
By James J. Kenney, PhD, FACN
Gardner CJ, Trepanowski JF, Del Gobbo LC, et. al. Effect of low-fat vs low carbohydrate diet on 12-month weight loss in overweight adults and the association with genotype pattern or insulin secretion: The DIETFITS Randomized Clinical Trail. JAMA 2018;319(7):667-79.
Stephanie Ronco has been editing for Food and Health Communications since 2011. She graduated from Colorado College magna cum laude with distinction in Comparative Literature. She was elected a member of Phi Beta Kappa in 2008.