Grand Rounds

The DIETFITS Trial: Can Personal Genetics Determine Whether a Low-Fat or Low-Carb Diet is a Better Weight Loss Choice?

BACKGROUND AND PURPOSE:

While differences between low-fat and low-carbohydrate diets have been slight, there can be larger variability between those enrolled in a particular diet type
It has been suggested in previous literature that these differences could be related to
- Underlying genetic make-up
- Baseline Insulin dynamics such that individuals with greater insulin resistance may have better success with low-carbohydrate diets
Gardner et al. (JAMA 2018) examined the effect of healthy low-fat (HLF) diet vs a health low-carbohydrate(HLC) diet on weight change and if genotype pattern or insulin secretion are related to dietary effects on weight loss

Single-site, parallel-group, randomized trial
Data collected from The Diet Intervention Examining The Factors Interacting with Treatment Success (DIETFITS) trial
Participants: Patients 18 to 50 years without diabetes and body mass index between 28 and 40
Patients were randomized to 12 months of
- HLF diet
- HLC diet
Behavior modification intervention via 22 diet-specific small group sessions
- Focused on ways to achieve the lowest fat or carbohydrate intake that could maintained over long periods of time
3 SNPs (previously identified in weight loss studies) and INS-30 (blood concentration of insulin 30 minutes after a glucose challenge) were tested for association with weight loss
Primary outcome: 12-month weight change and determination of whether there were significant interactions among diet type and genotype pattern, diet and insulin secretion, and diet and weight loss

Data was collected from 609 participants
- Mean age, 40 years | 57% women | mean BMI, 33
- 40% had a low-fat genotype and 30% had a low-carbohydrate genotype
- Mean baseline INS-30, 93 μIU/mL
In the HLF vs HLC diets, respectively, the mean 12-month macronutrient distributions were 48% vs 30% for carbohydrates, 29% vs 45% for fat, and 21% vs 23% for protein
There was no significant difference in weight change at 12 months between the two groups
- −5.3 kg for the HLF diet vs −6.0 kg for the HLC diet (mean between-group difference, 0.7 kg [95% CI, −0.2 to 1.6 kg])
There was no significant diet-genotype pattern interaction (P = .20) or diet-insulin secretion (INS-30) interaction (P = .47) with 12-month weight loss
Secondary Findings
- Blood lipid results
  - HLF group: More favorable LDL cholesterol profile
  - HLC group: More favorable HDL cholesterol and triglyceride profile

There was no significant change in weight loss between a healthy low-fat and low-carbohydrate diet
There was no significant interaction between genotype or insulin secretion with weight loss

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