I found this here
In the summer, body weight can go up by several pounds due to increased body water. This is accomplished through fluid-conserving hormones such as aldosterone, which allows the kidney to retain more fluid and reduces the amount of salt in sweat, a measure that also aids in water retention. The increase and stabilization of total body water can only be accomplished by continuing to exercise in hot weather and will not occur in people who spend most of their time indoors in air-conditioned environments.
There has already been quite a bit of discussion about it being NORMAL to retain more fluid in the heat so I have to ask why it is such a concern. We know we are losing fat and we know the scale is only one measure. We know our goals are long-term and the heat won't remain forever.
The plan works.
Now, if there is a reason for phasing off p1 for medical reasons or by someone who has regained control and wants to use whole foods instead of processed, I support that. The amt of soy/processed food in IP is what kept me from starting IP eight months sooner than I did. However, water retention during a heat wave really isn't a problem.