A Study Comparing the Health Effects of Two Diets Following UK Dietary Guidance in People Living with Overweight or Obesity

There are two parts to this study:

1. The investigators will study if the benefit from eating a healthy, balanced diet depends on the types of food processing in the diet. The investigators will do this by providing participants with two diets that follow the Eatwell Guide (referred to in this study as Diet A and Diet B to avoid unblinding), but containing foods with different types of food processing, for 8 weeks each. The investigators will collect data on blood pressure, body composition, physical activity and fitness, questions regarding quality of life, mental health and wellbeing, and blood samples at the start of each diet and at 4 and 8 weeks into each diet.
2. The investigators will then study whether participants are able to switch from their usual unhealthy diet to a healthy, balanced diet, and the benefits of doing so. The investigators will do this by providing participants with 6 months of personal support. The investigators will also look at what helps participants to maintain a healthy diet, and what makes it difficult. The investigators will also support participants to be more physically active.

Eating a healthy, balanced diet is important for health. A healthy, balanced diet involves eating a variety of foods in the right proportions. In the UK, the Eatwell Guide gives recommendations on what people should eat more of, and what people should eat less of. Unhealthy diets contain lots of foods high in saturated fat, added sugar and salt, which increases the risk of developing obesity, cardiovascular disease and type 2 diabetes. Advice for a healthy, balanced diet includes consuming five portions of fruit and vegetables per day and eating more high fibre foods. Following the healthy, balanced diet advice given in the Eatwell Guide can reduce the risk of poor health.

However, there are other parts of the diet that are not covered in the Eatwell Guide that may also be important for health. For example, research suggests that different types of food processing might influence health, but these types of food processing are not included within the recommendations.

It is unknown if the benefits of following the healthy diet advice in the Eatwell Guide depend on the types of food processing in the diet. It is important to find out if these types of food processing matter, in order to give people the best advice possible. It also means that the government and other health organisations may need to change the regulations around the food that people eat.

To answer this, the investigators will compare the effects of two healthy, balanced diets following the advice in the UK Eatwell Guide, but each based on a different type of food processing (referred to in this study as Diet A and Diet B to avoid unblinding). Participants will receive the first diet for 8 weeks, return to their normal diet for 4 weeks, and then receive the second diet for a further 8 weeks. Participants will have 6 months of support to help improve their diet and be more physically active. Participants will work with a behavioural scientist to create a personal plan to eat a healthier diet and be more physically active, using behaviour change techniques based on capability, opportunity, motivation - behaviour (COM-B) theory of behaviour change. This support will last for 6 months, with ongoing monthly telephone/video calls with the research team. The results from this study will be used to help inform UK dietary guidance and food policy.

There are two parts to this study:

1. The investigators will study if the benefit from eating a healthy, balanced diet depends on the types of food processing in the diet. The investigators will do this by providing participants with two diets (Diet A and Diet B) that follow the Eatwell Guide, but containing foods with different types of processing, for 8 weeks each.
2. The investigators will then study whether participants are able to switch from their usual unhealthy diet to a healthy, balanced diet, and the benefits of doing so. The investigators will do this by providing participants with 6 months of personal support. The investigators will also look at what helps participants to maintain a healthy diet, and what makes it difficult. The investigators will also support participants to be more physically active.

Eligibility Criteria:
Inclusion Criteria:

* Staff at University College London Hospitals (UCLH)
* Adults aged between \[18 and 65\] years old.
* BMI ≥25 kg/m2 (living with overweight or obesity)
* Weight stable for the past 3 months (≤5 % variation in body weight over preceding 3 months)
* Have a habitual dietary intake high in unhealthy food
* Able to read and write in English.
* Medically safe to participate in a dietary intervention programme.
* Willing and able to give written informed consent.
* Able to attend the relevant in person and online sessions.
* Able to comply with the study protocol (including dietary recommendations for each intervention and reporting adherence).
* Females of childbearing potential and males agree to use an effective method of contraception from the time consent is signed until the end of the intervention period and final follow-up assessment. Effective methods of contraception acceptable for this trial are outlined in Appendix 2.
* Females of childbearing potential must be on highly effective contraception and have a negative pregnancy test within 7 days of being randomised. NOTE: Participants are considered not of childbearing potential if they have undergone a hysterectomy, bilateral tubal ligation, or bilateral oophorectomy) or they are postmenopausal.

Exclusion Criteria:

* Contraindication for dietary intervention
* Participation in another clinical intervention trial
* Concomitant usage of medications that cause weight gain or weight loss
* Cardiometabolic comorbidities (e.g. diabetes, on insulin)
* Coeliac disease
* Inflammatory bowel disease
* A diagnosed eating disorder
* Planning a weight management programme in the next 3 months
* Any diagnosed food allergy, or other allergies which limit the ability to adhere to the intervention diet
* Dietary restrictions (e.g. vegan or vegetarian) which limit the ability to adhere to the interventions
* BMI \>40 kg/m2 or basal metabolic rate ≥2300/kcal/day (to ensure intervention diets are at least 300kcal/day greater than maintenance energy needs).
* Females who are pregnant, breast-feeding, or intends to become pregnant.
* A history of drug or alcohol abuse
* Any other factor making the participant unsuitable in the view of investigator.